An experimental investigation on influence of the shape of the nozzle for flow field and heat transfer characteristics between electronic equipment surface and confined impinging air jet.

Anwarullah, M. ; Rao, V. Vasudeva ; Sharma, K.V. 等

Introduction

Jet impingement is one of the flow techniques used to cool or heat target surfaces. It is widely used in industrial applications ranging from drying of textiles and films, metal sheet manufacturing, gas turbine cooling etc. Recently, due to its high heat transfer rate, impinging jet heat transfer has been applied in the field of electronic component cooling. The flow and heat transfer in an impinging jet depend on non-dimensional parameters such as the nozzle-to-chip spacing, H/d and the Reynolds number, Re. In addition, the effect of the nozzle geometry, flow confinement, and turbulence have all been shown to be significant by Jambunathan et. al [1].

Gardon and Akfirat [2] studied the effect of turbulence on the heat transfer between two dimensional jet and flat plate. Gardon and Akfirat [3] studied effect of multiple two-dimensional jets on the heat transfer distribution. Baughn and Shimizu [4] and Hrycak [5] conducted experiments of heat transfer to round jet from flat plate employing different methods of surface temperature measurement. A number of studies dealt with heat transfer enhancement due to impinging jets and extensive reviews are presented by Martin [6], Schwarz and Cosart [7] and Viskanta [8]. A numerical investigation on the effect of jet Reynolds number, and nozzle-to-plate distance has also been conducted. Lytle and Webb [9] experimentally investigated the flow structure and heat transfer characteristics of air jet impingement for nozzle-plate spacing less than one nozzle diameter in the range of 3600 < Re < 27,600. Beitelmal et al. [10] analyzed two-dimensional impinging jets and correlated heat transfers in the stagnation point, stagnation region and wall jet region with approximate solutions developed using simplified flow assumptions. Theoretical solutions in the wall jet region fit better at large distances from stagnation point. O'Donovan and Murray [11] investigated the turbulent fluctuations within the wall jet to study the fluid flow and convective heat transfer mechanisms that influence the magnitude and location of secondary peaks. They reported that at low nozzle to impingement surface spacings the mean heat transfer distribution in the radial direction exhibits secondary peaks. Colucci, and Viskanta, [12] investigated Effect of nozzle geometry on local convective heat transfer to a confined impinging air jet.

The objective of the present paper is to study the influence of the shape of the nozzle (circular, rectangular and square) on the local heat transfer distribution to normally impinging submerged air jet on surface of the electronic resistors. The effect of jet to plate spacing (2 to 10 nozzle diameters) and Reynolds number (6500-12500) are studied for all the nozzles investigated.

Experimental setup

The 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 277[degrees] 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 10 mm diameter located above the resistors is measured with a rotameter. Air at 20-bar is made available to the nozzle 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 10 mm diameter located above the resistors is measured with a rotameter. Air at 20-bar is made available to the nozzle

[FIGURE 1 OMITTED]

Experimental Procedure

The 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 1. Power is supplied to the resistors through a step down transformer and the aluminum plate through a dimmerstat. 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 95[degrees]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 40[degrees]C The procedure is repeated at different flow rates of air with temperature values recorded in the Reynolds number range of 5850 to 12500. 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 discussion

The mean velocity profile measured along the jet centerline for two jet Reynolds number are shown in Fig. 2. In this figure, all the data were non-dimensioned by the jet exit velocity [U.sub.e]. The normalized velocity decreases gradually with an increasing nozzle-to-resistor spacing, as shown in Fig. 4, independent of the jet Reynolds number. The jet for Re = 12500 has a higher normalized velocity with comparison to the case of Re = 6500 as the nozzle-to-resistor spacing is less than 5. The opposite trend occurs at the further downstream, indicating a strong mixing with the surrounding air here.

