Optical Diagnostics and Modeling of Different Reacting and Non-reacting Systems

Saptarshi Basu

Mechanical Engineering Department

University of Connecticut

Ph.D Research Projects

• Soot topography in diffusion flames

wrapped by line vortex

• Scalar mixing in vortex

• Innovative laser based non-intrusive diagnostics

in fuel cells

Ph.D Research Projects

Hydrodynamics and heat transfer in

liquid films over rotating disk

Plasma synthesis of ceramics

PublicationsFuel Cell Diagnostics [Experimental]

1. Basu, S., Xu, H., Renfro, M. W., and Cetegen, B. M. (2004). ASME Journal of Fuel Cell Science and Technology, Vol 3 February 2006 

2. Basu, S., H. Gorgun, Renfro, M. W., and Cetegen, B. M. (2005). Journal of Power Sources, 159 (2006)

3. Basu, S., Renfro, M. W., and Cetegen, B. M. Journal of Power Sources, 162 (2006)

Patents 

S.Basu, M.W Renfro, B.M Cetegen, Fiber Optic based in-situ diagnostics for PEM fuel cells. Patent pending

Vortex Dynamics [Modeling & Experimental] 4. Cetegen, B. M. and Basu, S. (2006). Combustion and Flame, 146(2006) 

5. Basu, S., Barber, T and Cetegen, B. M. (2006), In Review Physics of fluids, July 2006

Publications

Heat Transfer over Rotating Disk [Modeling] 

6. Basu, S. and Cetegen, B. M. (2005). ASME Journal of Heat Transfer, Vol 128, March 2006 

7. Basu, S. and Cetegen, B. M. (2005), In Press ASME Journal of Heat Transfer

Plasma Synthesis of Ceramics [Modeling & Experimental]8. Basu, S. and Cetegen, B. M. Accepted for publication in International

Journal of Heat and Mass Transfer, July 2006 

9. Basu, S., E.H Jordan and Cetegen, B. M., Invited paper in Journal of Thermal Spray Technology, January 2007 

10. Basu, S. and Cetegen, B. M., submitted to Acta Materialia, January 2007

Modeling and Diagnostics of Solution Precursor Plasma Spray Process

Funding Agency ONR

Grant No. N00014-02-1-0171

Thermal Barrier Coating

Padture et al, Science 2002• Gas Turbine Blades

• Temperature Reduction upto 300ºC

Solution Precursor Plasma Spray Process

Precursor : Aq Soln. Zr + Yittria Salts => 7YSZ

Alumina + YSZ

Important Parameters Affecting Coating Quality

• Initial Loading of Salts

• Droplet Size

• Plasma Temperature and Velocity Field

• Injection Velocity

• Type of Injection (Transverse or Co-axial)

• Precursor Property like 7YSZ or Alumina etc

• Standoff Distance

All affects droplet level transport

Effect of Processing

50 m

5 m

50 m50 m

5 m5 m

Hardness: 365Density: 73 %

Hardness: 760Density: 82 %

Hardness: 1020Density: 95 % +

Droplet level transport needs to be understood

for process improvement

This improvement takes years of trial and error

Modeling

Axial Injection

• Entire thermal history of the droplets

• Mass transport at the droplet level in the plasma

Droplet Vaporization and Precipitation Routes

Fig.1: Solute containing droplet vaporization and precipitationroutes: (A) Uniform concentration of solute and volumeprecipitation leading to solid particles; (B) super-saturation nearthe surface followed by (1) fragmented shell formation (lowpermeability through the shell, (2) unfragmented shell formation(high permeability), (3) impermeable shell formation, internalheating, pressurization and subsequent shell break-up andsecondary atomization from the internal liquid; (C) elastic shellformation, inflation and deflation by solids consolidation

(A)

(B)

(C)

(I)

(II)

(III)

(A) Volume precipitation (B) (I) fragmented shell formation (low permeability through the shell, (II)

unfragmented shell formation (high permeability), (III) impermeable shell formation

(C) Elastic shell formation, inflation and deflation

• Predict precipitation (surface or volume) of droplets• Model droplet level transport for better understanding of the

role of process parameters on droplet morphology • Predict shattering of shells after precipitation

SEM image of the deposition on a substrate after a single pass.

