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International Journal of Engineering Technology, Management and Applied Sciences

www.ijetmas.com December 2016, Volume 4, Issue 12, ISSN 2349-4476

69 Adimurthy. M, Vadiraj V Katti

Experimental Investigations on the Local Distribution of wall

static pressure coefficient Due To an Impinging Slot Air Jet on

a Confined Rough Surface

1Adimurthy. M

1BLDEA’s VP DR. P G Halakatti college of

Engineering and Technology, Vijayapura,

Karnataka, India

2Vadiraj V Katti

2 Principal, KLS Vishwanathrao Deshpande Rural

Institute of Technology, Haliyala, Karnataka, India,

ABSTRACT

Experimental investigations on the local distribution of wall static pressure coefficient due to an impinging slot air jet on

a confined rough surface is performed with the parameters: Reynolds number (2500-20000), jet to plate spacing (0.5 to

10), confinement ratio (6.8, 10.6, 13.2), size of the ribs (4mm, 6mm) and the distance of the point from the stagnation

point in the lateral direction on the plate. Static pressure probe and a differential pressure transducer is used to record

the pressure on the target plate. It is clearly identified that the presence of confinement and roughness on the surface

changes the flow behaviour and an enhancement in the coefficient of pressure for the confinement ratio of 6.8 and rib

size of 4mm at the range of Reynolds numbers studied. The results can help in design of a better cooling device for

industrial applications.

Keywords

Confinement ratio, surface roughness, Reynolds Number, coefficient of pressure, jet to plate spacing, stagnation point

pressure coefficient.

1.INTRODUCTION

Jet impingement heat transfer received considerable research attention due to their inherent characteristics of

high rates of heat transfer. Such impinging flow devices allow for short flow paths on the surface with

relatively high heat transfer rate. The industrial processes which employ impinging jets are drying of textiles,

film and paper; processing of some metals and glass; gas turbine blades and outer wall of combustors; cooling

of electronic equipment The Impinging jet can be defined as a high velocity coolant mass ejected from a hole

or slot that impinges on the heated plate. A characteristic feature of this flow arrangement is an intensive heat

transfer between the wall and fluid.

Various researchers have worked on impinging jets of different configurations in the past. A.H.Beitelmal et

al [1] studied the effect of surface roughness on average heat transfer at on impinging air jet. The temperature

was measured over a Reynolds number ranging from 9600 to 38500. Nozzle to plate spacing (z/d) ranging

from 1 to 10. They observed up to 6%increase in average Nusselt number due to surface roughness that

disturbs the boundary layer and promotes the turbulence of wall jet. The correlation between average Nusselt

number, Reynolds number and nozzle to plate spacing was proposed. V.A.Chirac and A.Ortega [2] used the

numerical finite difference method to determine steady and unsteady flow and heat transfer due to a continued

slot jet impingement on isothermal plate. The jet Reynolds number ranging from 250 to 750 of Prandtl

number 0.7 and jet to plate spacing (H/W) of 5 is studied. They observed that at Reynolds number 585-610 the

flow become unsteady. In the steady regime stagnation Nusselt number is directly proportional to the

Reynolds number and distribution of heat Transfer in the wall jet region was influenced by re-entrainment of

the flow. X.Gao and B.Sunden [3] studied the heat transfer characteristics of confined single and multiple

impinging slot jet experimentally. The liquid thermography was used map the temperature distribution on the

plate. The effects of nozzle to plate spacing, Reynolds numbers, and width of jet and presence of the exhaust




port on the local and average Nusselt number is studied. Comparing the single and multiple jets at two nozzles

to plate spacing (H/B) 8 and 24, all the local Nusselt number distribution shows same behaviour for all

Reynolds number from 1150-7550, which confirms that large slot to slot distance in multiple jets prevents

from jet interaction with each other. The effects of the nozzle to plate spacings on stagnation Nusselt number

of central jet of the three slot system and single slot jet at different Reynolds numbers observed and stagnation

Nusselt number is maximum at H/B=8 and both jets have same value up to H/B=8, increasing in H/B leads to

decrease in Nu0 values of multiple jet. The effect of slot width shows that higher width has higher local

Nusselt number and lower width have higher average Nusselt number, it occurs due to increase in turbulence

intensity as the slot width increases. V.Narayan et al [4] experimentally studied the flow field surface pressure

and heat transfer of turbulent slot jet. Two nozzle to surface spacing at 3.5 (transitional jet) and 0.5 (potential

core jets) nozzle exit hydraulic diameter are considered for exist Reynolds number at 23000. They reported

high heat transfer rate in the transitional jet impingement and non-monotonic decay in heat transfer

coefficient. For potential core jet impingement and generation of turbulence near the surface prior to

impingement and pressure at span wise vertices in the stagnation region with increase in near wall responsible

for increased in heat transfer in transition jet impingement. E.Bayadar and Y. Ozmen [5] conducted

experimental and numerical investigation on impinging air jet at Reynolds number ranging from 30000 to

