The Pennsylvania State University

The Graduate School

College of Engineering

A STUDY ON THE DEFORMATION AND BREAKUP OF

SUPERCOOLED LARGE DROPLETS AT THE LEADING EDGE

OF AN AIRFOIL

A Thesis in

Aerospace Engineering

by

Belen Veras-Alba

© 2017 Belen Veras-Alba

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

August 2017

The thesis of Belen Veras-Alba was reviewed and approvedú by the following:

Jose L. Palacios

Assistant Professor of Aerospace Engineering

Thesis Advisor

Michael P. Kinzel

Research Associate

Applied Research Laboratory

Philip J. Morris

Professor of Aerospace Engineering

Head of the Department of Aerospace Engineering

úSignatures are on file in the Graduate School.

ii

Abstract

Ice accretion is an issue that has a�ected aircraft since the early years of powered

flight. Although it was a known problem, the full extent was not known. Both small

and large droplets were of concern. The e�ects of both were countered with ice

protection systems based initially on computer codes that predict the size, shape,

and location of ice on aerodynamic surfaces for small droplets. The codes have been

tested and validated for the conditions described in Federal Aviation Regulation Part

25 Appendix C (small droplets, up to 50 µm) and aircraft only had to be certified

for those conditions. Supercooled large droplets (SLD) reach locations further aft

on the surfaces than small droplets making the ice protection systems insu�cient in

SLD icing conditions. The protection systems remove ice but do not reach the limits

of the SLD ice and ridges remain on the wing surfaces which continue to negatively

impact the performance of the aircraft. Certification regulations regarding SLD

have been implemented but the codes do not yet accurately predict ice accretion

due to SLD. To validate the codes, experimental data on the behavior of larger

droplets when impacting a lifting surface are necessary.

The results of an experimental study on the deformation and breakup of super-

iii

cooled droplets near the leading edge of an airfoil are presented. The experiment

was conducted in the Adverse Environment Rotor Test Stand (AERTS) facility

at The Pennsylvania State University with the intention of comparing the results

to prior room temperature droplet deformation results. To collect the data, an

airfoil model was placed on the tip of a rotor blade mounted onto the hub in the

AERTS chamber. The model was moved at speeds between 50 and 80 m/s while a

monosize droplet generator produced droplets of various sizes which fell from above,

perpendicular to the path of the model. The temperature in the chamber was set

to -20¶C. The supercooled droplets were produced by maintaining the temperature

of the water at the droplet generator under 5¶C. The supercooled state of the

droplets was determined by measurement of the temperature of the droplets at

various distances below the tip of the droplet generator. A prediction code was

also used to estimate the temperature of the droplets based on the size, vertical

velocity, initial temperature, and distance traveled by the droplets. The droplets

reached temperatures between -5 and 0¶C. The deformation and breakup events

were observed using a high-speed imaging system. A tracking software program

processed the images captured and provided droplet deformation information along

the path of the droplet as it approached the airfoil stagnation line.

It was demonstrated that to compare the e�ects of water supercooling on droplet

deformation, the slip velocity and the initial droplet velocity must be the same in the

cases being compared. A case with a slip velocity of 40 m/s and an initial droplet

velocity of 60 m/s was selected from both room temperature and supercooled

droplet tests. In these cases, the deformation of the weakly supercooled and warm

iv

droplets did not present di�erent trends when tested in room temperature and mild

supercooling environments. The similar behavior for both environmental conditions

indicates that water supercooling has no e�ect on particle deformation for the

limited range of the weak supercooling of the droplets tested and the selected

impact velocity.

v

Table of Contents

List of Figures viii

List of Tables xi

List of Symbols xii

Acknowledgments xv

Chapter 1Introduction 11.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . 11.2 Supercooled Large Droplet Studies . . . . . . . . . . . . . . . . . . 61.3 Thesis Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.4 Thesis Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.4.1 Chapter 2: Experimental Setup and Test Procedures . . . . 151.4.2 Chapter 3: Data Analysis . . . . . . . . . . . . . . . . . . . 161.4.3 Chapter 4: Experimental Results and Comparisons . . . . . 161.4.4 Chapter 5: Conclusions and Future Work . . . . . . . . . . . 16

Chapter 2Experimental Setup and Test Procedures 172.1 Airfoil Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Rotor Stand Facility . . . . . . . . . . . . . . . . . . . . . . . . . . 192.3 Droplet Generator System . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.1 Droplet Generator . . . . . . . . . . . . . . . . . . . . . . . 212.3.2 Frequency Generator . . . . . . . . . . . . . . . . . . . . . . 222.3.3 Air Pressure Controller . . . . . . . . . . . . . . . . . . . . . 222.3.4 Water Reservoir . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.4 High-Speed Imaging System . . . . . . . . . . . . . . . . . . . . . . 252.5 Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

vi

2.6 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.7 Droplet Supercooling . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.7.1 Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.7.2 Droplet Temperature Measurement Test Procedure . . . . . 36

2.8 Test Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.9 Previous Attempts . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.9.1 Attempt 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.9.2 Attempt 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Chapter 3Data Analysis 413.1 Tracking a Single Droplet . . . . . . . . . . . . . . . . . . . . . . . 413.2 Calculation of the Horizontal Velocity and Acceleration of the

Droplet against Time . . . . . . . . . . . . . . . . . . . . . . . . . . 463.3 Change of the Frame of Reference . . . . . . . . . . . . . . . . . . . 473.4 Relative Velocity between the Droplet and the Air (Slip Velocity) . 493.5 Calculation of the Reynolds Number, the Weber Number, and the

Bond Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Chapter 4Experimental Results and Comparisons 554.1 Droplet Supercooling . . . . . . . . . . . . . . . . . . . . . . . . . . 554.2 Comparison of the Experimental Data Sets . . . . . . . . . . . . . . 624.3 E�ect of Initial and Slip Velocity on the Deformation and Bond

Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664.4 E�ect of Temperature on the Deformation and Bond Number . . . 68

Chapter 5Conclusions and Future Work 725.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 725.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Bibliography 77

vii

List of Figures

1.1 Droplet breakup near the leading edge of the NACA 0012 6.1-mairfoil for droplets of 100 (top left), 500 (top right), and 1000 µm(bottom left) in diameter. The image in the bottom right is aclose-up of the droplet breakup in Area Z in the bottom left image . 4

1.2 Photograph of experimental setup used for the droplet deformationand breakup experiments conducted at INTA. Airfoil chord = 0.71m (27.95 in) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Photographs of the splashing of 94-µm MVD droplets . . . . . . . . 71.4 Collection e�ciency versus distance from the leading edge . . . . . . 81.5 Collection e�ciency versus distance from the leading edge . . . . . . 101.6 Close-up of SLD clear ice . . . . . . . . . . . . . . . . . . . . . . . . 111.7 Close-up of SLD horns . . . . . . . . . . . . . . . . . . . . . . . . . 111.8 Close-up of SLD nodules . . . . . . . . . . . . . . . . . . . . . . . . 12

2.1 Plot of coordinates of airfoil model profile. . . . . . . . . . . . . . . 182.2 CAD model with dimensions in inches. . . . . . . . . . . . . . . . . 182.3 Top and side views of an airfoil model mounted on a rotor blade. . . 192.4 Rotor stand and nozzles in ceiling . . . . . . . . . . . . . . . . . . . 202.5 Droplet generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.6 B&K Precision 4011A function generator. . . . . . . . . . . . . . . 232.7 MicroFab CT-PT-21 Pneumatics Controller. . . . . . . . . . . . . . 232.8 Photograph of the reservoir lid showing modifications. . . . . . . . . 242.9 Schematic of the bucket showing components inside of the reservoir. 252.10 Photron SA-Z high speed camera with 200 mm Micro Nikkor lens

and doubler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.11 Front view of triangular mirror. . . . . . . . . . . . . . . . . . . . . 272.12 Schematic of the camera setup. . . . . . . . . . . . . . . . . . . . . 272.13 Photograph and schematic of the hub structure and light source setup. 292.14 Photograph of the light source in testing location. . . . . . . . . . . 302.15 Photograph of the experimental setup. . . . . . . . . . . . . . . . . 31

viii

2.16 Photograph of the camera image calibration setup. . . . . . . . . . 322.17 Sample calibration image. Resolution: 560Hx384V, 29 pix/mm.

Line spacing is 1 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . 322.18 Schematic of droplet supercooling experimental setup. . . . . . . . . 362.19 Laser reflected o� of circular mirror showing path to be followed by

light source to camera lenses. . . . . . . . . . . . . . . . . . . . . . 40

3.1 Droplet breakup prior to impact. Chamber Temperature = -20¶C,V

airfoil

= 80 m/s, droplet diameter (left) = 473 µm, droplet diameter(right) = 401 µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2 First and last frame of video of a droplet being tracked. (Left:droplet tracking begins, Right: droplet tracking ends) Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 473 µm,165 frames in video, 1.83 milliseconds in length. . . . . . . . . . . . 43

3.3 Sequence of droplet deformation images. Chamber temperature =-20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm. . . . . . . . . 443.4 Horizontal displacement versus time with curve fit. Chamber tem-

perature = -20¶C, Vairfoil

= 80 m/s, droplet diameter = 401 µm.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.5 Droplet velocity versus distance from airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm. 483.6 Droplet acceleration versus distance from airfoil leading edge. Cham-

ber temperature = -20¶C, Vairfoil

= 80 m/s, droplet diameter = 401µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.7 Curve fit of air velocity measurements taken at various distancesfrom the leading edge of an airfoil (chord = 0.047) . . . . . . . . . 50

3.8 The droplet, air, and slip velocities of the droplet as it approachedthe airfoil model. Chamber temperature = -20¶C, V

airfoil

= 80 m/s,droplet diameter = 401 µm. . . . . . . . . . . . . . . . . . . . . . . 51

3.9 Reynolds number versus distance from the airfoil leading edge.Chamber temperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter= 401 µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.10 Weber number versus distance from the airfoil leading edge. Cham-ber temperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.11 Bond number versus distance from the airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm. 54

4.1 Sample of image obtained using the IR camera to measure thedroplet temperature along with schematic. . . . . . . . . . . . . . . 56

ix

4.2 Droplet temperature versus distance from the droplet generator.Results of experimental method used for measuring droplet temper-ature at various vertical distances from the droplet generator. . . . 57

4.3 Droplet temperature versus distance from the droplet generator.Results of the prediction code used for determining droplet tempera-ture at various vertical distances from the droplet generator. InitialTemperature = 1¶C. . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.4 Photograph of ice shape formed on airfoil model during tests. . . . . 604.5 Droplet temperature versus distance from the droplet generator.

