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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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