Temperature distributions at H/d = 2 are shown in Fig.3. It is observed that with increase in the Reynolds numbers, temperatures decreases at all time locations for all the three nozzle configurations. This may be due to increases of velocity of jet with Reynolds number. Air jet from the nozzle is forced over the resistors when they have attained a maximum steady temperature of 98[degrees]C in the range of 6500 < [Re.sub.d] < 12500 It is also observed at a H/d of 2, that the surface temperature of the resistors drop down rapidly in 75 seconds from the time of starting of air flow. As expected the temperature gradient is higher at rectangular nozzle as compared to square and circular nozzles. It is also observed that the rapid decrease in temperature is also due to large temperature potential between the surface and the ambient Fig. 4 shows the distribution of local Nusselt numbers for various Reynolds number at H/d = 4. However, the stagnation point Nusselt number values are almost same for all the three nozzles for a particular Reynolds number. The distributions of the local Nusselt number for the square and circular nozzle are almost same and are marked by the beginning of the transition region at r/d = 1, which further extends up to an r/d = 2. For rectangular nozzle, the Nusselt number values in the stagnation region remain almost constant upto r/d = 0.5 and thereafter decreases monotonically. In the stagnation region, the Nusselt number values are higher for the rectangular jet as compared to the square and the circular jet

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The local Nusselt number at a Reynolds number of 23 000 and H/d of 6 is compared with those of the earlier published data as shown in Fig.5. It compares well with the results of Lytle and Webb [9] and Gao et al. [14] which use thin metal foil technique. It also compares well with the heat transfer results of Baughn and Shimizu [4], but is higher in the region away from the stagnation point. The heat transfer data of Lee et al. [11] and Yan and Saniei [15] are lower than the results of the present work. These differences may be attributed to the differences in the measurement techniques. Fig. 6 shows the influence of H/d on the stagnation point Nusselt number at different Reynolds numbers for the square, rectangular and circular and nozzle. It is observed that stagnation point Nusselt numbers increase with H/d from H/d = 2.0 up to H/d = 4.0 and then slightly drop. This trend may be due to increase in turbulent intensity at the stagnation point with increase in H/d.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Data reduction and uncertainty analysis

Present experimental data equation:

Nu = 0.027[Re.sup.0.586] [Pr.sup.0.33] (1)

The local heat transfer coefficient is calculated using the following equation:

h = q/ ([T.sub.S] - [T.sub.a]) (2)

The local Nusselt number on the resistor surface is defined by

Nu = h.d/[k.sub.air] (3)

Nozzle Reynolds number is defined as follows:

Re = Vd/v (4)

The uncertainty associated with the experimental data is estimated using the standard single-sample uncertainty analysis recommended by Kline and McClintock [16] and Moffat [17]. In the present experiments, the temperature measurements were accurate to within [+ or -] 0.5[degrees]C, the uncertainty of [Re.sub.d] and [Nu.sub.o] for the ranges of parameters studied under steady-state conditions is within [+ or -] 2% and [+ or -] 4.5 %, respectively. The present experimental data is subjected to regression and given in a simplified form as

[Nu.sub.Reg] = 1.2965[Re.sup.0.4] [Pr.sup.0.4] [(H/d).sup.-0.012] (5)

valid in range 2< H/d< 10, and 6500< Re< 23000 with average deviation 8% and standard deviation 10 %.

The present experimental data is in good agreement with the values of Nusselt obtained with Eq.5 as shown in Fig.7

[FIGURE 7 OMITTED]

Conclusions

The effects of the shape of the nozzle on impinging jet heat transfer is experimentally investigated at different Reynolds numbers and nozzle-to-resistor spacing. Three different nozzles cross sections namely, square, rectangular and circular are covered in this study. The following are the main conclusions that may be drawn from this study. The effect of Reynolds number on the performance of noncircular jets is similar to that for the circular jet; with increase of Reynolds number, the heat transfer rate increases. The heat transfer characteristics of square and circular jets show much similarity. Increase in Reynolds number increases the heat transfer at all the radial locations for a given H/d. Based on the present experimental conditions, the jet Re, the nozzle tip- to-resistor spacing and cooling time have an important influence on the heat transfer of impinging circular jet nozzle, especially on the wall jet and impingement region. The heat transfer rate increases as the jet spacing decreases owing to the reduction in the impingement surface area

Acknowledgement

The first author is working as faculty in the Department of Mechanical Engineering and grateful to the management of Muffakham Jah College of Engineering and Technology, Hyderabad for the financial support in the fabrication of the experimental setup

