EXPERIMENTAL STUDY OF HIGH HEAT REMOVAL BY ALUMINUM PIN FIN HEAT SINK USING MULTI-JET AIR IMPINGEMENT

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1 International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN(P): ; ISSN(E): Vol. 4, Issue 5, Oct 2014, TJPRC Pvt. Ltd. EXPERIMENTAL STUDY OF HIGH HEAT REMOVAL BY ALUMINUM PIN FIN HEAT SINK USING MULTI-JET AIR IMPINGEMENT A. J. MORE, D. S. MANAKAR & N. P. BHONE Department of Mechanical Engineering, AISSMS s Institute of Information Technology, Pune, India ABSTRACT This Experimental Study utilizes the multi-jet impingement technique to investigate the thermal performance of pin fin heat sink under unconfined and semi confined conditions. In the present Experimental investigates the effect of Aluminum Heat sink, nozzle jet exit Velocity, the jet to jet spacing and the nozzle plate to Flat plate heat sink or pin fin base plate Separation distance(z/d) in a 3 3Square Multi-jet impinging array on average Nusselts number. Thermal Performance of Pin fin heat sink of mm for the Reynolds numbers range 6000 to is studied. The thermal performance of 4 4array circular Pin fin heat sink is studied with the thermal Parameters like Average Nusselts number, total thermal resistance, Average base temperature, Reynolds numbers. KEYWORDS: Aluminium Heat Sink, Multi-Jet Impingement, Pin Fin Heat Sink, Electronic Cooling INTRODUCTION As power densities continue to increase, electronic component cooling becomes a more serious design problem. One of the most common means for cooling electronic modules is a finned heat sink that enhances convection heat transfer to ambient air. There are a variety of heat sink types, with differing fin geometries, and operating with natural or forced convection. A common geometry is a pin-fin array heat sink. The research described in this paper focuses on the thermal performance of a pin-fin array heat sink which is oriented such that the air flow enters the fin-bundle perpendicular to the fin base (impinging flow)[1]. In the literature review studies basically 3 stages followed optimization of pin fin heat sink, Single jet impingement on heat sink and multi jet impingement. H. H. Jung 2 and J. G. Maveety Studied the optimization of pin fin heat sink by optimization ratios [2] and experimental and numerical investigation on different pin fin heat sink array [2, 3]. For Multi jet impingement Seo Young Kim Studied the effects of the aluminum heat sinks, the jet velocity, the jet-to-jet spacing and the nozzle plate-to-heated surface separation distance in a 3 3 square multi-jet impinging array on the averaged Nusselt number[4]. Koonlaya Kanokjaruvijit studied on eight-by-eight jet array impinging onto a staggered array of dimples at Reynolds number 11,500 was investigated by the transient wide band liquid crystal method [5]. The distance between the perforated plate and the target plate was adjusted to be 2, 4 and 8 jet diameters to examine its effect on the heat transfer performance [5]. When multiple jets are used, the local distribution of heat transfer coefficients changes depending on the number and spacing of jets in addition to the nozzle-to-target spacing, jet Reynolds number, and spent fluid exhaust. Huber and Viskanta [6] studied heat transfer in multiple air jet impingements on smooth surfaces. They showed that single jets resulted in lower local heat transfer coefficients than multiple jets, and attributed this degradation in heat transfer to jet editor@tjprc.org