Results of the prediction code used for determining droplet tempera-ture at various vertical distances from the droplet generator. InitialTemperature = 3¶C. . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.6 Schematic of droplet generator showing parts inside of droplet gen-erator [8]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.7 Droplet deformation and Bond number versus droplet diameter.Data collected at INTA and in AERTS facility plotted on samegraph for comparison. V

slip

= 60 m/s. Blue Circle (AERTS):Chamber Temperature = -20¶C, V

airfoil

= 80 m/s. Orange Square(INTA): Chamber Temperature = 20¶C, V

airfoil

= 90 m/s. . . . . . 644.8 Deformation versus diameter varying the initial droplet velocity with

a constant slip velocity of 50 m/s. Chamber Temperature = -20¶C. 674.9 Deformation versus diameter varying the slip velocity with a constant

initial droplet velocity of 70 m/s. Chamber Temperature = -20¶C. . 684.10 Droplet deformation versus droplet diameter in warm (20 ¶C) and

cold (-20¶C) environments. Initial drop velocity = 60 m/s. Slipvelocity = 40 m/s. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.11 Bond number versus droplet diameter in warm (20 ¶C) and cold(-20¶C) environment. Initial drop velocity = 60 m/s. Slip velocity= 40 m/s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.12 Droplet deformation versus droplet diameter in warm (20 ¶C) andcold (-20¶C) environment. Initial drop velocity = 70 m/s. Slipvelocity = 50 m/s. . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.13 Bond number versus droplet diameter in warm (20 ¶C) and cold(-20¶C) environment. Initial drop velocity = 70 m/s. Slip velocity= 50 m/s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5.1 Schematic of setup with new design for droplet supercooling. . . . . 76

x

List of Tables

2.1 Test matrix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

xi

List of Symbols

a Ellipse minor axis

AERTS Adverse Environment Rotor Test Stand

AIAA American Institute of Aeronautics and Astronautics

b Ellipse major axis

B Coe�cient of thermistor

b/a Droplet deformation = ellipse major axis/ellipse minor axis

Bo Bond number

CAR Civil Air Regulations

CFR Code of Federal Regulations

D Droplet diameter

f Frequency

FAA Federal Aviation Administration

fps Frames per second

INTA Instituto Nacional de Técnica Aeroespacial

IR Infrared

IRT Icing Research Tunnel

xii

LED Light-emitting diode

LEWICE Lewis Ice accretion software program

LWC Liquid Water Content

µair

Air absolute viscosity

MVD Median volume diameter

NACA National Advisory Committee for Aeronautics

NASA National Aeronautics and Space Administration

PFV Photron FASTCAM Viewer

Q Flow rate

R Resistance

R0 Resistance at room temperature

Re Reynolds number

RPM Revolutions per minute

flair

Density of air

flwater

Density of water

SLD Supercooled Large Droplets

‡waterair

Water surface tension for droplet

T Temperature

T0 Room temperature

Tw

Temperature of water

TAB Taylor Analog Breakup

We Weber number

WSU Wichita State University

V Velocity

xiii

Vair

Velocity of the air

Vairfoil

Airfoil velocity

Vdroplet

Droplet velocity

Vslip

Slip velocity

xiv

Acknowledgments

I would like to thank Dr. Jose Palacios for his support and advice while preparing

for and conducting this research. I appreciate the time you took to mentor me

throughout the last few years.

To all of my AERTS lab mates, thank you for always being willing to help with

the setup and data collection and for patiently answering my questions about your

research. Thank you also for your support.

I also want to thank Dr. Michael Kinzel for reading the thesis and providing

valuable feedback.

Thank you, Mami, for all of your support from the very beginning. Thank you

for always loving me, believing in me, and for encouraging me to keep going during

the hard times. Thank you also to the rest of my family and to my friends for your

continued love and support and for pushing me to be and do my best.

Most importantly, I would like to thank my God for all of the opportunities I

have had to explore and to learn. I am grateful for the adventures I have had with

God by my side and look forward to all of the adventures to come.

xv

Chapter 1 |

Introduction

1.1 Background and Motivation

Ice accretion on aircraft surfaces a�ects the performance of the aircraft and handling

becomes more di�cult [1]. There is an increase in weight and drag and a decrease

in lift as the airfoil profile changes. The problem of ice accretion on aircraft was

recognized early in the history of powered flight, and icing condition intensity

scales and terminology have been defined since the 1940s [2]. For safety reasons,

aviation regulations and aircraft certification requirements were developed and in

1926, the Civil Air Regulations (CARs) were introduced [3]. In 1936, the airplane

airworthiness regulation (CAR 4a) was created. The CARs were replaced by

Title 14 of the Code of Federal Regulations (14 CFR) and Part 25, airworthiness

standards of transport category airplanes, was introduced in 1966 as a combination

of the CARs that pertained to airworthiness standards. Although introduced as

CFRs later, the regulations in Appendix C of Part 25 had been used since 1964 to

design ice protection systems [4].

1

Appendix C of Part 25 describes the icing envelopes used in the design of ice

protection systems of aircraft. The median volume diameter (MVD) of the droplets

in the clouds described range between 15 and 50 µm. Despite being Appendix C

certified, aircraft still su�ered from the e�ects of ice accretion, causing accidents and

incidents. The Appendix C certification each large tranport aircraft was required

to obtain excluded supercooled large droplets (SLD, droplet diameter greater than

50 µm). At the time of the implementation of Appendix C in Part 25, little was

known about SLD and their e�ect on ice accrection [5]. As more information

was learned from accidents and incidents caused by icing, the Federal Aviation

Administration (FAA) saw the need for additional regulations regarding icing. In

2010, the FAA proposed an amendment to 14 CFR Part 25 which would add SLD

condition regulations for the certification of transqport aircraft. This amendment

is Appendix O of 14 CFR Part 25 [6].

One example of an accident caused by icing involved American Eagle Flight

4184 on October 31, 1994. The aircraft, an American Eagle ATR-72, departed

from Indianapolis, Indiana at 2:15pm local time and was expected to arrive in

Chicago, Illinois 45 minutes later. After almost 30 minutes of flight, the pilots

began a descent and were instructed by air tra�c control to hold. Other pilots

had already reported icing conditions in the area and Flight 4184 did encounter

freezing drizzle. Although the deicing system was activated, all of the ice could

not be removed. Freezing drizzle falls under the category of SLD. Larger droplets

a�ect a greater airfoil area upon impingement than smaller droplets which explains

the reason for the ine�ectiveness of the deicing system over the entire accretion

area. The ice that remained on the wing surfaces caused poor aircraft performance

2

and handling qualities. Control of the aircraft was lost, and it crashed into a field

in Roselawn, Indiana [5].

Pilots of large transport aircraft have observed the breakup of large droplets

near aircraft wing surfaces [7]. Due to the small amount of research conducted

on SLD, little was known about the dynamics of the large droplets. In 2005, Tan,

Papadakis, and Sampath, sponsored by the FAA, published a report on the behavior

of droplets near the leading edge of an airfoil at a Mach number of 0.3 [7]. They

computationally studied pressure distributions and the behavior of droplets near

the leading edge of an airfoil using the Taylor Analog Breakup (TAB) droplet

breakup model in FLUENT. The team simulated droplet breakup using a NACA

0012 airfoil with chord lengths of 0.91 m (3 ft.) and 6.1 m (20 ft). They used

three di�erent droplet sizes: 100, 500 ad 1000 µm. They also studied the pressure

distribution of a 6.1-m (20-ft.) three-element airfoil and the droplet breakup.

The study determined that 100 µm droplets did not breakup near either airfoil

type, regardless of length. The larger droplets did experience breakup and the

breakup was observed in di�erent locations along the chord of the airfoils, mostly in

regions of severe pressure gradients. Results of the breakup near the leading edge

of the NACA 0012 6.1-m (20-ft.) chord airfoil are shown in Figure 1.1. Breakup

is observed only ahead of the airofoil leading edge when the droplets are 500 and

1000 µm in diameter. There is a larger breakup region when simulating the 1000

µm diameter droplets than when simulating the 500 µm diameter droplets. The

authors found no experimental data related to the simulations presented in the

report and suggested that experiments be conducted to determine if the results

obtained using the TAB breakup model are accurate.

3

Figure 1.1: Droplet breakup near the leading edge of the NACA 0012 6.1-m airfoilfor droplets of 100 (top left), 500 (top right), and 1000 µm (bottom left) in diameter.The image in the bottom right is a close-up of the droplet breakup in Area Z inthe bottom left image [7].

In 2007, the National Aeronautics and Space Administration (NASA) Glenn

Research Center and the Instituto Nacional de Técnica Aeroespacial (INTA) in

Madrid, Spain, began working together and developed an experimental research

program to experimentally study the deformation and breakup of droplets near

the leading edge of an airfoil [8] [9]. A test rig was designed with a single rotating

arm and a counterweight. For this experiment, three airfoil models were used with

chord lengths of 0.21, 0.47, and 0.71 meters. An airfoil model representative of the

blunt leading edge of large transport aircraft was placed at the tip of the rotating

arm. A droplet and frequency generator were used to produce monosize droplets

between 100 and 1800 µm in diameter that fell perpendicular to the path of the

airfoil model. The deformation and breakup of the droplets near the leading edge of

4

the airfoil model were captured using a high-speed imaging system. A photograph

of the experimental setup used is provided in Figure 1.2. The airfoil models moved

at speeds between 50 and 90 m/s at the location of impact.

Figure 1.2: Photograph of experimental setup used for the droplet deformation andbreakup experiments conducted at INTA. Airfoil chord = 0.71 m (27.95 in) [9].

The tests were conducted to experimentally determine the behavior of droplets

near the leading edge of blunt airfoils by tracking the deformation of the droplet as

it impacted the airfoil model and by calculating important parameters pertaining

to droplets. Such parameters are the Reynolds number, the Weber number, and

the Bond number. To calculate these parameters, the horizontal and vertical

displacements of the droplets were measured to obtain the velocity and acceleration

required in the calculations of the droplet parameters. The tests were conducted in

a room temperature environment. Although the droplets encountered by aircraft

in icing conditions are supercooled, the experiment provided valuable experimental

data on large droplets. This experiment was the inspiration of the research presented

5

in this thesis although the goal was to experiment with supercooled droplets and

determine the di�erences in behavior.

1.2 Supercooled Large Droplet Studies

A limited number of studies have been conducted related to supercooled water

droplets and the behavior in di�erent conditions. Early in SLD research, cer-

tain struggles faced by those looking to study SLD, both experimentally and

computationally, were documented, and a possible aid is presented by Tan and

Papadakis [10]. Yet, this did not prevent researchers from continuing to look for

information regarding SLD. In their 2003 SAE International conference paper,

Papadakis, Rachman, Wong, Bidwell, and Bencic present their experimental inves-

tigation of SLD splashing and impingement [11]. The splashing experiments were

conducted in the Goodrich Icing Wind Tunnel and the impingement experiments

were done in the NASA Glenn Icing Research Tunnel (IRT). A sample of images

obtained in the Goodrich Icing Wind Tunnel of the splashing of 94-µm MVD

droplets obtained in the experiments is shown Figure 1.3. The images indicate that

as the velocity increased, the splashing also increased. Large and small droplet

impingement test data were collected and compared to impingement data obtained

using the LEWICE-2D code. Agreement between the test and simulation data

existed for the small droplets (11 and 21µm MVD) but for the larger droplets (79,

137, and 168 µm MVD) higher impingement was predicted by the simulation than

observed in the experiments.

