jet impingement

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International Journal of Applied Engineering Research

ISSN 0973-4562 Volume 4, Number 2 (2009), pp. 161174

Research India Publications

http://www.ripublication.com/ijaer.htm

Effect of Aspect Ratio of Air Jet on Heat Transfer

Rate in the Impingement Cooling of Electronic

Equipment - An Experimental Study

M. Anwarullah1, V. Vasudeva Rao

2and K.V. Sharma

1Research Scholar, 3 Professor1,3

Centre for Energy Studies, JNTU College of Engineering,

Hyderabad-500034, India1E-mail address: [email protected].

2Professor, Department of Mechanical Engineering, SNIST, Hyderabad. India

AbstractThe objective of this work is to carry out an experimental study to examine

the effect of geometric parameters on the confined impinging jet heat transfer

characteristics of an array of electronic resistors with a single circular jet of

different diameters. The effect of Reynolds number and aspect ratio H/d, on

Nusselt number has been studied. Measurement of surface temperatures of the

resistors is made in the range of 5850 < Red < 12200 and 2 < H/d


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Effect of Aspect Ratio of Air Jet on Heat Transfer Rate in the Impingement 163

Zumbrunnen et al. [15] carried out studies on convective heat transfer from a plate

cooled by water jets. Lytle and Webb [16] investigated the flow structure and heat

transfer characteristics of air jet impingement for nozzle-plate spacing of less than a

nozzle diameter in the range of 3600 < Re < 27,600. Baydar [17] carried out an

experimental investigation for low Reynolds number up to 10,000 at various nozzle-

to-plate ratios. An expression for the stagnation Nusselt number was derived by Vader

et al. [18] given by376.05.0

PrRe505.0=ONu (5)Zhou and Ma [19] experimentally investigated the radial heat transfer behavior of

impinging submerged circular jets. An expression for the local Nusselt number valid

at the stagnation point in the radial direction is obtained as33.0499.0

PrRe32.1=ONu (6)Lienhard et al. [20] experimentally investigated the splattering and heat transfer

during impingement of a turbulent liquid jet. The recommended equation for the local

Nusselt number at the stagnation location is given by33.05.0

PrRe24.1=ONu (7)

Siba et al. [21] experimentally studied impingement cooling of a flat circular disk

made of conducting material SS304. Recently, the flow characteristics of both

confined and unconfined air jet impinging normally onto a flat plate have been

experimentally investigated by Baydar and Ozmen [22]. Schwarz and Cosart [23]

presented measurements and and theoretical analysis on fluid flow characteristics of

impinging slot jet, but only for the turbulent wall jet zone. The present study is

concerned with the experimental investigation of the confined impinging jet flowfields at various nozzle-to-resistor surface distances. The main objective of the present

work is to study the effect of geometric parameters on the heat transfer characteristics

of resistor surface normal to impinging air jet

Experimental setupThe experimental set up as shown in Fig. 1, consists of five cylindrical electrical

resistors fixed to an insulating plate of diameter 100mm and 2mm thick located

centrally on an aluminum heater plate. A chip assembly on PCB is simulated with the

electrical resistors which are 25 mm long and 4 mm in diameter. The resistors each of

5 W rating are connected to supply through volt and ammeter. Five J-type

thermocouples are attached to measure the surface temperature of each resistor.

Thermocouples of Type J would normally have an error of approximately 0.75% of

the target temperatures when used at a temperature lower or higher then 277O

C. A

heater plate of 240 mm diameter and 20 mm thick is connected to a heating coil of

500 W rating through a dimmerstat to enable the temperature of the insulating plate to

be higher than ambient. Two thermocouples are connected to the heater plate and

another one measures the ambient temperature. All these eight thermocouples are

connected to a temperature indicator through a scanner to observe the readings and

store the values in a personal computer. The air flow rate through a nozzle of different

diameters located above the resistors is measured with a rotameter. Air at 20-bar is


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164 M.Anwarullah, V. Vasudeva Rao and K.V. Sharma

made available to the nozzle from a reciprocating air compressor of 220 liter

stororage capacity through the rotameter. Provision is made tovary the distance

between the nozzle tip and the test surface. The axis of the nozzle is always aligned

with the centre resistor and normal to the plane on which heat sources are mounted.

Figure 1: Schematic diagram of Experimental Setup.

Experimental ProcedureThe air jet emanating from the nozzle and impinging on the resistors is depicted as

free jet and wall jet regions respectively and shown in Fig 2. Power is supplied to the

resistors through a step down transformer and the aluminum plate through adimmerstat.

Figure 2: Schematic diagram of flow emanating from nozzle and impinging on chip

surface.

Aluminum disc

Wall Jet region

HNozzle,

Free Jet region

Impingement region

d

1 2 3 4 5


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Effect of Aspect Ratio of Air Jet on Heat Transfer Rate in the Impingement 165

The volumetric energy generation due to heating of the resistors using AC current

is assumed to be uniform. The temperature of the resistors is allowed to rise up to 950

C and then cooled by forced convection mainly from the top surface by the air stream

flowing in the wall jet region. The surface temperature of the resistors are recorded till

they attain 400C The procedure is repeated at different flow rates of air with

temperature values recorded in the Reynolds number range of 5850 to 12200. The

velocity of jet is measured using a Pitot tube. The heat loss from the bottom of the

resistors is assumed to be negligibly small.

Results and discussionAir jet from the nozzle is forced over the resistors when they have attained a

maximum steady temperature of 98o

C in the range of 5850 < Red < 12200 and 2

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