Cracked and dried Shell

Motivation

Xie et al 2003

• High Relative Velocity

• Low Surface Tension

• Large Droplet Size

Timescale of microsecs

Timescale of millisecs

Timescale of microsecs

Three Stages of Droplet Modeling

Aerodynamic Breakup

Aerodynamic breakup of 50 micron initial diameter 7YSZ droplets for a surface tension of 0.036 N/m

Aerodynamic Breakup (II)

Not an issue for droplets of diameter less than 50 microns

Heating and Vaporization (2D)

Models tried are

• 2D transient heat and species transport coupled with plasma

• Spherically symmetric heat and species transportEnergy (2D)

Tsin

sin

1

T1TrVPe5.0

T

dt

dr5.0rVPe5.0

Tr

2

22

sL

ssrL

2s

Species (2D)

sinsin

11

rVLePe5.0

dt

drLe5.0rVLePe5.0rLe

22

2

sLL

sLsrLL

2sL

Heating and Vaporization (1D)

Energy (1D)

Species (1D)

r s2 T

0.5

drs

dtT

1

2

2 T

LeLr s2

0.5LeL

drs

dt

1

2

2

• Precipitation based on homonuclear hypothesis

• Precipitation triggered whenever solute concentration at surface reaches super saturation limit

Heating and Vaporization (2D)

Computed droplet temperature (left) and solute mass fraction (right) for a 40 micron droplet injected into a DC arc plasma at 1 ms (top) and 3 ms (bottom)

Precipitation (2D)

d = 5 m.

d = 10 m.

d = 20 m.

Xie et al Mater. Sci. Eng A 2003

What is shell porosity

What is the morphology/property of the shell

Internal Pressurization

• The model of the interior of the particle is divided into three zones• Internal pressure buildup due to heating and lack of evaporation• Porosity of the shell

Internal Pressurization

• 10 micron initial sized droplets

• Pressure rise decreases with porosity due to venting

• 40 micron initial sized droplets

• Pressure rise decreases with porosity due to venting

Conclusions

• Droplets below 50 micron do not undergo aerodynamic breakup

• Precipitation occurs when surface concentration of solute reaches supersaturation

• Small droplets < 5 microns tend to volume precipitate

• Large droplets surface precipitate forming thin shells

• Low porosity shells fracture on further heating in the plasma

• High porosity shells do not fracture and arrive at the substrate as unpyrolized material with watery core

Limitations

• No precipitation kinetics is modeled

• Literature show precipitation is not homonuclear

• No experimental data is available for precipitation at high heating rates

• Behavior of droplets in an array or vicinity of other droplets is not modeled

Future Research Direction 

 • Physical evolution of particulate formation in droplets at rapid heating rates 105 to 107 K/s. [FUNDING NSF : CBET/ONR : Collaboration AMPAC/UCONN]

• The morphology, microstructure and chemical composition of droplets evolution as a function of time [FUNDING NSF : CBET /ONR: Collaboration AMPAC/UCONN]

• Advanced modeling with improved precipitation mechanism [FUNDING NSF : CBET /ONR : Collaboration UCONN]

• Provide general guidelines for processing routes for different types of precursors for different microstructure for advanced applications • In situ monitoring of splat dynamics and transient composition variation [FUNDING NSF : CBET /ONR]

Future Research Direction Laser heating of droplets

Advantages• Similar heating rates as in the plasma • Similar timescales for precipitate formation• Diagnostics are easier to carry out

Similar profiles

Future Research Direction 

• High speed imaging of precipitation

• Different kinds of precursors investigated

• Ex situ analysis of morphology of collected sample using SEM, FTIR and Raman

Future Research Direction

High Temperature resistant Fiber Optic

• In situ analysis of splat by infrared fiber optic evanascent spectrsocopy

• Transient composition variation can be monitored

In-situ Simultaneous Measurements of Temperature and Partial Pressure in a PEM Fuel Cell under Steady and Dynamic cycles

Funding Agency

U. S. Army Research RDECOM

Presentation Outline Development and calibration of diode laser based absorption measurement technique for

water vapor concentration measurement

Application of the measurement technique to an operating fuel cell

Theoretical method to extract temperature and partial pressure simultaneously using a single laser scan

Experimental Results of partial pressure and temperature in a PEM fuel cell under steady conditions

Experimental Results of partial pressure and temperature in a PEM fuel cell under dynamic conditions

Extensions and improvements of the measurement technique to spatially resolved measurements

Concluding Remarks

Introduction to PEM fuel cells

At the anode the hydrogen gas ionizes releasing electrons and protons (H+). 