With the goal of improving ice accretion prediction codes, Tan and Papadakis

computationally investigated the breakup, splashing, and reimpingement of droplets

6

Figure 1.3: Photographs of the splashing of 94-µm MVD droplets [11].

on three di�erent airfoils [12]. The droplet median volume diameter used ranged

between 137 and 236 µm. The three airfoils used were the MS(1)-0317, the GLC–

305, and the NACA 23012. In the study, the MS(1)-0317 and the GLC–305 airfoils

were clean airfoils while the NACA 23012 was simulated with a glaze ice shape

formed over 22.5 minutes using LEWICE 2.2. The breakup model used was the

TAB model, the Wichita State University (WSU) splash model was used to predict

the splashing of the droplet, and the rebound model used was based on sand

particles bouncing on a surface. All three models were validated and it was found

that including the breakup, splashing, and reimpingement events of a droplet was

beneficial in the simulation of droplet impingement distribution.

In 2015, Bilodeau, Habashi, Fossati, and Baruzzi were published in the Journal

of Aircraft for their work on an Eulerian model of SLD splashing and bouncing [13].

7

The numerical approach modeled postimpact SLD that splashed and bounced on

aircraft surfaces. This method was used on clean and iced NACA 23012 airfoils

and on the MS(1)-0317 airfoil. Agreement existed between the numerical approach

and the experimental data collected and presented by Papadakis and his team [11].

The collection e�ciency along the surface of the NACA 23012 airfoil is shown in

Figure 1.4. The results of the study further show, using a three-element high-lift

configuration airfoil, that the reimpingement and splashing of the larger droplets

cannont be ignored.

Figure 1.4: Collection e�ciency versus distance from the leading edge [13].

Also in 2015, a Lagrangian computational method pertaining to supercooled

water droplets was presented in the Journal of Aircraft by Wang, Chang, and

8

Wu [14]. The Lagrangian method was used to develop a droplet tracking method

that includes droplet splashing, bouncing, and reimpingement in the form of a

mass ratio. In the case of the Lagrangian tracking method, both small and large

droplets were studied. Results using the tracking method were validated using

experimental data and agreement is seen in the collection e�ciency for both

small and large supercooled droplets (Figure 1.5). Along with the LEWICE and

experimental results, the results of the droplet tracking method are shown, including

and excluding the splashing and reimpingement e�ects. In each of the plots, the

results that include splashing and reimpingement e�ects more closely agree with

the experimental data.

Aside from conducting wind tunnel experiments and simulations to attempt to

predict the splashing and reimpingement of SLD, tests were done to characterize

SLD ice accretions on unprotected surfaces and study the aerodynamic e�ects. In

2005, Broeren, LaMarre, Bragg, and Lee presented the work done in the IRT and

at the University of Illinois. The IRT was used to accrete SLD ice onto a portion of

a commuter-class aircraft wing while the aerodynamic testing was completed at the

University of Illinois using a small-scale model of the airfoil and ice shape. In the

IRT, the tunnel temperature was maintained at -2.2¶C and the MVD used in the

tests was 133 µm. Two velocities, 61.7 and 92.6 m/s, were used with liquid water

content (LWC) values of 0.55 and 0.32 g/m3, respectively. Since, based on the

SLD cloud data available, the LWC values used in the wind tunnel tests may have

been higher than the LWC in SLD clouds, the cloud conditions were scaled to more

accurately represent SLD clouds. Three key features of SLD ice were observed:

horns, nodules, and clear ice. Clear ice was seen in the region of the stagnation

9

Figure 1.5: Collection e�ciency versus distance from the leading edge [14].

point (Figrure 1.6), the horns (Figure 1.7) formed downstream of the clear ice in

certain cases, and the nodules (Figure 1.8) were downstream of the horns and were

comparable to glaze or rime ice feather structures [15].

The aerodynamic portion of the test was done using the same airfoil section

as was used in the IRT although the chord was 0.46 m (18 in) while the chord of

the model used in the IRT was 1.96 m (77.25 in). To recreate the ice shapes on

10

Figure 1.6: Close-up of SLD clear ice [15].

Figure 1.7: Close-up of SLD horns [15].

the model, simple geometric shapes representative of the SLD ice accretions were

appropriately scaled and added to the surface of the model. The three key features

were tested on the model individually and each decreased the lifting capabilities

of the airfoil, with the horns having the greatest e�ect on the decrease in lift. All

three features were then combined and tested. Once again, the horns were the

features that a�ected the lift the most while the nodules aft of the horns had little

e�ect. Yet, it was observed that with the combination of the clear ice and the

horns, the performance was better than that of the horns only.

The e�ects of supercooled cloud, drizzle, and rain drop icing was also studied.

Ashenden, Lindberg, and Marwitz used wind tunnel tests to determine the perfor-

11

Figure 1.8: Close-up of SLD nodules [15].

mance degradation on a NACA 23012 airfoil from the three icing conditions [16].

The droplet size determined the icing condition. Cloud conditions were defined by

droplet diamters less than 40 µm, drizzle conditions by droplet sizes between 40

and 400 µm, and rain conditions were defined by droplet sizes larger than 400 µm.

The airfoil angle of attack ranged between -2 and +18¶.

Each icing condition a�ected the drag and the lift coe�cient of the airfoil over

the range of angles of attack. The lift coe�cient decreased between 12% and 38%

and the drag increased between 6% and 36% in the supercooled cloud condition.

The supercooled drizzle ice shape decreased the lift between 4% and 43% and

increased the drag between 49% and 56%. The rain drop ice shape changed the

lift between +19% and -42% and increased the profile drag between 10% and 42%.

According to the results presented, freezing drizzle is the most severe condition for

aircraft.

Miller, Addy, and Ide ran tests in the IRT using a full-scale Twin-Otter wing

section and a NACA 23012 wing section [17] [18]. The test was completed in two

12

parts. The first part (Twin-Otter) was done with the goals of documenting the

capabilities of the IRT regarding a large droplet icing cloud and of determining how

ice accretion on the surface varied based on di�erent values for the variables. The

second part (NACA 23012) was also conducted with the purpose of determinig the

e�ects of the di�erent parameters. In addiction, the second part had the objective of

comparing the results of the experiment to the results obtained with the Twin-Otter

wing section in the first part. In both parts, the parameters that were varied to

determine the e�ects were the temperature, the deicing system cycle, and the angle

of attack.

The temperature range for the investigation was -15 to 2.78¶C (5 to 37¶F). This

ranged from the lowest temperature where the large droplet icing condition was

thought to exist in nature to the point where there was no ice accretion. The angle

of attack ranged from -2 to 3.9¶. The deicing system cycle time was 42 seconds,

three minutes, and 6 minutes.

In the three cases, temperature, angle of attack, and deicing system cycle time,

the behavior of both of the airfoils was similar. A ridge of ice formed just aft of

the deicing system for both wing sections. The reactions to the environmental

temperature of each of the airfoil sections were also similar. Between -1.1 and

1.1¶C (-30 and 34¶F) the ice downstream of the deicing system self shed. Between

-4.4 and -2.2¶C (24 and 28¶F) the ice did not shed as easily and the tallest ridges

were formed at these temperatures. As the angle of attack increased, the amount

of ice attached to the pressure side of the airfoil also increased and this was seen

with both the Twin-Otter and the NACA 23012 airfoil wing sections. The deicing

system cycling time was only tested at 0¶C (32¶F) on the Twin-Otter and for this

13

test point, both of the airfoil wing sections behaved similarly. in that. No e�ect

was observed on the ridge that formed downstream of the deicing system due to

the activation cycles of the deicing system.

In all of the literature that was reviewed, the behavior (deformtion and breakup)

of supercooled droplets was not investigated near the leading edge of an airfoil.

The tests described by Vargas in references [8] and [9] did study the deformation

and breakup of large droplets but the droplets were not supercooled. The work

presented in this thesis is the first time the deformation of supercooled water

droplets near the leading edge of an airfoil is explored.

1.3 Thesis Objectives

The purpose of this research was to further understand the e�ects of slight super-

cooling (268 < Tdroplet

< 273) on the behavior of droplets impacting the leading

edge of an airfoil. Small supercooled droplets are known to cause ice accretion on

aircraft surfaces and a�ect the aerodynamic performance and handling qualities [1].

To address the issue, ice protection systems are installed on aircraft where ice is

known to accumulate. The designs are initially based on the results of computer

codes that predict the shape, size, and location of ice on aerodynamic surfaces for

droplets ranging between 5 and 50 µm in diameter. The codes have been tested for

the icing conditions described in the 14 CFR Part 25 Appendix C but also need to

be tested and validated when making predictions for icing conditions with larger

droplet diameters (SLD, Appendix O). To validate or improve the predictions,

experimental data on the behavior of larger droplets when impacting a lifting

surface are necessary. The research described in the following chapters aims to

14

begin studying supercooled droplet behavior at the leading edge of an airfoil.

The first objective of this research was to obtain supercooled water droplets

of various sizes at the point of impact, which is the stagnation line of an airfoil

model located 19.05 cm (7.5 inches) below the tip of a droplet generator. Once

supercooled droplets could reliably be obtained, the next objective was to visualize

the deformation and breakup of the droplets. This was accomplished using a

high-speed imaging system.

An experimental setup was developed in the Adverse Environment Rotor Test

Stand facility that would facilitate the visualization of the supercooled droplets as

they interacted with a generic-shape airfoil model. Once data on the deformation

and breakup of droplets interacting with an airfoil model was collected and processed,

the objective of comparing the behavior of supercooled water droplets to water

droplets at room temperature could be met. This led to the final objective of

studying the e�ects of temperature on the behavior of the droplets.

1.4 Thesis Overview

The work presented in this thesis is divided into the following chapters:

1.4.1 Chapter 2: Experimental Setup and Test Procedures

The experimental setup in the Adverse Environment Rotor Test Stand facility is

explained. Each component of the setup is described in this chapter. The test

procedures for the experimental methods and the test matrix are also presented.

Prior attempts at conducting the experiment are briefly mentioned.

15

1.4.2 Chapter 3: Data Analysis

The method of analyzing the data collected during the experiment is described.

Various terms used and methods of calculating important parameters are introduced

and explained.

1.4.3 Chapter 4: Experimental Results and Comparisons

The methods of determining the temperature of the droplets used in the experiment

are described and the results are discussed. The results of the experiment are also

presented and are compared to the results of prior tests done in a room temperature

environment.

1.4.4 Chapter 5: Conclusions and Future Work

Conclusions based on the results obtained from the research presented in this thesis

are stated. Suggestions for future work are also included.

16

Chapter 2 |

Experimental Setup and Test

Procedures

The following chapter gives details on the equipment used for the tests and the test

procedure. The experiment consists of four main components: the airfoil models,

the rotor stand, the droplet generator, and the high-speed imaging system.