2 14 A. J. More, D. S. Manakar & N. P. Bhone interactions prior to impingement (in the free-jet region); inter-jet interactions became less pronounced as the nozzle- to-target spacing was decreased. A decrease in jet-to-jet spacing from 8 to 4 was found to result in an increase in the average heat transfer coefficient; in fact, 4 has been suggested as an optimum spacing in several studies. N. K. Chougule G. V. Parishwad, Studied [11] Numerical investigation is carried out on multi air jet array of 3x3, 5x5 and 10x10 (nozzle diameter 5, 3 and 1.5mm) impinging on aluminum pin fin heat sink (pin fin array 4x4, 6x6 and 11x11 respectively) for H/d ratio of 3, 4 and 5 keeping the nozzle exit to pin fin tip distance 1d for each case. Numerical analysis carried out for the Reynolds number range from to for constant heat supply of 30W (Constant heat flux 8333W/m 2 ). N. K. Chougule, G.V. Parishwad studied numerical simulation of the 4x4 pin fin heat sink with single jet and 3x3 multi air jet impingement. Thermal performance of both single jet of diameter 15mm and 3x3 array multi air jets of 5mm diameter are evaluated in terms average Nusselt number (Nu avg ). Since the total flow area of the nozzle holes for the single jet impingement is the same as that for multi jet impingement, the square root of area is adopted as a characteristic length scale throughout the study. The Reynolds number is varied from 7000 to at Z/d =6, 8 and 10[12]. PROBLEM DESCRIPTION The schematic of a multi jet impinging on a pin fin heat sink which is to be analyzed is shown in Figure 1. The air jet is discharged through the round nozzle having length l and diameter d is directed normally towards the pin finned target plate with base 60 x 60 x 6mm, the pin fin are provided on top of plate on 50 x 50mm area, the sink is subjected to constant heat input (30W) from bottom and except top surface all other walls are adiabatic. Figure 1: Pin Fin Heat Sink and Nozzle Arrangement The material of the heat sink is Aluminum with thermal conductivity 230 W/mK. The jet after impingement on pin fin heat sink target surface as shown in Figure 1. EXPERIMENTAL SET UP AND APPARATUS A schematic diagram of the experimental set up is shown in Figure 2. It is an Air flow bench, which provides controlled and measurable flow of air through nozzles or jet plate It consists of a 0.5 HP blower, air straightener (air box), contraction section, and structure and Data Acquisition System (DAQ) to measure temperature, pressure. The experimental set up consists of pin fin heat sink, 100W capacity Impact Factor (JCC): Index Copernicus Value (ICV): 3.0

3 Experimental Study of High Heat Removal by Aluminum Pin Fin Heat Sink Using Multi-Jet Air Impingement 15 electric heater, 3 3 arrays nozzle jets assembly. The experimental set up includes the 4 4 array circular pin fin aluminum heat sink of size mm base of dimension pin fin heat sink. One 3 mm thickness, 100 W Capacity 60 60mm square electric heater was used to heat pin fin base. Due to the same base dimensions of the heat sink and electric heater same heat distribution at the base of the heat sink. Back side and 4 edges of the heater and pin fin heat sink covered with the asbestos 7 mm thickness insulator at the base and 3 mm at each edge of the heat sink and heater to less heat loss at the bottom side and edges. Air at ambient condition drawn into the variable speed generating blower, the air flow measured by the use of Pitot tube manometer difference. Air from the air box flow through air strengthener comes out to through the 3 3 nozzle exit. Air impinging on fin base (target plate) which contains heat source assembly. The nozzle to the target plate or fin base Spacing is maintained by use plastic spacers. Experiments Carried out for 3 different Z/D ratios says 6,8,10 for 2 different flow configurations unconfined, semi confined, all side confined at constant heat flux supply 30 W[2]. The average heat transfer coefficient for the heat sinks was calculated according to Newton s laws of cooling, h avg = Q total /A t (T bavg -T a ) (1) Total thermal resistances ( C/W) may be calculated from these definitions using: R th = (T bavg - T a )/Tt otal (2) Temperature measurement on the heat sink was at 6 different locations at the base and one at the pin fin tip. Due to use of 3 3 multi jet air impinging along the fin base temperature distribution is same due to the symmetry of the heat sink area, nozzle spacing, and constant heat flux supply from the base The thermocouples mounted at one corner of the heat sink equally spaced. For measurement of the temperature K-type thermocouple were used. Figure 2: Experimental Set up Pin-fin heat sinks provide a large surface area for the dissipation of heat and effectively reduce the thermal resistance of the package at the cost of higher pumping power. They often take less space and contribute less to the weight and cost of the product. RESULTS AND DISSCUSSIONS Experiments were performed for the Reynolds number range for the 2 flow configuration editor@tjprc.org