50000. The mean velocity, turbulence intensity and pressure distribution were obtained for given Reynolds

number and a nozzle to plate spacing ranging between 0.2-6. They observed pressure distribution on the

impingement plate was independent of the Reynolds number. Sub atmospheric region occurs on impinging

plate for nozzle to plate spacing up to 2 and it become stronger and more deductible with decreasing the

nozzle to plate spacing. They reported maximum turbulence intensity and secondary peak in Nusselt number

occurred in the sub atmospheric region. V.Katti and S.V.Prabhu [6] studied enhancement of heat transfer on a

flat plate surface using axis symmetrical ribs by normal impingement of circular jet. Effect of ribs width (w)

ribs height (e) pitch between the ribs (p) location of first rib from stagnation point and clearance under the rib

(c) on local heat transfer distribution is studied jet to plate distance varying from 0.5 to 0.6. They observed

that the enhancement in heat transfer due to acceleration of fluid in that region created by clearance under the

rib and Nusselt number of stagnation point increases by shifting rib nearer to the stagnation. With increasing

in width of rib major flow impinges on top surface of rib and may not reach to target surface it may lead to

decrease in heat transfer and increasing rib height flow velocity along the target surface decaying at faster

rate. They also observed wall static pressure gradient of stagnation point is higher for surface with detached

ribs than smooth surface. Puneet Gulati et al [7] studied the effects of shape of nozzle, jet to plate spacing and

Reynolds number on impinging air jet. Reynolds number varied between 5000 to 15000 and jet to plate

spacing from 0.5 to 12 Nozzle diameters. Length to equivalent diameter ratio of 50 is chosen. The local heat

transfer is determined using thermal images obtained from IR thermal imaging method. They observed that

rectangular nozzles show high heat transfer rate due to high turbulence intensity at nozzle exit and it requires

more pumping power due to larger pressure loss coefficient. Pressure drop measurements across the nozzle

are made and pressure loss coefficient for all nozzle configurations reported. Nirmalkumar et al [8] conducted

experimental investigation of local heat transfer distribution and fluid flow properties on a smooth plate by

impinging slot air jet. Reynolds number based on slot width varied from 0.5 to 12. They identified three

regimes on the flat surface viz. stagnation region (0 ≤ x ≤ 2), transition region (2 ≤ x/b ≤ 5) and wall jet region

(x/b ≥ 5). Semi empirical correlation for Nusselt number in the stagnation region and wall jet region is

suggested. Katti, Adimurthy and Ashwini [9] experimentally studied the local wall static pressure distribution

on smooth and rough surface by impinging slot jet. They observed that wall static pressure is independent of

Reynolds number and its strength decreases with increase in nozzle to plate spacing, they also observed

enhancement in heat transfer rate at rough surface using detached ribs compared to smooth surface.

Adimurthy and Katti [10] investigated the local distribution of wall static pressure and Heat transfer on a

smooth plate impinged by a slot air jet. They adopted Infrared thermographs to study the heat transfer

distribution and static pressure probe to analyze the flow distribution and behaviour over the plate. They

presented a correlation to estimate the Normalized value of the Nusselt number in particular to the stagnation

region. Adimurthy, Katti and Venkatesha [11] experimentally studied the fluid flow and heat transfer

characteristics of a rough surface, impinged by a confined laminar slot air jet. Heat transfer distribution was




analyzed with the IR images and static pressure probe is used to capture the pressure distribution over the

rough surface in presence of confinement. They correlated flow pattern and heat distribution in their study.

They also presented an empirical formula to estimate the Stagnation point Nusselt number in the potential

core as well as beyond the potential core region separately.

Present work is intended, to study the influence of Reynolds number, jet to plate spacing, surface roughness

and confinement of the jet on the local distribution of wall static pressure coefficient due to impingement of a

confined slot air jet on the rough surface.

3. EXPERIMENTAL SETUP AND METHODOLOGY

The experimental set-up for wall static pressure distribution study is as shown in 1 and Air is supplied by a

blower through a well calibrated orifice meter and venturimeter. The flow rate is controlled by a flow control

valve, to achieve the uniform flow rate at the exit of the nozzle.