2.1 Airfoil Models

Although various chord length airfoil models were used in the experiments conducted

at INTA by Vargas et al. [9], a single chord length was used in the experiment

described in the following sections. The profile of the airfoil model used is repre-

sentative of the blunt leading edge of airfoils used on large transport aircraft [9],

and matches one of the airfoil shapes used in the room temperature tests done

at INTA. A plot of the non-dimensional coordinates can be seen in Figure 2.1.

The coordinates were modified to obtain the desired chord length of 0.47 m (18.5

in). SolidWorks, a computer aided design software, was used to generate a three-

17

dimensional representation of the model. The model is displayed in Figure 2.2 with

dimensions in inches. The symmetric airfoil models are made of extrude polystyrene

foam and have a chord of 0.47 m (18.5 in) , a span of 0.30 m (12 in), and a thickness

of 0.19 m (7.4 in).

Figure 2.1: Plot of coordinates of airfoil model profile.

Figure 2.2: CAD model with dimensions in inches.

The models were mounted on the tips of two truncated QH-50D DASH UAV

18

blades and have a cutout of the blade airfoil. Epoxy was used to mount the models

to the blades. A thin sheet of vinyl reinforced the leading edge of the models. A

top and side view of the model can be seen in Figure 2.3.

Figure 2.3: Top and side views of an airfoil model mounted on a rotor blade.

2.2 Rotor Stand Facility

As mentioned in the previous section, the airfoil models were mounted on to the

tips of two rotor blades. The blades were then mounted to the grips of a QH-50

lower hub. This hub is located in the center of a large freezer that makes up part

of the Adverse Environment Rotor Test Stand (AERTS) facility.

The AERTS facility is located at The Pennsylvania State University. A 6 m x 6

m x 4 m (20 ft x 20 ft x 13 ft) chamber houses a 93.2 kW (125 hp) motor which

rotates the QH-50 lower hub. The hub can be connected to the 48-channel slip

ring to transmit power and signals as necessary. Surrounding this motor and hub

is an octagonal ballistic wall. The protective wall is made up of weather-resistant

lumber and steel and is covered by an aluminum weather protection layer. The

ballistic wall allows for a rotor diameter of 3 m (10 ft). The chamber can be

19

cooled to temperatures between -25 and 0 ¶C (-22 and 32 ¶F) and the ceiling of the

facility is equipped with 15 NASA standard icing nozzles. The rotor stand and the

location of the nozzles in the ceiling (concentric rings centered above rotor stand)

are shown in Figure 2.4. The nozzles can be controlled to produce clouds of various

median volume diameters which can generate icing conditions representative of the

environments aircraft encounter during flight. The facility was designed to test new

ice protection system technologies, measure the ice adhesion strength of coatings,

and correlate ice shapes to ice shape prediction codes [19]. While the facility is

used for its original purposes, it can be modified and used for other purposes, such

as centrifugal testing of components or droplet splashing visualization as it is done

in this e�ort.

Figure 2.4: Rotor stand and nozzles in ceiling [19].

For the current experiment, the chamber temperature was set to -20¶C. An

icing cloud was not necessary and therefore, the nozzles were not used. Instead,

a monosize droplet generator was used to produce the droplets needed in various

sizes.

20

2.3 Droplet Generator System

The droplet generator system worked together to produce droplets of similar size for

each test. It was part of a system consisting of the droplet generator, a frequency

generator, an air pressure controller, and a water reservoir. The droplet generator

was the same one used by Vargas et al. [9] at INTA.

2.3.1 Droplet Generator

The TSI Monosize Droplet Generator Model (MDG-100) was used to generate a

stream of droplets of various diameters using orifice sizes ranging from 100 to 500

µm. A correlation for the droplet size generated is given in Equation 2.1,

D(µm) =5Q(cc/min)

f(kHz)

6 13 (2.1)

where D is the droplet diameter in micrometers, Q is the flow rate in cubic

centimeters per minute, and f is the frequency in kilohertz. It is used to calculate

the droplet diameter based on the orifice size, the excitation frequency (input

frequency to the droplet generator), and the flow rate. A photograph of the droplet

generator is provided in Figure 2.5. As displayed in the photograph (Fig. 2.5), the

droplet generator was connected to a frequency generator. The frequency generator

provided a square wave disturbance which caused a piezoelectric transducer within

the droplet generator to vibrate. The vibration caused the stream flowing through

the generator to break up into droplets of similar size.

The droplet generator was also connected to a heated and pressurized water

21

reservoir. The pressure was set to a specific value such that the pressure, orifice

size, and vibration frequency worked together to provide the droplet size desired.

Figure 2.5: Droplet generator.

2.3.2 Frequency Generator

The frequency generator was a B&K Precision 4011A model 5MHz function gener-

ator (Figure 2.6). It is capable of generating sine, square, and triangle waves at

frequencies from 0.5 Hz to 5 MHz [20]. A square wave with frequencies between 5

and 20 KHz were used for the experiment.

2.3.3 Air Pressure Controller

The air pressure controller was the MicroFab CT-PT-21 Pneumatics Controller. It

was used with a Keyence AP-30 Series two-color digital display pressure sensor. The

unit can provide both positive and negative purge pressure, but specific pressures

22

can also be set and controlled [21].

Figure 2.6: B&K Precision 4011A function generator.

Figure 2.7: MicroFab CT-PT-21 Pneumatics Controller.

2.3.4 Water Reservoir

The water reservoir was formed by a 5 gallon bucket with an air-tight lid. The

bucket was covered with wool insulation. The insulation was necessary since the

reservoir remained in the cold chamber during the tests. The reservoir was equipped

23

with a bendable immersion heater, input air tubing, and a plastic hose fitting to

allow the water to exit. For the reservoir to accommodate the heater, the water,

and air hose, the lid was modified by introducing input and output connections. A

photograph of the lid is shown in Figure 2.8.

Figure 2.8: Photograph of the reservoir lid showing modifications.

The opening for the air hose was simply a hole with the diameter of the air

hose, 3.175 mm (0.125 in). The water hose fitting was a plastic barbed fitting for

a 6.35-mm (0.25-in) inner diameter hose. The valve and cap were used together

while adding water. The valve was opened to relieve the pressure while water was

pumped into the bucket through the cap to avoid removing the lid. A ceramic

insulator between the ends of the heater and lid prevented contact and, therefore,

melting of the plastic lid. The heater terminals were pushed through holes made

in the lid and secured with the hex nuts included with the heater. Insulated ring

terminals were also secured to the terminals using the hex nuts and were used to

connect a power cord to the heater.

24

Power to the heater was not provided continuously. An OMEGA CN7500 Series

PID controller along with a solid state relay were used to control the power provided

to the heater. A temperature measurement was supplied to the controller by a

Type-T thermocouple that was inserted into the water hose near the reservoir. A

schematic showing the heater and the water and air hoses is presented in Figure

2.9.

Figure 2.9: Schematic of the bucket showing components inside of the reservoir.

2.4 High-Speed Imaging System

An objective of this thesis was to visualize the deformation and breakup of super-

cooled droplets near the leading edge of an airfoil model. To accomplish this goal,

two Photron SA-Z high-speed cameras were employed. Two cameras were used

to widen the field of view and to increase the number of breakup events captured

25

during each test. The cameras are able to capture images at frame rates of 1,000

to 1,000,000 frames per second (fps). This capability exceeded the requirements

for the tests described in this work but the range was necessary for capturing the

series of images required to meet the objective.

Each camera used a 200 mm Micro Nikkor lens and a 2x doubler between the

200 mm lens and the camera to double the focal length of the lens. A photograph

of a camera with the lens and doubler attached is shown in Figure 2.10. To prevent

sagging of the lens due to the weight, U-channel bolted to the case of the camera

and threaded rods were used to support the weight of the lens.

Figure 2.10: Photron SA-Z high speed camera with 200 mm Micro Nikkor lens anddoubler.

A triangular mirror, pictured in Figure 2.11, was placed between the lenses of

the cameras to direct the view of the cameras to the center of the chamber. Due

to space limitations between the ballistic wall and the wall of the chamber, the

cameras had to face each other, requiring the use of the mirror. A schematic of the

setup is shown in Figure 2.12.

The camera software used to control the cameras and record the high-speed

26

Figure 2.11: Front view of triangular mirror.

Figure 2.12: Schematic of the camera setup.

27

images was Photron FASTCAM Viewer (PFV). The cameras were triggered to

begin recording using the software. The frame rate and resolution of the cameras

were also set within the software. The software is also capable of editing the images,

creating videos using the images captured, and playing other videos in the software

image viewer. These capabilities were used in the process of analyzing the data.

2.5 Lighting

A shadowgraph technique was used in the experiments. The droplets were illumi-

nated from behind to have the droplets appear in black with a lighter, white/grey

background. To illuminate the droplets from behind, the light source needed to

be located on the rotating hub. A stationary light source would not have met the

needs of this experimental setup. The second rotor blade with the airfoil mounted

on the tip and the rotor hub would have obstructed the light and created other

unwanted shadows in the images.

A structure was designed for the hub that would allow the light source to rotate

with the blade and continuously provide illumination of the area at and ahead of

the leading edge of the airfoil. The hub structure consisted of two 1.27-cm (0.5-in)

thick, 68.6-cm (27-in) diameter aluminum rings that were secured to the top and

bottom of the blade grips. The rings were machined as two parts and a schematic

of the top view of half of a ring is shown in Figure 2.13. The light source was

mounted onto this structure, between the two rings, and at a location near the

leading edge of the root of the blade, illuminating the blade tip. A picture of the

setup is shown in Figure 2.13.

The light source was a white 9500 lumens CXA2590 High-Power light-emitting

28

Figure 2.13: Photograph and schematic of the hub structure and light source setup.

diode (LED) from CREE. The individual wafers were wired to a power supply,

which provided the 69 volts required of the LED. The wafers were secured to an

aluminum plate using a plastic Ledil lighting connector. A thermal conductive

adhesive was placed between the LED wafer and the aluminum plate to maximize

the heat transfer from the LED to the aluminum and to a heat sink attached to the

back side of the aluminum plate. The lighting connector was also used to hold a

reflective cone that was used to direct the light to the camera lenses. A photograph

of the LED with the reflective cone mounted onto the hub structure is shown in

Figure 2.14. Power was transmitted to the LEDs through the slip ring while the

power supplies remained in the control room.

29

Figure 2.14: Photograph of the light source in testing location.

2.6 Test Procedure

To begin, all of the components of the experiment were installed. The camera

technician began the setup of the imaging system while the droplet generator

system, the light source, and the rotor blades with the airfoil models were set

up. A photograph of all of the components installed in the chamber is shown in

Figure 2.15. The frequency generator and the air pressure controller were then

turned on to settings that had been determined prior to testing. These pressure

and frequency parameters were established for specific orifice diameters and used

to obtain droplets of similar sizes during each test. The droplet generator then

began to jet and it was used in the calibration of the cameras by capturing images

of a known distance against the focused falling droplets.