2H2 4H+ + 4e- 

This reaction releases energy. At the cathode, the oxygen reacts with electrons taken from the electrode, and H+ from the electrolyte, to form water. 

O2 + 4e- + 4H+ 2H2O

Bipolar plate contains the gas channels for circulating the air and hydrogen

Courtesy : Dr. J. Fenton

Water management in PEM fuel cells

Water diffusion from cathode to anode

Water supplied by externally humidification

Water removed by circulating hydrogen

Water produced within the cathode

Water transport due to electro-osmotic drag by protons

The depleted air leaving the fuel cell remove water

Water supplied by externally humidifying the air/O2 supply

Main Challenges

• Flooding/drying up of membrane

• Real time humidity level control in inflow gas streams

• Proton flux through the membrane directly proportional to the current density in a MEA.

• Careful balancing of the supply and removal of water vapor to prevent FLOODING/ DRYING and BETTER PERFORMANCE and DURABILITY

Protonic conductivity of several membrane materials as a function

of relative humidity at T=100 ºC studied by Alberti et al

Water management in PEM fuel cells

Water vapor measurements in fuel cells

Current state-of-the-art limited only to inlet and outlet measurements or expensive instrumentation

Fourier transform IR (FTIR) spectrometer Gas chromatograph Neutron Scattering

Most measurements performed at the INLET and OUTLET of the cell

Intrusive techniques require extraction of the sample from the fuel cell and sending it to the external measurement device

Both techniques provide slow, temporally and spatially averaged measurements

There is a GREAT NEED to measure water vapor concentration non-intrusively with good spatial and temporal resolution

Water Management & Transport (DOE)

Water Management & Transport (DOE)

Water Transport

Impurity sensor like CO sensor

Novel concepts

Cold start

Tunable Diode Laser Absorption Spectroscopy (TDLAS)

Non-intrusive, in-situ first of its kind of measurements

Line of sight absorption in individual flow channels

Measurement time: ~seconds

Off-the-shelf telecommunications cheap lasers (~$500)

Total system cost ~$5000

Straightforward extension to other species such as CO, CO2

Ability to measure multiple channels or species simultaneously via multiplexing

Ability to measure temperature across each channel using a single laser scan

Potential use in feedback control systems

Potential use in high temperature fuel cells like SOFC

Potential use in Direct Methanol Fuel Cells

Beer-Lambert law

b

0s

,0

i dlpexpI

I

i

s I

I

bp 0ln

1

Absorption of light by a sample

= Transmitted light intensity

= Incident light intensity

= Partial pressure of water vapor

= Absorption coefficient

iI

0I

sp

The measured partial pressure represents a path-averaged valueWavelength (nm)

1306 1308 1310 1312 1314

Lin

e In

tens

ity

(cm

/mol

)

0

10-23

2x10-23

3x10-23

4x10-23

5x10-23

6x10-23

T=300 T=400

1100 1200 1300 1400 1500 1600 1700

Lin

e In

tens

ity

(cm

/mol

)

10-23

10-22

10-21

10-20

10-19

T=300 T=400

Proper Selection of Wavelength is crucial

Simulated signal Voigt profile simulation with HITRAN database parameters

Test cell for system calibration Simple humidity and temperature controlled absorption cell constructed

for system calibration and validation

Signal Photodiode

DAQ

Diode Laser

AIR

Signal photodiode

Auxiliary heater

Exhaust valve

Heater controller

Windows

Absorption cellTC4

TC3

TC2

TC1

Reference Photodiode

Fiber coupler

Diode Laser

Optically accessible bipolar plate

Same plate design for

anode and cathode

Optically accessible plates

Effective flow path is 7 cm

BIPOLAR PLATE

Measurement averaged

along channel length

Laser system Laser temperature tuned over ~5 nm to couple with different

H2O transitions Laser current tuning affects both power and wavelength (0.4

nm) Laser power monitored by photodiode All measurements on cathode side of the fuel cell