1.Traversing table, 2.Target plate, 3.Detached rib, 4.Nozzle, 5.Plenum chamber, 6.Diffuser, 7.Control

valve, 8.Orifice meter, 9.Air blower, 10.Venturimeter, 11.U-Tube manometer, 12.Differential pressure

transducer, 13.Confinement plate

Figure.1 Experimental setup for wall static pressure distribution with rough

Surface

The airflow is directed to the plenum through a diffuser and two meshes in the plenum. Velocity profile

remains uniform because of the diffuser upstream of the plenum. Nozzle is made of an acrylic sheet with 45

mm height and 4 mm wide. The aspect ratio of the nozzle cross section is maintained very high around 24 so

that the effect of nozzle height is neglected. Jet air temperature is measured using a Chromel-Alumel

Thermocouple (K-type) positioned at the inlet of the nozzle. The output of the thermocouple is measured by

milli voltmeter. The target plate is kept on 2-D traverse table, so it can be easily moved for various jet to plate

spacings by operating the traversing system. Orifice meter and venturimeter are used to measure the rate of air

flow through the jet. The target plate made up of acrylic sheet of 10mm thickness. A static pressure tap

approximately 0.5mm in diameter is drilled in 10mm thick acrylic plate for 3mm deep from impingement

surface and then counter bored to 3mm diameter for the remaining depth. The jet-to-plate distances (Z/Dh) of

0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 10 are considered in the present study. The ribs are fabricated from the

thermally non active material Plexiglas (K=0.23w/mk). Two ribs of size 4mm and 6mm width, 45mm height

and thickness is 2mm are chosen for the study. The spacer of thickness 1mm is used to mount the rib over the

target plate to maintain uniform gap between the plate and the rib.




Nomenclature

symbol Description

A area of jet (m2)

a1 cross sectional area of inlet pipe (m2)

a2 cross sectional area of throat (m2)

B breadth of nozzle (m)

Cd coefficient of discharge

Cp co-efficient of pressure

Dh hydraulic diameter of nozzle in (m)

g acceleration due to gravity 9.81 m/sec2

L length of nozzle (m)

Qact actual discharge of air (m3/sec)

Vj velocity of jet (m/s)

x manometer deflection (m)

μ dynamic viscosity of air in (kg/m sec)

ρm density of manometer fluid (kg/m3)

p Recorded value of pressure from DP

4. DATA REDUCTION

The important parameters of the study are computed using the various equations listed below. The description

for the notations is given in the nomenclature.

Discharge of venturimeter (Qact), is calculated using the

equation, Qact = Cd

)(

2

2

2

2

1

21

aa

gHaa

(1)

Equivalent head for the flow measurement is computed

using , H=

1

p

m

x

(2)

Hydraulic diameter (Dh) for the slot jet is computed using

the equation,

Dh= BL

LB

2

(3)

Reynolds number of jet (Rejet) is given by, Rejet=

hj DV

(4)

Velocity of jet (Vj) can be measured using the formula, Vj=

j

act

A

Q

(5)

Co-efficient of wall static pressure(Cp) is calculated from

equation,

Cp= 2

2

jaV

p

(6)




5 RESULTS AND DISCUSSION

The results of an investigation to determine the local wall static pressure distribution of a normally impinging

confined slot air jet on flat rough surface for Reynolds number ranging from 2500 to 20000 and for the jet to

plate spacing (Z/Dh) of 0.5 to 10 are presented. Pressure distribution is measured using Differential Pressure

(DP) transducer.

5.1 Influence surface roughness and confinement on wall static pressure distribution at Re=2500

Fig 4.2.1(a) to Fig 4.2.1(f) shows variation of local wall static pressure along the stream wise direction for

particular Z/Dh values at Reynolds number 2500 of different confinement ratios of 4mm and 6mm rib.

From fig 4mm rib with confinement ratio of 6.8 shows maximum stagnation wall static pressure and sub

atmospheric region at Z/Dh=0.5, sub atmospheric region and secondary peaks are depend on the Z/Dh value.

(a)

(b)

(c) (d)

Fig 4.2.1 Comparison of wall static pressure of different rib width and confinement ratios




5.2 Influence of Confinement on smooth and rough surface (4mm rib)

From Fig 4.2.2(a) to Fig 4.2.2(f) shows effect of confinement ratios on variations of local wall static pressure

along stream-wise direction of smooth and rough surface (4mm rib) for different nozzle to plate spacing

(a)

(b)

(c)

(d)




(e) (f)

Fig 4.2.2 effect of confinement on smooth and rough surface (4mm rib)

The maximum stagnation wall static pressure occurred for confined ribbed surface compare confined smooth

surface. The sub-atmospheric region occurred up to Z/Dh=4 for confined ribbed surface, Z/Dh=0.5 for confined

smooth surface.

It clear that strength of sub-atmospheric region depend on ribbed configuration at Z/Dh>0.5, and secondary

peaks observed for confined ribbed configuration and peaks height increases with increase in Z/Dh. The

spreading rate of confined smooth jet is more than ribbed and confined.

5.3 Influence of confinement ratio on stagnation point wall static pressure

Fig 4.2.3(a) to Fig 4.2.3(e) shows variation of stagnation point wall static pressure with Z/Dh for different

confinement ratio of 4mm rib. At lower Reynolds number Lc/Dh=6.8 shows better values.