After all was set up and the droplet generator began jetting, the cameras were

further tuned to focus on the droplets. The rotor was rotated and an airfoil model

30

Figure 2.15: Photograph of the experimental setup.

was placed at the location where the stream of droplets just touched the airfoil

model. The cameras were then aligned such that the line of view of both of the

lenses, with the use of the triangular mirror, was parallel to the airfoil model

stagnation line. The cameras were secured to x-y direction positioning tables which

were bolted to a beam that was fixed on top of two tripods. With the cameras in

the proper position, the lenses were focused on the spanwise location where the

stream of droplets fell. Once the cameras were properly positioned and focused,

the image seen in the camera software could be calibrated.

The calibration was done using a transparent ruler that was placed at the

location of the focal point of the lenses (see Figure 2.16). At this location, the

cameras could focus on the measuring lines (millimeter and centimeter lines) of the

ruler. An image was recorded for each camera and was used in the data analysis

31

Figure 2.16: Photograph of the camera image calibration setup.

process to determine the number of pixels per millimeter and the diameter of the

droplets seen in the images. An example of an image taken by a camera during the

camera image calibration can be seen in Figure 2.17.

Figure 2.17: Sample calibration image. Resolution: 560Hx384V, 29 pix/mm. Linespacing is 1 mm.

32

For the experiment described in this work, the resolution of each camera was 384

pixels in the horizontal direction and 464 pixels in the vertical direction. Using the

calibration image, the conversion was measured to be 33.55 pixels per millimeter.

The resolution provided a field of view of 11.45 mm in the horizontal direction

and 13.83 mm in the vertical direction for each camera. The use of two cameras

allowed for a wider field of view, increasing the chance of capturing the complete

droplet breakup event. The views of the lenses were set to slightly overlap using

the numbers on the ruler. The overlap was created to ensure that the lenses had

a continuous image and that the images could be stitched together during data

processing if necessary. The overlap reduced the width by about 1 millimeter.

With the calibration of the cameras completed, the cameras were ready to begin

recording. A Kapture Group trigger control system was set up in the chamber for

use with the cameras. The triggering was necessary for capturing images of the

droplets near the leading edge of the airfoil whenever necessary. The laser and

receiver were set up in the chamber while the control module was placed in the

control room. At this point, the tests could begin and they were controlled and

monitored from the control room.

The rotor could be observed using di�erent cameras installed in the chamber

and the high-speed cameras were controlled using the PFV software installed on

a laptop. The frame rate and shutter speed were established using the software.

The software is capable of writing information on each recorded frame, which

was helpful in identifying the di�erent test parameters during the data processing.

The following information was recorded on each frame: frame rate, total time of

recording, image resolution, number of frames in the image sequence, environmental

33

temperature, airfoil velocity, and target droplet size.

When all was set with the software and the operators were ready to begin, the

rotor was started. After it reached the desired speed at the droplet impact location,

the revolutions per minute (RPM) were maintained. By controlling the trigger

system, the cameras only recorded after the airfoil models reached the desired

speed. The recording was then allowed to begin. Once the recording was completed,

the rotor was stopped. The recording was analyzed to determine if droplets were

captured that could be used in the data analysis. If no droplets were observed

in the recording, the test point was repeated. If droplets were observed near the

leading edge of the airfoil, the recording was saved and the operators prepared for

the next test point.

All was initially tested in a room temperature environment. The chamber

was then cooled to begin collecting data pertaining to supercooled droplets. In

addition to the procedure described above, when the temperature in the chamber

was decreased to -20¶C, the temperature of the water in the reservoir and near the

droplet generator was monitored and recorded during testing. The temperature

needed to be monitored to be able to avoid or deal with any freezing in the

system immediately and to obtain droplet data while the water reached the lowest

temperature possible before freezing.

The temperature in the reservoir was provided by the Type-T thermocouple in

the water hose near the reservoir. The temperature of the water near the droplet

generator was provided by a thermistor inserted into the end of the hose near the

droplet generator. The thermistor was located about 14 cm (5.5 in) above the tip

of the droplet generator. The resistance output by the thermistor was recorded

34

and converted to temperature using Equation 2.2

1T

= 1T0

+ 1B

ln

AR

R0

B

(2.2)

where T is the temperature in Kelvin, T0 is the room temperature in Kelvin, B

is the coe�cient of the thermistor (B = 3560 K), R is the measured resistance,

and R0 is the resistance at room temperature for the thermistor used (R0 = 2000

Ohms).

2.7 Droplet Supercooling

It was important to determine the temperature of the water as it interacted

with the rotating airfoil models. An attempt was made to experimentally obtain

measurements of the temperature of the water at various vertical distances from

the tip of the droplet generator.

2.7.1 Test Setup

The droplet generator was set up in the same location as was used during the

tests. The frequency generator and the air pressure controller were set to allow the

droplet generator to jet at the desired test conditions.

A ruler was then attached to the support beam where the droplet generator was

located such that the ruler was parallel to the stream of droplets. An infrared (IR)

camera was used to record the temperature of the water. A black plastic surface,

1.9 mm (0.075 in.) thick, was used as the background for the IR camera.

35

2.7.2 Droplet Temperature Measurement Test Procedure

To begin, the black plastic surface was placed inside of the chamber and allowed to

reach the environmental temperature of -20¶C. The locations where the temperature

was to be measured, 2.54, 7.62, 12.7, and 19.05 cm (1, 3, 5, and 7.5 in) below the

tip of the droplet generator, were then determined on the ruler. The IR camera was

calibrated using ice water. Since the IR camera could not provide the temperature

of the water using the droplet stream only, the black plastic surfase was used

such that the water was allowed to pool up. The IR camera could then provide

a temperature measurement. As soon as the camera could distinguish the water,

an image with the temperature measured was recorded. The measurement was

repeated multiple times at each location. A schematic of the test setup is shown in

Figure 2.18.

Figure 2.18: Schematic of droplet supercooling experimental setup.

36

One temperature measurement was collected at each distance below the tip

of the droplet generator at a time. The low temperature inside of the chamber

limited the amount of time a person could remain in the chamber so the priority

for each test was to collect a temperature measurement at each distance. Prior to

entering the chamber, the temperature of the water exiting the reservoir and the

temperature of the water just above the droplet generator was recorded. The same

was done when exiting the chamber after collecting data.

2.8 Test Matrix

The test matrix for the experiments is provided in Table 2.1. The test matrix

was designed to obtain data for rotor tip speeds of 50, 60, 70, and 80 m/s and for

droplet generator orifice sizes of 200 and 400 µm. The pressure in the reservoir

and the frequency of the piezoelectric transducer were varied to obtain the desired

droplet sizes when using the di�erent orifices. The higher frame rates of 150000 and

140000 fps were used initially and compared to the lower frame rate of 90000 fps to

determine the widest view available with a resolution that allowed for observation

of the droplet from no deformation to deformation and/or breakup near the airfoil

leading edge. The chamber temperature was maintained above freezing while

adjusting the cameras to establish the testing setup. Once the cameras were ready

to begin testing, the chamber temperature was decreased to create the environment

that would allow the droplets to supercool at the impact location. Initially, the

temperature of the water was recorded only at the inlet to the droplet generator

and the thermocouple was used to obtain the measurement. On the last day of

testing, the temperature was collected at the inlet to the droplet generator using a

37

Table 2.1: Test matrix.

Date(2017) Run

OrificeSize(µm)

FPS Press.(bar)

Freq.(kHz)

R(k�)

Tw¶C)

Tc(¶C) RPM V

(m/s)

2/21 1 200 140000 1.00 14.00 - 17.90 - 376 60.012/21 2 200 140000 0.91 14.00 - 15.90 - 376 60.012/21 3 200 140000 0.89 14.00 - 17.20 - 439 70.062/21 4 400 90000 0.89 50.00 - 17.30 - 314 50.112/21 5 400 90000 0.90 50.00 - 17.30 - 376 60.012/21 6 400 90000 0.90 50.00 - 17.20 - 376 60.012/21 7 400 90000 0.90 50.00 - 17.20 - 439 70.062/21 8 400 90000 0.90 50.00 - 17.20 - 439 70.062/21 9 400 150000 0.89 5.00 - 17.20 - 439 70.062/21 10 400 140000 0.89 5.00 - 17.20 - 439 70.062/22 1 400 90000 1.18 5.00 - 2.50 -20.0 313 49.952/22 2 400 90000 1.18 5.00 - 2.00 -20.0 313 49.952/22 3 400 90000 1.18 5.00 - 2.00 -22.6 313 49.952/22 4 400 90000 1.17 5.00 - 2.00 -22.7 376 60.012/22 5 400 90000 1.17 5.00 - 1.40 -22.7 376 60.012/23 1 200 90000 1.04 20.00 - 6.20 -17.2 314 50.112/23 2 200 90000 1.21 20.00 - 1.60 -21.4 314 50.112/23 3 200 90000 1.01 20.00 - 1.90 -14.0 376 60.012/23 4 200 90000 1.01 20.00 - 1.20 -14.0 376 60.012/24 1 200 90000 1.17 20.00 5.40 2.25 -24.6 314 50.112/24 2 200 90000 1.16 20.00 5.16 3.22 -21.0 376 60.012/24 3 200 90000 1.16 20.00 5.28 2.73 -21.2 439 70.062/24 4 200 90000 1.16 20.00 5.30 2.65 -20.4 439 70.062/24 5 200 90000 1.16 20.00 4.95 4.12 -19.6 501 79.962/24 6 200 90000 1.14 20.00 5.16 3.22 -19.0 500 79.802/24 7 200 90000 1.14 20.00 5.16 3.22 -18.9 500 79.80

thermistor while the thermocouple provided the temperature of the water near the

reservoir.

38

2.9 Previous Attempts

Prior to conducting the experiment described in this thesis, two other attempts

were made using the same components described above except for the light source

and the water reservoir.

2.9.1 Attempt 1

The issue in the first attempt was the lighting. Initially, a stationary light source

was used along with a round mirror that rotated with the rotor hub. The light

source was placed on a shelf inside of the chamber and reflected o� of a circular

mirror and into the camera lenses. A photograph of a laser being reflected into the

camera lenses to adjsut the position of the light source is shown in Figure 2.19.

When the light source was powered, the light followed the path shown by the laser.

When the setup was tested in a -20¶C environment, the plastic mirror became

brittle and shattered.

2.9.2 Attempt 2

After the first attempt, the lighting was changed and designed to rotate with

the rotor hub. With this change, data was successfully collected in both room

temperature and -20¶C environments. In the second attempt, the water reservoir

used was a small 12-ounce glass bottle with external heaters. The thermocouple

used to monitor the temperature of the water was also placed on the outside of the

reservoir. In this position, the thermocouple did not measure the temperature of

the water only. The measurement was a�ected by the environmental temperature.

39

Figure 2.19: Laser reflected o� of circular mirror showing path to be followed bylight source to camera lenses.