Nomenclature

A                    surface area of
                     the resistor, [m.sup.2]
[C.sub.p]            specific heat at constant pressure, J/(kg K)
H                    distance between nozzle tip to resistor, m
Nu                   local Nusselt
                     number, Eq. (1)
Re                   jet Reynolds
                     number, Vd/v
q                    heat flux, W/m
t                    cooling time,
                     seconds
[T.sub.s]            surface temperature of the resistor before
                     cooling, [degrees]C
[T.sub.[infinity]]   ambient
temperature, OC
V                    velocity of air,
                     m/sec
Hd                   nozzle-to-resistor
                     spacing to nozzle
                     diameter
NuO                  Nusselt number at stagnation point
K                    thermal conductivity of air, W/(m K)
Pr                   Prandtl number
r                    radial distance measured from the
                     stagnation point, m
Uc                   local mean stream wise velocity on the
                     jet centerline
Ue                   jet exit velocity

Greek symbols

[rho]                density of air, kg/[m.sup.3]
v                    kinematic viscosity of air, [m.sup.2]/s

Subscripts

Reg regression, Exp experimental

References

[1] K. Jambunathan, E. Lai, M.A. Moss, B.L. Button., 1992, "A review of heat transfer data for single circular jet impingement," Int. J. Heat Fluid Flow 13: 106-115

[2] R. Gardon, C. Akfirat, 1966,Heat transfer characteristics of impinging two dimensional air jets, Journal of Heat Transfer 88, 101-108.

[3] R. Gardon, C. Akfirat, 1965 "The role of turbulence in determining the heat transfer characteristics of impinging jets," International Journal of Heat and Mass Transfer 8, 1261-1272.

[4] J.W. Baughn, S. Shimizu, 1989 "Heat transfer measurements from a surface with uniform heat flux and an impinging jet," Journal of Heat Transfer 111, 1096-1098.

[5] P. Hrycak, 1983, "Heat transfer from round impinging jets to a flat plate," International Journal of Heat and Mass Transfer 26, 1857-1865.

[6] H. Martin, 1977 "Heat and mass transfer between impinging gas jets and solid surfaces", Advances in Heat Transfer 13, 1-60.

[7] W.H. Schwarz, W.P. Cosart., 1961, "The two-dimensional turbulent wall jet," J. Fluid Mech 10,481-495.

[8] R. Viskanta,1993, "Heat transfer to impinging isothermal gas and flame jets", Experimental Thermal and Fluid Science 6, 111-134.

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[10] A.H. Beitelmal, A.J. Shah, M.A. Saad, 2006, "Analysis of an impinging two dimensional jet", Journal of Heat Transfer 128, 307-310.

[11] T.S. O'Donovan, D.B. Murray, 2007, "Jet impingement heat transfer--Part I: Mean and root-mean-square heat transfer and velocity distributions, International Journal of Heat and Mass Transfer 50, 3291-3301.

[12] Colucci, D., Viskanta, R., 1996. E.ect of nozzle geometry on local convective heat transfer to a confined impinging air jet. Experimental Thermal and Fluid Science 13, 71-80.

[13] D.H. Lee, J. Song, C.J. Myeong, 2004, "The effect of nozzle diameter on impinging jet heat transfer and fluid flow, Journal of Heat Transfer 126, 554-557.

[14] N. Gao, H. Sun, D. Ewing, 2003, "Heat transfer to impinging round jets with triangular tabs, International Journal of Heat Mass Transfer 46, 2557-2569.

[15] X. Yan, N. Saniei, 1997, "Heat transfer from an obliquely impinging circular air jet to a flat plate, International Journal of Heat and Fluid Flow 18, 591-599.

[16] S. J. Kline and F. A. McClintock., 1953, "Describing uncertainties in single-sample experiments," Mech. Engng 3-8.

[17] R. J. Moffat., 1988, "Describing the uncertainties in experimental results," Expl.Therm.Fluid Scii. 1, 3-17

M. Anwarullah (1), V. Vasudeva Rao (2) and K.V. Sharma (3)

(1) Research Scholar, Centre for Energy Studies, JNTU College of Engineering Hyderabad-500034, India

Corresponding author E-mail address: manwar_sana@yahoo.com.

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

(3) Professor, Centre for Energy Studies, JNTU College of Engineering, Hyderabad. India