Beamsplitter

Reference Photodiode

To experiment/Test cell

LaserCurrent Controller

Laser current modulated by 100 Hz ramp function Single scan range of 0.4 nm Laser wavelength calibrated with a ring interferometer system Multiple scans averaged

Measurement Methodology

Test Cell

Signal Photodiode

Reference Photodiode

Current Ramp

Output Ln (sig/ Ref)

Signal processing (1) Measurements of reference and signal photodiode are imposed on current

ramp

Signal processing (2) Four Lorentzian profiles and a polynomial background fit to measured signal Sum of four Lorentzian represents Log(Signal/Reference)

Partial pressure extraction• Widths of two strong transitions are assumed equal based on

HITRAN Simulation

• Intensities of two strong transitions are assumed constant multiples of one another for all Ps and T.

• Six curve fit parameters: 2 widths, 3 intensities, background

• Empirically, self-broadening and air broadening are related by:

• Partial pressure directly related to measured transition width

airwwaterw hh ,, 2.5

Partial pressure determination Calibration experiments performed in fuel cell geometry with

controlled temperature and relative humidity

10 % Error

Operating Fuel Cell Setup

Laser

Data Acquisition and Laser controller

Electric Load Box

Temperature Controller

Current Controller

Fuel CellControl

H2

Air

TC1

TC2 TC3Humidifiers

PD1

PD2

Scribner Fuel Cell Test Station

Experimental conditions

• Absorption data collected twice for each current setting & averaged

• For each data set curve fit was done

• Partial Pressure predicted from measured half width using calibration line

Measurements in active fuel cell (Cathode side)

Measured water partial pressure in operating fuel cell at inlet water partial pressures of (a) Ps = 0.19 atm and (b) Ps = 0.26 atm for a cell temperature of 80

ºC Experimental data collected show a similar increase in partial pressure with current density Diagnostic technique works for high current densities if the inlet relative humidity is less than 80 %.

Temperature insensitive measurement

Theoretical study for the determination of temperature and partial pressure simultaneously

• Present laser source stimulated transitions insensitive to temperature

• For 20oC rise in temperature a temperature sensitivity of atleast 20 % is needed.

• 23 % variation in intensity detected for transition in the range of 1470 nm

Calibration with new Laser for simultaneous temperatureand partial pressure detection

Partial Pressure and Temperature Calibration

• Partial Pressure calibration same as before• Temperature is calibrated from peak intensity

Measurement Under Steady State Conditions

• Temperature and partial pressure detected from single laser scan• For a steady state cell with controlled temperature the measured temperature is about 55 ºC

Measurement under dynamic fuel cell operation

• Partial pressure rise upto 0.3 atm from the inlet value of 0.19 atm entire dynamic cycle of 300 secs • Temperature rise is recorded for each cycle above the steady state value of 80 ºC. Temperature as high as 90 ºC is observed.

Spatially Resolved Measurements (Steady State)

• Measurements across two channels simultaneously using a single laser scan• Difference in partial pressure between two channels increases with current load

Spatially Resolved Measurements (Steady State)

• Difference in partial pressure between two channels increases with current load

• Temperature across both channels are similar

Spatially Resolved Measurements (Dynamic)

• Measurements across two channels simultaneously using a single laser scan

• Load is varied sinusoidally

• Partial pressure closely follows the load profile without phase lag

Concluding Remarks

In situ water vapor concentration measurements demonstrated using

tunable diode laser absorption spectroscopy in PEMFC

Calibration measurements permit partial pressure determination within

10% accuracy and +/- 2 ºC for temperature

Measurements in operating fuel cell show strong signals with low noise

Simultaneous detection of temperature and H2O partial pressure

Measurements in the fuel cell under dynamic and steady state conditions

Spatially resolved measurements permit simultaneous determination of

temperature and pressure across two channels

FM spectroscopy will allow higher degree of accuracy in H2O

measurements as well as allowing to measure other species such as CO,

CO2

Acknowledgments

This work was sponsored by a grant from U. S. Army Research RDECOM, CERDEC (Fort Belvoir, VA) through Connecticut Global Fuel Cell Center (CGFCC)