As in the setup described above, the heaters were powered based on the tem-

perature measured by the thermocouple. With the thermocouple being cooled by

the environment, the heaters were continuously receiving power and increasing the

temperature of the water in the reservoir. During the experiment, the average tem-

perature measured by the thermocouple was 10¶C, meaning that the temperature

of the water was higher due to the e�ects of the environment on the thermocouple.

40

Chapter 3 |

Data Analysis

The entire process of analyzing the data was completed using the camera software

used to control the cameras and record the data (PFV) and a MATLAB script.

The original recorded data was viewed, edited, and separated into shorter videos.

The shorter videos were then read by a MATLAB script which provided output in

a text file format. The text file was then viewed as a spreadsheet and was used

to obtain specific values for the di�erent parameters of interest. This process is

explained in more detail in this chapter.

3.1 Tracking a Single Droplet

To learn information about the behavior and deformation of droplets, individual

droplets were studied as they interacted with the leading edge of the airfoil models.

As PFV could also be used to analyze and edit images, the raw images collected

during the experiments were viewed in PFV with the intention of selecting droplets

that could be analyzed. A summary of the requirements for selection is given below

in some detail. During each run, the camera was triggered to begin recording only

41

once. Yet, a single recording contained a total of 64 complete blade passages. Each

blade passage was allotted 500 frames to ensure that the droplets and blade could

be observed with empty frames before and after.

Although 64 blade passages were recorded during each run, each visible blade

passage did not provide valuable data. At times, the droplets were too far out of

focus. In other instances, the droplets could not be followed over an appropriate

path for analysis. The droplets that could be used in the analysis needed to be

observable from no or very little deformation (spherical shape, circular in the

2-dimensional images) to deformation or breakup prior to impact. A droplet that

had already begun the process of deforming prior to entering the field of view or a

droplet that did not complete the process of deformation within the view of the

cameras could not be used since only partial information would be obtained. For

this reason, careful inspection of the recordings was required and only a sample of

all of the droplets seen in the frames was separated for further analysis.

The droplets that could be followed from little to no deformation to deforma-

tion/breakup just prior to impact were separated into shorter videos to be able to

track individual droplets. The first step in the process of choosing droplets was to

find a droplet that could still be seen as the leading edge of the blade was crossing

the view of the cameras. At this point, some droplets would have already impacted

the airfoil model while other droplets would only be a few frames from impact. The

droplets that had not yet splashed would be deformed and some may have already

begun breaking up. Two droplets near the leading edge of the airfoil model are

shown in Figure 3.1.

The droplets that could be observed breaking up prior to impact and then

42

Figure 3.1: Droplet breakup prior to impact. Chamber Temperature = -20¶C,V

airfoil

= 80 m/s, droplet diameter (left) = 473 µm, droplet diameter (right) = 401µm.

impacting as the airfoil model continued to pass would be marked and the frame

number would be recorded. The frames would then be run backwards while visually

tracking the droplet. If the droplet could be followed back to the point where no

or very little deformation was observed, it was selected as a droplet that would

be further analyzed. The number of the first and last frames were recorded. An

example of such frames is shown in Figure 3.2. The blade moves from left to right

in all of the recordings. This process was repeated throughout the recordings to

obtain more droplets that met the conditions required for selection.

Figure 3.2: First and last frame of video of a droplet being tracked. (Left: droplettracking begins, Right: droplet tracking ends) Chamber temperature = -20¶C,V

airfoil

= 80 m/s, droplet diameter = 473 µm, 165 frames in video, 1.83 millisecondsin length.

43

Once a number (at least 20) of droplets were selected, they were separated from

the complete set of raw data and saved as shorter videos in .avi file format. The

shorter videos were visually studied more closely to observe the behavior of the

droplets as they interacted with the airfoil model. A sequence of images showing

di�erent stages of the deformation of a droplet is shown in Figure 3.3. The droplet

begins with little deformation and as the airfoil approaches it begins to change

shape.

The front of this droplet (the part closest to the airfoil, left side in images)

begins to flatten and continues to do so as the airfoil model moves closer. The

droplet then reaches the point where the diameter in the horizontal direction is

at the minimum. From there, the e�ects of the di�erence in the droplet and air

velocity acting on the droplet begin to shear the edges of the droplet. The diameter

in the horizontal direction then begins to increase and, at the velocity used, this

droplet takes on a hat-like shape. This shape persists until impact in this case. In

other cases, the e�ect of the di�erence in velocities is large enough to shear the

droplet and thin films of water are observed behind the droplet. An example is

shown in the image on the left of Figure 3.1.

Figure 3.3: Sequence of droplet deformation images. Chamber temperature =-20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

After looking into the shape of the droplets as they impacted the airfoil model,

the quality of the images was assessed. If necessary, the brightness and the contrast

between the droplet and the background were adjusted. This was done to provide

44

the MATLAB script with videos that had droplets with the clearest boundaries

possible. This would improve the accuracy of the program in determining the

diameter and other parameters of the droplet.

After the shorter videos were saved in the correct format and placed in the

directory with the script, the script was run to analyze each video frame by

frame. The program required the following input for each video: the file name,

the conversion between pixels and millimeters, the frame rate (frames per second),

the airfoil chord (millimeters), the velocity of the airfoil (meters per second), the

droplet number in the first frame (numbering from top to bottom then going left

to right, in the case of Figure 3.2, the droplet to be tracked is droplet number 1 in

the frame), the number of frames in the video, and the number of the frame where

the airfoil first appears.

The program began by searching for and reading the file specified. It then

calculated the appropriate threshold value to convert the grayscale images to binary

images using the Otsu method and prepared the images for the subsequent parts in

processing [22]. The program then determined the number of objects in the images.

The droplet to be tracked in the video was identified and an ellipse was placed over

it. The location of the centroid of the ellipse was calculated and used to determine

the motion of the droplet. From the tracking of the centroid, the motion in pixels

relative to the first frame is recorded. For each frame, the frame number, the time

with respect to the first frame and the tracking frame, the horizontal and vertical

displacements of the centroid, the area, diameter, and perimeter of the ellipse, and

the ellipse major and minor axes lengths were recorded. The major and minor axes

45

of the ellipse were used to calculate the diameter of the droplets using Equation 3.1

d = 3Ôb2 · a (3.1)

where d is the droplet diameter, a is the ellipse minor axis, and b is the ellipse

major axis. The information obtained was then used to calculate the parameters of

interest described in the following sections.

3.2 Calculation of the Horizontal Velocity and Ac-

celeration of the Droplet against Time

As previously stated, the horizontal displacement of the droplet was determined by

calculating the displacement of the ellipse that was used to represent the droplet

in the MATLAB script. A curve fit of the droplet displacement against time

was generated within the script using a MATLAB function called easyfit.m along

with a user-defined exponential function. The horizontal displacement data points

calculated from the motion of the ellipse were plotted in the program and the curve

fit is shown with the data points in Figure 3.4. The curve fit equation was used to

obtain the horizontal velocity and acceleration of the droplet using the first and

second derivatives respectively. Both are taken against time, with the stationary

frame of reference located at the triangular mirror.

46

Figure 3.4: Horizontal displacement versus time with curve fit. Chamber tempera-ture = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

3.3 Change of the Frame of Reference

It was mentioned in the previous section that the stationary frame of reference

was located at the triangular mirror, a stationary object with respect to the airfoil

model. It is simpler to change this and begin viewing the motion of the droplets

with a frame of reference that is stationary on the airfoil model. The origin is

located at the stagnation line of the model. As noted in [Vargas, 2011 [8]], a

coordinate system at rest with respect to the airfoil is not an inertial frame because

the airfoil model in motion is continuously accelerating. The droplet deformation

and breakup occurs at a distance less than or equal to the airfoil chord making that

section of the airfoil model path nearly straight. Therefore, the frame of reference

that is stationary on the airfoil is assumed as inertial as the airfoil model and

droplet interact. The Galilean transformation using the airfoil velocity was used to

obtain the velocity and acceleration of the droplet in a frame of reference that is

47

stationary on the airfoil model (Figures 3.5 and 3.6).

Figure 3.5: Droplet velocity versus distance from airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

Figure 3.6: Droplet acceleration versus distance from airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

48

3.4 Relative Velocity between the Droplet and

the Air (Slip Velocity)

The slip velocity is calculated as the magnitude of the di�erence between the

velocity of the droplet and the velocity of the air. The velocity of the droplet was

obtained using the first derivative of the curve fit of the horizontal displacement as

previously explained. The velocity of the air was measured experimentally and a

curve fit was applied to the data.

The experimental data necessary for obtaining the velocity of the air was

collected in the INTA 0.3 m x 0.2 m wind tunnel using an airfoil of similar shape

as the airfoil used in the experiment described in this thesis, although it was scaled

down to 10% of the size (0.047m chord). A Laser Droplet Velocimeter was used to

measure the velocity of the air at various distances ahead of the stagnation line of

the airfoil model [8]. The results are shown in Figure 3.7 and the curve fit equation

is given in Equation 3.2:

f = 0.4201 ◊ (1 ≠ e22.5663x) + 0.5532 ◊ (1 ≠ e4.1106x) (3.2)

where x is the distance from the stagnation line of the airfoil non-dimensionalized

by the airfoil chord (x/chord). The value obtained, f, is the ratio of the velocity of

the air to the free stream velocity (velocity of airfoil with respect to a reference

frame on the triangular mirror).

The horizontal velocity of the droplet and the velocity of the air, using Equation

3.2, are then known at every position ahead of the airfoil model. The relative

49

Figure 3.7: Curve fit of air velocity measurements taken at various distances fromthe leading edge of an airfoil (chord = 0.047) [8].

velocity between the droplet and the air can be calculated. This velocity, |Vair

≠

Vdroplet

|, is called the slip velocity. The three velocities, the droplet, air, and slip

velocities, are shown together in Figure 3.8. As seen in the plot, the slip velocity

increases exponentially as the air velocity decreases exponentially to zero.

3.5 Calculation of the Reynolds Number, the We-

ber Number, and the Bond Number

The slip velocity was used in the calculation of the main droplet parameters, the

Reynolds number (Eq. 3.3), the Weber number (Eq.3.4), and the Bond number

(Eq. 3.5). The parameters were measured along the path of the droplet only

taking into account the horizontal displacement as there was no sign of deformation

due to the vertical motion of the droplet. The droplets fell at a nearly constant

50

Figure 3.8: The droplet, air, and slip velocities of the droplet as it approached theairfoil model. Chamber temperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter= 401 µm.

velocity ranging between 3 and 4 m/s (vertical velocity calculated using vertical

displacement of droplet). The shape of the droplet during deformation was due

to the response of the droplet to the horizontal velocity and acceleration as was

the case for Vargas and Feo [8]. In all of the measurements and calculations the

vertical velocity was not considered.

The parameters were calculated along the entire path of the droplet as it

interacted with the airfoil model. As the program analyzed the video frame by

frame, it kept track of each frame and the number of frames each was from the

frame where the droplet and the airfoil impacted. Using the velocity of the airfoil

and the time between each frame, the program was able to determine the distance

the droplet travelled between each frame and could calculate the distance between

the droplet and the airfoil model at each frame.