4 16 A. J. More, D. S. Manakar & N. P. Bhone Unconfined, Semiconfined cross flow arrangements with 3 Z/D ratios 6,8,10 fixed With constant heat flux supply 30 W[2,3]. The effects on the results of varying this spacing (Z/D Ratios) as well as the Reynolds Numbers were studied with the heat sink clamped on to the heat source. The 3 3 nozzle array with diameter of nozzle d=5mm used for experimentations. Pin fin heat sink used were 4 4 array with diameter of pin fin D= 5 mm height of fin H p = 25mm, pitch X p =Y p =15 mm [2,3].Experiments were performed for Reynolds numbers ranging from 6000 to with the nozzle-to-target plate spacing fixed at H p = 25 mm. It is interesting to note that for pin fin heat sink, the multi jet array nozzles produce the higher heat transfer coefficient for Z/D=6 using semi confined cross flow arrangement. UNCONFIMED CROSS FLOW ARRANGMENT Effect of Average Heat Transfer Coefficents in Unconfined Cross Flow Arrangment(UC) Figure 3: Effect of Nozzle Plate-To-Heated Surface Separation Distance on Heat Sink Average Heat Transfer Coefficient for Multiple Nozzles at Re = 6000 to There are two types of cross flow arrangement were studied viz unconfined cross flow arrangement (UC), partially cross flow arrangement (PC) Figure 4 Shows that effect of average heat transfer coefficient on different Reynolds number for flat plate and pin fin heat sink. So average heat transfer coefficient h W/m 2 K is function of Reynolds number. Figure 4.shows that unpinned heat sink, pin fin heat sink for unconfined cross flow arrangement and for three Z/D nozzle plate to the target heated d surface viz 6, 8, 10 Figure 4 Shows that average heat transfer coefficient increases linearly with Reynolds number for all Z/D ratios and heat sink. In the multiple jets flat plate heat sink yield higher convective coefficients than pin fin heat sink. For the unpinned heat sink the multi-jet impingement results in heat transfer coefficients that are 16 % higher than those pin fin heat sink. Because material volume of pinfin heat sinks is more than flat plate heat sink. Figure 5 Shows that total thermal resistance R th C/W for flat plate or unpinned and pin fin heat sink decreases linearly with increasing Reynolds number for unconfined cross flow arrangement(uc) for 3 different Z/D ratios. For moderate flow conditions, Re~10000 the effect of nozzle placement on cooling performance is important for both geometries. The minimum thermal resistance occurs when the nozzle is placed at a vertical height ranging at Z=6. With increased Reynolds number Re>9000, nozzle placement is seen to have little effect on cooling performance. The reduction in R th C/W with increased Re is attributed to enhancement in the heat transfer coefficient by the turbulence generated by the jet itself. As the jet mixes with the surrounding air, the turbulence intensity increases with the nozzle-to-heat sink vertical spacing until it reaches the maximum level. Impact Factor (JCC): Index Copernicus Value (ICV): 3.0

5 Experimental Study of High Heat Removal by Aluminum Pin Fin Heat Sink Using Multi-Jet Air Impingement 17 Figure 4: Effect of Nozzle Plate-To-Heated Surface Separation Distance on Heat Sink Total Average Thermal Resistance for Multiple Nozzles at Re = 6000 to With increased Reynolds number, the turbulence intensity is high and the effect of nozzle-to-heat sink spacing is reduced, It means that when Z/D is low turbulence intensity is increased thus heat transfer coefficient was increased so total thermal resistance R th C/W is reduced. Total thermal resistance is decreased from 6000 to in the range of 40 to 55%. SEMICONFINED CROSS FLOW ARRANGMENT Figure 5 shows that total thermal resistance R th C/W for Pin fin heat sink was decreased from 0.41 to 0.27 C/W. For the good design of heat sink, it should have less thermal resistance and moderate heat transfer rate, heat dissipation rate is more to the surrounding air so that heat sink temperature should be nearly equal to the atmosphere. Figure 5: Effect of Nozzle Plate-To-Heated Surface Separation Distance on Heat Sink Average Heat Transfer Coefficient H W/M 2 k for Multiple Nozzles Jets at Re = 6000 to editor@tjprc.org