I acknowledge the contributions of the following people:

Profs. B.M Cetegen (Advisor), M.W Renfro (Co-advisor), E.H Jordan (Co-advisor)

Prof. J. Fenton, Drs. M. Williams and V. Ramani for allowing us to use their fuel cell set-up

Mr. T. Mealy for technical help in machining Dr. Haluk Gorgan and Prof. F. Barbir for allowing us to use their

fuel cell set-up

Limitations of Current Technique and Future Plans

• The spatial resolution was limited to average value along channel length. Pointwise measurements needed

• The temporal resolution of the technique inadequate for high frequency load cycle

• Cyclic loading can lead to unforeseen effects in species transport and temperature distribution.

• Timescale of load variation and species kinetics mismatch. Result malfunctioning like water logging or drying up of the cell

• The modes of failure for a cyclic load compared to steady state operation Durability will be different for same load.

Future Research Directions (I)• Pointwise resolvable measurements across each channel

• Extension of the measurement technique to include species like CO

• Extension of the methodology to direct methanol fuel cell (DMFC)

• Improved temporal resolution of the technique to quantify the response characteristics of the gas phase subjected to high frequency load cycle

• Potential extension to include high temperature fuel cell like SOFC

• Developing novel diagnostics techniques for two-phase flow characteristics, thermal transport within the gas channels and the MEA

• Novel imaging techniques and optical sensors for fuel cell testing and real time feedback control strategy

• Extension to fuel cell stacks

• Measurements in cold weather start up

Future Research Directions (II)

• In-situ infrared fiber evanescent wave spectroscopy using tunable laser for pointwise measurements of concentration both in the vapor and liquid phases FUNDING [CBET/DOE[EERE]/ US ARMY/ FSEC]

• High speed imaging for monitoring growth and dynamics of liquid water from the MEA at high power FUNDING [CBET/ DOE[EERE]/ PRIVATE SECTOR/COLLABORATION FSEC]

• PIV measurements to observe velocity and clogging in the channels FUNDING [CBET/ DOE[EERE]/ PRIVATE SECTOR/ COLLABORATION FSEC]

• FM Spectroscopy to increase temporal resolution of the measurements FUNDING [CBET/ DOE[EERE]/ US ARMY/ FSEC]

• 3D modeling to understand the response especially the short time transients [COLLABORATION UCONN]

CCD

Camera

Laser Sheet

Embedded, Unclad, Flattened Optical Fiber for Evanascent Spectroscopy

Strobe Light

Water Droplets

Water Film

Water Vapor ZoneTransparent optically accessible Plate with Anti-Fog coating

Current Collector

COMPLETE UNDERSTANDING OF WATER TRANSPORT

Frequency Modulation (FM) Spectroscopy

Laser Temperature Controller

Current Controller

Data Acquisition

Lock-in amplifier

Fuel cell

Fiber coupler

Sine wave generator

Ramp generator

Mixer

Signal Photodiode

Schematics of a single laser FM spectroscopy

• To measure weak transitions in other wavelength regions which are very sensitive to temperature

• Reduce the error band arising principally due to large background.

Preliminary FM Spectra

Simulated Experimental

Initial data(2f) taken under ambient conditions at a temperature of 25oC at a partial pressure of 0.013 atm.

The profile looks very clean with negligible background.

Outcomes• Complete understanding of the transport of water vapor and other species under steady and dynamic conditions

• Chemical kinetics of the of the fuel cell under high frequency load cycle as in automobile

• Inception of two phase flow

• Growth and transport of liquid water

• Simultaneous measurement of temperature

• Extension to SOFC and DMFC

• Validation of the computational models

• Partial understanding of the durability issue of the fuel cell arising through water transport

• Extension to similar measurements inside the MEA

• Measurements in fuel cell stacks

• Measurements during cold weather start up

Combined Funding Options

• NSF [CBET: Transport and Thermal Fluids Phenomena, Energy for Sustainability]

• DOE [Office of Energy Efficiency and Renewable Energy (EERE) ]

• ONR

•  U.S. Army Research, Development and Engineering Command

• Collaboration with Private Company

• Collaboration with FSEC, UCONN, CGFCC

• Startup Funding