The following are the definitions of the main parameters measured along the

51

path of the droplet:

Reynolds Number:

Re = flair

|Vair

≠ Vdroplet

|Dµ

air

(3.3)

Weber Number:

We = flair

|Vair

≠ Vdroplet

|2D‡water

air

(3.4)

Bond Number:

Bo = flwater

D2

‡waterair

3dV

droplet

dt

4(3.5)

where D is the droplet diameter, Vdroplet

is the droplet velocity, Vair

is the air

velocity at the location of the droplet, |Vair

-Vdroplet

| is the slip velocity, flair

is the

density of the air, µair

is the air absolute viscosity, flwater

is the density of water

for the droplet, ‡waterair

is the water surface tension for the droplet. The Reynolds

number is the ratio of inertial forces to viscous forces and is related to the drag

force on the droplet. The Weber number is the ratio of inertial forces to the surface

tension force and relates to the droplet breakup and breakup modes. The Bond

number is used to characterize the shape of bubbles or droplets moving in a fluid.

The Bond number is the ratio of gravitational forces to the surface tension force

and was defined with respect to the droplet acceleration. The Reynolds number,

Weber number, and Bond number of the droplet as it approached the airfoil model

are shown in Figures 3.9, 3.10, and 3.11 respectively. The main parameters were

calculated with changing values for the diameter of the droplet to take into account

the e�ects of the deformation.

52

Figure 3.9: Reynolds number versus distance from the airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

Figure 3.10: Weber number versus distance from the airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

53

Figure 3.11: Bond number versus distance from the airfoil leading edge. Chambertemperature = -20¶C, V

airfoil

= 80 m/s, droplet diameter = 401 µm.

54

Chapter 4 |

Experimental Results and Com-

parisons

The experiment described in previous chapters of this thesis was done with the

purpose of comparing the results obtained using supercooled droplets to the results

obtained using droplets at room temperature to study the e�ects of the droplet

temperature on the behavior of the droplets. The results of the experiments and

comparisons will be presented and discussed in this chapter.

4.1 Droplet Supercooling

Before continuing to the results of the experiment, it is important to look into the

temperature of the droplets as they exited the droplet generator and moved towards

the point of impact (the stagnation line of the airfoil model). The temperature of

the droplets was collected experimentally using an IR camera. The capabilities of

the IR camera did not allow for the stream to be resolved (the stream was too thin)

and therefore, the temperature of the droplet stream could not be determined by the

55

camera alone. Instead, a black surface that was allowed to reach the environmental

temperature inside of the chamber was used to collect enough water such that the

camera could provide a temperature measurement. An example of an image from

the IR camera and a simple schematic of the image are shown in Figure 4.1. The

maximum temperature observed in the box in the center is shown in the top left

corner while the temperature scale of the entire image is shown along the right side.

Figure 4.1: Sample of image obtained using the IR camera to measure the droplettemperature along with schematic.

The temperature measurements provided by the thermocouple (near the water

reservoir) and the thermistor (located above the inlet of the droplet generator)

were recorded prior to and after each entry. Data was taken using water at two

temperatures (thermocouple reading averaged 9.5 and 7¶C) to observe the water

temperature changes with distance of two di�erent temperatures and compare

trends. When using the colder water, the temperature of the water at the inlet of

the droplet generator was an average of 3¶C and at the warmer temperature, the

56

temperature of the water at the inlet of the droplet generator was an average 4¶C.

For the image shown in Figure 4.1, the temperature of the water was 6.7¶C at the

exit of the reservoir and 2.62¶C at the inlet of the droplet generator, which was about

14 cm (5.5 inches) above the tip of the droplet generator. Multiple measurements

were taken at each of the four distances from the tip of the droplet generator (2.54,

7.62, 12.7, and 19.05 cm). The results of the experimental method are shown in

Figure 4.2. The set of data collected with the warmer water temperature (Tw

=

4¶C) is represented by the orange triangle while the set of data collected with the

colder water temperature (Tw

= 3¶C) is represented by the blue triangle.

Figure 4.2: Droplet temperature versus distance from the droplet generator. Resultsof experimental method used for measuring droplet temperature at various verticaldistances from the droplet generator.

57

The experimental approach to obtaining the droplet temperature did not provide

accurate measurements. The black surface was at the environmental temperature

(-20¶C) which has thermal energy that is expected to a�ect the measured water

temperature. Despite these shortcomings, the results indicate that, at each distance

from the tip of the droplet generator, the temperature of the water was below 0¶C.

Another factor that would introduce error into the experimental measurements is

the fact that the exact test environment could not be replicated for the measurements

of the water temperature. During the tests, the rotor was spinning causing the

air in the chamber to mix. The mixing of the air inside of the chamber removed

the temperature gradient known to exist in the chamber when the rotor is not

spinning. The temperature gradient may be the explanation to the odd behavior of

the droplet temperature when the distance from the tip of the droplet generator is

19.05 cm (7.5 in). An increase in the temperature is observed which is not expected

as the temperature of the droplet should continue to decrease as more time is spent

in the cold environement of the chamber. The large standard deviation in the

data and the unexplained increase in droplet temperature at the 19.05 cm (7.5

in) location made it apparent that the results obtained experimentally would not

su�ce to ensure that the droplets used during the tests were supercooled.

In response, another approach was used to determine the temperature of the

droplets at the four locations below the droplet generator. A prediction code by

Tadas P. Bartkus, a NASA Glenn Research Center contractor, was used to give

further confidence that the droplets were indeed supercooled [23]. The program

required the environmental temperature, the droplet size, the droplet vertical

velocity, which was set to 4 m/s, and the initial droplet temperature. The vertical

58

velocity of 4 m/s was used in the program because the vertical velocity of the

droplets analyzed in the cold chamber ranged between 3 and 4 m/s. The initial

droplet temperature used was 1¶C. The results from the code are shown in Figure

4.3 for droplets ranging from 200 µm to 700 µm in diameter.

Figure 4.3: Droplet temperature versus distance from the droplet generator. Resultsof the prediction code used for determining droplet temperature at various verticaldistances from the droplet generator. Initial Temperature = 1¶C.

The initial temperature was the temperature of the droplets at the tip of the

droplet generator. 1¶C was chosen because the temperature of the water at the

input of the droplet generator for the data used ranged between 2.65 and 4.12¶C

during the tests. The nearest temperature measurement device was at the droplet

generator water inlet and the heat transfer between the water and the droplet

59

generator between the water inlet point and the tip was not known. An assumption

for the temperature at the tip was needed and a value lower than the the values at

the droplet generator inlet was chosen. During the experiments, if the temperature

of the water at the generator inlet fell below 2.25¶C, the water at the tip of the

droplet generator would freeze. Using the assumption of the temperature of the

water at the tip of the droplet generator, the results indicate that droplets up to

700 µm could be below 0¶C at the impact location. This would confirm that the

droplets analyzed were supercooled as the diameter of all of the droplets analyzed

from the cold chamber tests were smaller than 700 µm. The ice formed during

testing is shown in Figure 4.4. The white spikes seen in the photograph resemble

rime ice feathers indicating that supercooled droplets were obtained during the

tests.

Figure 4.4: Photograph of ice shape formed on airfoil model during tests.

A second, more conservative approach was taken to predict the temperature

60

of the water at the four distances below the tip of the droplet generator. This

approach removed the droplet generator as a factor and instead assumed that only

convective cooling occurred between the location where the water was measured

and the impact location at the stagnation line of the airfoil model. Instead of

only falling 19.05 cm (7.5 in) while in the cold environment, the droplets, using

this approach, fell 33.02 cm (13 in). The initial temperature used was set to 3¶C

since the temperatures measured at the droplet generator inlet during the tests was

always between 2 and about 4¶C. The results to the second approach are shown

in Figure 4.5. The results suggest that only droplets that were smaller than 500

µm in diameter could reach temperatures below 0¶C. It should be noted that the

droplet generator has a small reservior where the water collects just above the

orifice (Figure 4.6). While in this small reservoir, the temperature of the water is

expected to decrease before it is formed into the stream of droplets that impacts

the airfoil model.

61

Figure 4.5: Droplet temperature versus distance from the droplet generator. Resultsof the prediction code used for determining droplet temperature at various verticaldistances from the droplet generator. Initial Temperature = 3¶C.

4.2 Comparison of the Experimental Data Sets

In this section, the experimental data collected at INTA will be compared to

the data collected in the AERTS facility. In their 2012 American Institute of

Aeronautics and Astronautics (AIAA) conference paper, Vargas, Sor, and Garcia-

Magariño presented test data for the case that was compared to a case of data

obtained in the AERTS facility [9].

A room temperature (20¶C) environment was used in the facility at INTA. The

velocity of the airfoil was 90 m/s, the slip velocity chosen was 60 m/s, and the

62

Figure 4.6: Schematic of droplet generator showing parts inside of droplet generator[8].

range of droplet diameters presented was between 550 and 1800 µm. The results of

the deformation, which is the ratio of the ellipse major axis to the ellipse minor axis

(b/a), and Bond number were presented and are displayed, using orange squares,

in Figures 4.7a and 4.7b respectively. The AERTS case that was most similar to

the conditions of the INTA test case was compared. Due to rotor stand limitations,

the maximum airfoil velocity that could be reached in the AERTS facility for

this experiment was 80 m/s. A sample of droplets from cases where the airfoil

velocity was 80 m/s was analyzed and the deformation and the Bond number were

plotted. The slip velocity was also chosen as 60 m/s, as in the INTA test case. The

environmental temperature was now -20¶C. The results are shown in blue circles in

Figure 4.7a and 4.7b.

For the comparison, the sets of data were plotted on the same graphs. One can

63

(a) Deformation versus droplet diameter.

(b) Bond number versus droplet diameter.

Figure 4.7: Droplet deformation and Bond number versus droplet diameter. Datacollected at INTA and in AERTS facility plotted on same graph for comparison.V

slip

= 60 m/s. Blue Circle (AERTS): Chamber Temperature = -20¶C, Vairfoil

=80 m/s. Orange Square (INTA): Chamber Temperature = 20¶C, V

airfoil

= 90 m/s.

easily observe that there is a lack of overlap in the data (line drawn in to point out

lack of overlap). The droplet diameter ranges are di�erent for the two sets of data.

64

Although the ranges of deformation and Bond number are similar, conclusions

regarding the behavior of the droplets due to temperature di�erences can not be

made. In general, the behavior of the droplets is similar in that the deformation

and Bond number for larger droplets are higher than that of the smaller droplets.

Although the range of droplet sizes was obviously di�erent, the other di�erences in

the data (velocity and temperature) should be analyzed.