6 18 A. J. More, D. S. Manakar & N. P. Bhone Figure 6: Effect of Nozzle Plate-To-Heated Surface Separation Distance on Heat Sink Total Average Thermal Resistance for Multiple Nozzles at Re = 6000 to CONCLUSIONS The following conclusions can be derived from the above results and discussion After studying the results experimental methods following points are concluded H/d ratio had a significant impact on heat transfer. At smaller H/d ratios with higher Re, there is vortex flow near the target surface in wall jet region, causing increase in heat transfer coefficient. At H/d=6, average base temperature is much lower, compared to other cases. The Reynolds number of the impinging jet plays an important role in the thermal resistance. Increasing the Reynolds number consistently diminishes the thermal resistance. Thermal resistance decreases with increasing Reynolds number. Nozzle plate to the target heated surface distance Z/D=6 was most good agreement for both unconfined and partially confined cross flow arrangement. 4)Experimental results of jet impingement on a flat plate heat sink and Pin fin heat sink revealed the fact that better heat transfer enhancement occurs for partially confined cross flow compared to unconfined cross flow arrangement was 4 to 5% more. NOMENCLATURE A t Total heat transfer area m 2, d D Z/D H n N Nozzle diameter, mm Pin fin diameter for the multi-jet impingement mm Nozzle plate-to-heated surface separation distance Fin height mm, Numbers of pin fin, number of nozzles Impact Factor (JCC): Index Copernicus Value (ICV): 3.0

7 Experimental Study of High Heat Removal by Aluminum Pin Fin Heat Sink Using Multi-Jet Air Impingement 19 Z Q Vertical distance from nozzle outlet to flat plate m Heat input to the heater W h avg Average heat transfer coefficient W/m 2 K. K Thermal conductivity of aluminum W/m K W or L Width or Length of heat sink mm.. Nu UC PC Nusselt number Unconfined Cross Flow Arrangement Semi Confined (Partially Cross Flow Arrangement. Re Nozzle Jet Reynolds number T bavg Ta Base average temperature for pin fin C. Supply air temperature at the nozzle C REFERENCES 1. Waqar Ahmed Khan, Modeling of Fluid Flow and Heat Transfer for Optimization of Pin-Fin Heat Sinks, ASME, Jim G. Maveety, Henry H. Jung, Heat Transfer From Square Pin-Fin Heat Sinks Using Air Impingement Cooling Vol. 25, no. 3, September H. H. Jung and J. G. Maveety, Pin-Fin Heat Sink Modeling And Characterization 2000 IEEE 4. Seo Young Kim, Myung Ho Lee, and Kwan-Soo Lee, Heat Removal by Aluminum-Foam Heat Sinks in a Multi-Air Jet Impingement, 2005 IEEE 5. Srinath V. Ekkad, David Kontrovitz, Jet impingement heat transfer on dimpled target surfaces, International Journal of Heat and Fluid Flow 23 (2002) A. M. Huber and R. Viskanta, Effect of jet-jet spacing on convective heat transfer to confined, impinging arrays of axisymmetric air jets, Int. J. Heat Mass Transf., vol. 37, pp , Zeinab S. Abdel-Rehim, Optimization and Thermal Performance Assessment of Pin-fin Heat Sinks, Journal of Applied Sciences Research, 3(3): , H. A. El-Sheikh,S V. Garimella, Heat Transfer from Pin-Fin Heat Sinks under Multiple Impinging Jets, IEEE Transactions On Advanced Packaging, Vol. 23, No. 1, February C.J. Kobus, T. Oshio, Development of a theoretical model for predicting the thermal performance characteristics of a vertical pin-fin array heat sink under combined forced and natural convection with impinging flow, International Journal of Heat and Mass Transfer 48 (2005) N. K. Chougule G. V. Parishwad A. R. Nadgire, Numerical Investigation of Multijet Air Impingement on Pin Fin Heat Sink with Effusion Slots, WCECS 2013, October, 2013, San Francisco, USA. editor@tjprc.org

8 20 A. J. More, D. S. Manakar & N. P. Bhone 11. N. K. Chougule, G.V. Parishwad, Numerical Analysis of Pin Fin Heat Sink with a Single and Multi Air Jet Impingement Condition, ISSN: (IJEIT)Volume 1, Issue 3, March Y. Kondo, H. Matsushima, Optimization of Pin-Fin Heat Sinks for Impingement Cooling of Electronic Packages, , Vol. 122, September 2000 by ASME. 13. Dae Hee Lee, Heat transfer enhancement by the perforated plate installed between an impinging jet and the target plate. International Communications in Heat and Mass Transfer 45(2002) Impact Factor (JCC): Index Copernicus Value (ICV): 3.0