(a) (b)




(c) (d)

(e) (f)

(g) (h)




(i) (j)

Fig 4.2.3 Influence of confinement ratio on stagnation point wall static pressure

At Re =5000 to 20000 The Lc/Dh=13.6 shows higher stagnation wall static pressure within potential core

region and Lc/Dh=10.2 shows better values behind potential core region.

Fig 4.2.3(f) to Fig 4.2.3(j) shows variation of Cp0 with Z/Dh for different confinement ratio of 6mm rib. The

Lc/Dh=13.6 shows better Cp0 values at Z/Dh<4. After potential core the Cp0 values of all configuration merge

with smooth plate values

6. CONCLUSION

An experimental study is carried out to investigate local wall static pressure distribution and local heat transfer

distribution between the normally impinging confined slot air jet and flat rough surface. A single fully

developed jet issued from a slot jet is chosen. Reynolds number based on nozzle exit condition is varied from

2500 to 20000. Different configurations of detached ribs and confinement plates are chosen. Sequential

parametric study is carried out to investigate the influence confinement on the local wall static pressure

distribution on rough surface. The jet to plate distance is varied from 0.5 to 10 times the Hydraulic diameter of

the jet.

The conclusions drawn from the investigation are as follows:

Maximum wall static pressure occurs at stagnation point and its value decreases with increase in nozzle to

plate spacing for all the configurations investigated.

Sub-atmospheric region is observed up to Z/Dh=4 for all rough surface configurations studied.

Effect of confinement is pronounced more at Z/Dh=0.5 for all configurations since at lower jet to plate

spacing, more recirculation of the jet happens.

Secondary peaks in wall static pressure are visible invariably at all jet to plate spacings and Reynolds

numbers for rough surface due to the effect of flow acceleration under the detached ribs.

Increase in confinement length increases the potential core length with increasing Reynolds number.

7. REFERENCES [1] Beitelmal, Abdlmonem H., Michel A. Saad, and Chandrakant D. Patel. "Effects of surface roughness on the average

heat transfer of an impinging air jet." International Communications in Heat and Mass Transfer 27.1 (2000): 1-12.

[2] Chiriac, Victor A., and Alfonso Ortega. "A numerical study of the unsteady flow and heat transfer in a transitional

confined slot jet impinging on an isothermal surface." International Journal of Heat and Mass Transfer 45.6 (2002):

1237-1248.




[3] Gao, Xiufang, and B. Sunden. "Experimental investigation of the heat transfer characteristics of confined impinging

slot jets." Experimental heat transfer 16.1 (2003): 1-18.

[4] Narayanan, V., J. Seyed-Yagoobi, and R. H. Page. "An experimental study of fluid mechanics and heat transfer in an

impinging slot jet flow." International Journal of Heat and Mass Transfer 47.8 (2004): 1827-1845.

[5] Baydar, E., and Y. Ozmen. "An experimental and numerical investigation on a confined impinging air jet at high

Reynolds numbers." Applied thermal engineering 25.2 (2005): 409-421.

[6] Katti Vadiraj, and S. V. Prabhu. "Heat transfer enhancement on a flat surface with axi-symmetric detached ribs by

normal impingement of circular air jet." International Journal of Heat and Fluid Flow 29.5 (2008): 1279-1294.

[7] Gulati, Puneet, Vadiraj Katti, and S. V. Prabhu. "Influence of the shape of the nozzle on local heat transfer

distribution between smooth flat surface and impinging air jet." International Journal of Thermal Sciences 48.3

(2009): 602-617.

[8] Nirmalkumar, M., Vadiraj Katti, and S. V. Prabhu. "Local heat transfer distribution on a smooth flat plate impinged

by a slot jet." International journal of heat and mass transfer 54.1 (2011): 727-738.

[9] V.V.katti, Adimurthy.M. & Ashwini. .M. Tamagond, “Experimental Investigation on Local Distribution of Wall

Static Pressure Coefficient due to Impinging Slot Air Jet on Smooth and Rough Surface”. Journal of Mechanical

Engineering Research and Technology, Volume2, Number 1, (2014) pp500-508.

[10] Adimurthy M and Katti V.V “Local distribution of wall static pressure and heat transfer on a smooth flat plate

impinged by a slot air jet.” Heat and Mass Transfer [2016], DOI 10.1007/s00231-016-1847-9

[11] Adimurthy M, Katti V.V and Venkatesha, “Experimental Study of Fluid Flow and Heat Transfer Characteristics of

Rough Surface, Impinged by a Confined Laminar Slot Air Jet.” International Journal of Engineering Research &

Technology (IJERT), ISSN: 2278-0181, Vol. 5 Issue 06, (2016), JERTV5IS060418

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