It is interesting to note that, for the droplet parameters noted in the previous

chapter (Reynolds number, Weber number, and Bond number), the temperature

decrease has little e�ect on the variables that are temperature dependent. The

density of the water changes by less than 2% and the density and viscosity of

air change by less than 10% over the 40¶C (-20 to 20¶C)range [24]. The surface

tension also has a small change in the temperature range used. Based on the

experimental values of the surface tension of supercooled water, the change in the

surface tension between water at 20¶C and water at -5¶C is less than 5% [25]. In

the weakly supercooled range reached during the tests, the parameters are not

expected to change due to the decrease in temperature and therefore can be set as

constants. The e�ects of the temperature on the deformation of the droplets will

still be discussed, but before that is done, the e�ects of both the airfoil velocity

and the slip velocity on droplet deformation will be explored.

65

4.3 E�ect of Initial and Slip Velocity on the De-

formation and Bond Number

To determine the e�ect of the initial velocity on the deformation of the droplets,

the variables among the cases to be compared were identical, except for the velocity

of the airfoil. The environmental temperature was -20¶C and the data compared

corresponded to a slip velocity of 50 m/s. The initial velocity of the droplets with

respect to the stationary reference frame on the airfoil model was 80, 70, and 60

m/s. The deformation for the impacting droplets tested at the di�erent initial

velocities against the droplet diameters is plotted in Figure 4.8. Trendlines were

added to each set of data to assist with the interpretation of the results.

As seen in the plot, the sets of data overlap, with a few outliers. The general

trend is again that the deformation is greater in droplets with larger diameters. In

comparing the sets of data, the slopes of the data are in agreement in magnitude

and there is no indication of a particular trend due to the initial velocity. To further

explore the e�ects of velocity, the e�ects of the slip velocity were investigated.

Instead of di�erent initial velocities, the initial velocity remained constant while

the slip velocity was varied. The initial velocity was 70 m/s and the slip velocities

considered were 30, 40, 50, and 60 m/s. The deformation of the droplets at the

di�erent slip velocities against the droplet diameters is plotted in Figure 4.9. The

chamber temperature was -20¶C. Once again, trendlines are added for each set of

data.

For an initial velocity of 70 m/s and a slip velocity of 30 m/s, the slope of the

trendline is 0.0011. As the slip velocity was increased, the slope of the trendline

66

Figure 4.8: Deformation versus diameter varying the initial droplet velocity with aconstant slip velocity of 50 m/s. Chamber Temperature = -20¶C.

also increased and reached a value of 0.0065 at a slip velocity of 60 m/s. These

results indicate that there is an e�ect on the slope of the droplet deformation with

respect to the droplet diameter due to the di�erence between the initial droplet

velocity and the slip velocity. As the slip velocity approaches the initial droplet

velocity, the slope increases. Given this observation, the initial droplet velocity and

the slip velocity need to be the same in cases that are compared with the purpose

of determining the e�ects of temperature on the deformation.

67

Figure 4.9: Deformation versus diameter varying the slip velocity with a constantinitial droplet velocity of 70 m/s. Chamber Temperature = -20¶C.

4.4 E�ect of Temperature on the Deformation and

Bond Number

It was established that to determine the e�ects of temperature on the deformation

of droplets the initial droplet velocity and the slip velocity need to be the same in

both cases. This was accomplished using droplets with an initial droplet velocity

of 60 m/s and at a slip velocity of 40 m/s. In addition, an e�ort was made to

compare droplets of similar diameter. The sets of data presented in Figure 4.10

were collected in the AERTS facility. The di�erence in the data collected is the

environmental temperature during the tests, 20¶C versus -20¶C.

68

Figure 4.10: Droplet deformation versus droplet diameter in warm (20 ¶C) andcold (-20¶C) environments. Initial drop velocity = 60 m/s. Slip velocity = 40 m/s.

The data presented overlaped since the droplets analyzed have similar diameters.

The range of values of the deformation in each set of data was also similar. A

di�erence in the deformation of the droplets due to di�erences in the environmental

temperature was not observed. As the temperature did not have a large e�ect

on the di�erent parameters involved in calculating the Bond number, a di�erence

between the two cases is not expected.

The Bond number of droplets for both cases is plotted in Figure 4.11. The

Bond number in both the warm environment and the cold environment follow an

equal trend. Square polynomial fittings (since the Bond number is proportional to

the square of the droplet diameter) were added, further showcasing the continuous

trend between the results. The sample of experimental data analyzed suggests

that supercooling has no e�ect on the particle deformation for the airfoil velocity

69

investigated.

Figure 4.11: Bond number versus droplet diameter in warm (20 ¶C) and cold(-20¶C) environment. Initial drop velocity = 60 m/s. Slip velocity = 40 m/s.

The deformation and Bond number of droplets with an initial velocity of 70 m/s

and a slip velocity of 50 m/s are presented in Figures 4.12 and 4.13. As was the

case with the data presented in Figures 4.10 and 4.11, the results do not indicate

that the environmental temperature has an impact on the trend followed by the

deformation and the Bond number.

70

Figure 4.12: Droplet deformation versus droplet diameter in warm (20 ¶C) andcold (-20¶C) environment. Initial drop velocity = 70 m/s. Slip velocity = 50 m/s.

Figure 4.13: Bond number versus droplet diameter in warm (20 ¶C) and cold(-20¶C) environment. Initial drop velocity = 70 m/s. Slip velocity = 50 m/s.

71

Chapter 5 |

Conclusions and Future Work

The purpose of the work presented in this thesis was to determine the e�ects of

supercooling on the behavior of water droplets near the leading edge of an airfoil.

The conclusions of the work are presented along with suggestions for related future

work.

5.1 Conclusions

To accomplish the task of exploring the e�ects of supercooling on the behavior

of droplets as they impact an airfoil, an experimental setup was developed. The

setup used during the experiments conducted at Instituto Nacional de Técnica

Aeroespacial provided some parts, such as the droplet and frequency generators

as well as the airfoil model profile. The Adverse Environment Rotor Test Stand

(AERTS) facility at Penn State was used as the testing chamber since it is capable

of reaching and maintaining temperatures between 0 and -25¶C. It houses a rotor

test stand, which provided the motion of the airfoil model necessary to create

a setup similar to the one used at INTA. The lighting and high-speed imaging

72

system were di�erent than in the setup at INTA. A stationary light source could

not be used in the AERTS facility since the opposite rotor blade and the rotor hub

would have been obstacles between the light source and the camera lenses. Instead

of a single camera, two high-speed cameras were used to widen the field of view

and increase the number of deformation and breakup events captured. The water

reservoir was designed to keep the water inside of the cold chamber from freezing

using an immersion heater and by insulating the container.

The setup was designed to achieve with the experimental objective of visualizing

the droplet events. The high-speed imaging system enabled the recording of the

droplet deformation and breakup events as the airfoil model impacted the droplets

at various speeds throughout the experiment.

The droplets were required to be supercooled. By monitoring and controlling the

temperature of the water in the reservoir and at the inlet of the droplet generator,

the water was maintained at temperatures between 2 and around 4¶C as it entered

the droplet generator. The cold environmental temperature of the chamber (-20¶C)

assisted in supercooling the droplets. Although supercooling temperatures were

reached, only a small portion (-5 - 0¶C) of the entire supercooling range (-40 -

0¶C) was reached. The range reached was on the warmer side of the range and the

supercooled droplets obtained were considered weakly supercooled.

Using this experimental setup, the objective of data comparison between the

supercooled droplets and the droplets at room temperature could be met. Data was

collected of the deformation and breakup of the droplets in the weakly supercooled

state. The behavior of the mildly supercooled droplets observed could then be

compared to that of the water droplets at room temperature collected at INTA and

73

in the AERTS facility. The completion of this objective led to the final objective of

determining the e�ects of the temperature on the droplets.

To begin, the room temperature droplet data that was collected at INTA was

compared to the data collected of supercooled droplets in the AERTS facility. The

two sets of data could not be used in the comparison of the behavior as the data

did not match in terms of the di�erence between the values for the slip velocity and

initial droplet velocity. Although the same slip velocity was compared, the airfoil

velocity (initial droplet velocity) and the ranges of droplet diameters were di�erent.

To explore the e�ects of the droplet velocity on the deformation of the droplets,

experimental cases with constant slip velocity and varying airfoil velocities were

investigated. A specific trend was not observed in the slopes of the data, but when

the airfoil velocity was held constant and the slip velocity was varied, a clear e�ect

on droplet deformation was observed. As the slip velocity approached the airfoil

velocity, the slope of the deformation with respect to the droplet diameter increased.

For an airfoil velocity of 70 m/s and a slip velocity of 30 m/s the slope was 0.0011

and it increased to 0.0065 at a slip velocity of 60 m/s. It was demonstrated that to

compare the e�ects of water supercooling on droplet deformation, the velocities

(slip and droplet velocities) should be equal in each set of data compared.

Two cases with equal initial droplet velocity were selected for room temperature

and supercooled droplet conditions. The data from the two cases that were compared

also corresponded to the same slip velocity. The airfoil velocity was 60 m/s and the

slip velocity for both sets of data was 40 m/s. In these cases, the deformation of

the supercooled droplet and that of the room temperature droplets did not present

di�erent trends for the limited range, weakly supercooled states of water droplets,

74

tested. The similar behavior for both environmental conditions indicates that weak

supercooling had no e�ect on particle deformation at the selected impact velocity.

A comparision between data with an intial droplet velocity of 70 m/s and a slip

velocity of 50 m/s was also presented and the same conclusions were drawn as for

the comparison of the data with an initial velocity of 60 m/s and a slip velocity of 40

m/s. A di�erence in the behavior of the droplets within the range of temperatures

used was not observed.

It should be noted that the results and conclusions presented apply only to

droplets in the conditions described. The conclusions cannot be extrapolated to

droplets that are further supercooled or that have a higher initial velocity.

5.2 Future Work

General conclusions on the behavior of supercooled droplets at the leading of an

airfoil cannot be made based on the work presented. The behavior of the droplets

analyzed cannot be used to describe the behavior of droplets that are further

supercooled or that are impacted at higher velocities.

To be able to make a general conclusion, the experiment should be repeated with

certain changes. An increase in the droplet impact velocity and further supercooling

of the droplets to cover the entire supercooling range would provide the data

necessary to make more general conclusions on supercooled droplet behavior. While

upgrades to the facilities would allow the rotor to reach higher tip speeds, a small

cooling chamber inside of the test facility could allow the droplets to reach colder

temperatures in the supercooled droplet range. A schematic of the setup including

the small cooling chamber is shown in Figure 5.1.

75

The smaller cooling chamber would be designed to reach temperatures in

the range of -20 and -100¶C. It would be placed around and below the droplet

generator to increase the residence time of the droplets in a colder environment.

With this chamber surrounding the droplet generator and covering the majority

of the distance between the droplet generator and the impact location, it may be

possible to increase the distance between the droplet generator and the impact

location without negatively a�ecting the path of the droplet. This would increase

the residence time and allow the temperature of the droplet to decrease further

increasing the range of droplet sizes that can be supercooled.

Figure 5.1: Schematic of setup with new design for droplet supercooling.

76

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