Experimental Research on the Heat Transfer and Flow Performance of a Composite Heat Sink

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1 Experimental Research on the Heat Transfer and Flow Performance of a Composite Heat Sink Yu Xiaoling, Wang Qianlong, Feng Quanke School of Energy and Power Engineering, Xi an Jiaotong University, Xi an , China Phone: Fax: xlingyu@mailst.xjtu.edu.cn Abstract In this paper, a new plate-fin heat sink with staggered pins is proposed in order to satisfy requirements of air-cooled heat sinks used in electronic equipment. By staggering two rows of circular pins at proper places in every tunnel between two plate fins of the plate-fin heat sink, this composite heat sink has some advantages of both plate-fin and staggered-pin fin heat sinks. Because pins disturb the flow in tunnels, the heat dissipation effect of this heat sink is much better than that of the plate-fin heat sink. Results of experiments show that at the same heat dissipation rate and wind velocity, the thermal resistance of the composite heat sink is 10%~20% lower than that of the plate-fin heat sink. When the wind velocity is 2 m/s, its flow resistance is about 10% higher than that of plate-fin heat sink, but is much lower than the flow resistance of the staggeredpin fin heat sink of comparative size. Key words Composite heat sink, thermal resistance, flow resistance, pressure drop 1. Introduction Plate-fin heat sinks are widely used in electronic equipment cooling because they have many advantages, such as easy machining, simple structure, low flow resistance and low cost. However, they have lower heat transfer effect compared to other air-cooled heat sinks. The main reason is that the flow pattern through the straight tunnels of the heat sink is mainly laminar flow, with a thick laminar sub-layer on the wall of the heat sinks that have a large thermal resistance. The International Journal of Microcircuits and Electronic Packaging, Volume 25, Number 3, 3rd Quarter, 2002 (ISSN ) 279

2 Staggered-pin fin heat sinks are not widely used in electronic equipments as plate-fin heat sinks because they are difficult to machine and are more expensive. However, they have a higher heat transfer effect than plate-fin heat sinks because staggered pins destroy the laminar sub-layer on the wall. The subsequent airflow, passing through alternately contractive and outstretched tunnels, is more turbulent than through straight tunnels. It was assumed that optimum staggered-pin fin heat sinks offer the best heat dissipation capacity to weight ratio of any common heat sink structure. However, they are also acknowledged as offering relatively high flow resistance [1].Models of the thermal and hydraulic performance of plate-fin and pin-fin heat sinks are presented in Ref. [2]. According to fluid mechanics and heat transfer, if two types of heat sinks are combined and pins are inserted in the proper locations of the tunnels in the plate-fin heat sink, the heat transfer effect of the plate-fin heat sink will be enhanced. Furthermore, the flow is restricted in straight tunnels and the abruption and eddies only occur near pins, so the flow resistance will be lower than that through the staggered-pin fin heat sink. In this paper, the flow performance of uniform flow passing a circular cylinder is analyzed first to determine the optimum pin placement. Then, experiments in a wind tunnel are performed on this composite heat sink and the above two heat sinks. Final results of experiments indicate clearly the improved heat transfer and flow performance of the composite heat sink. 2 Determination of pin placement Both flow and heat transfer performances of the composite heat sink are affected greatly by pin placement in the tunnels of the plate-fin heat sink. The function of the pins is to destroy the laminar sub-layer on the wall and increase the flow turbulence. Therefore, two sides of the plate fins should be in the turbulence area caused by pins. The laminar flow area should be as small as possible. This is the rule that was used to determine the pin placement. Optimum pin placement can be determined by analyzing the turbulence area caused by the flow passing through a cylinder. Many scholars have studied the performance of circumcolumnar flow whose flow pattern is decided by the Reynolds number (Re) based on pin diameter. For Re < 5, flow abruption does not occur. For 5 < Re < 40, a pair of steady eddies appear after the cylinder. For 40 < Re < 150, the eddies expand. Then one eddy begins to break off followed by the other one. New eddies are generated behind the cylinder. Finally two rows of eddies that swing and interlace periodically are developed. This is the well-known Karmen vortex street. For Re < 150, the vortex street is laminar flow. For 150 < Re < 300, it changes from laminar flow to turbulent flow, and for 300 < Re < , the flow is called sub-critical flow. The flow pattern in the boundary layer on the surface of the cylinder is laminar flow, but that of the vortex street after the cylinder is transformed to turbulent flow completely and more vortexes are formed periodically in that vicinity. For < Re < , the flow is called transitional flow. At this point, he flow pattern in the boundary layer on the surface of the cylinder is changed to turbulent flow, the abruption point moves backward, the flow resistance decreases distinctly, vortexes are formed irregularly and the forming frequency is at random. For Re > , the flow is called supercritical flow, and the vortex forming frequency reverts to the regular pattern. The heat sinks mentioned in this paper are used in electronic equipment cooling with the 280

3 flow passing through the heat sink at the subcritical point. Yaling Wang, et al have performed a three-dimensional numerical simulation to the flow past a cylinder with Re equal to 1000 [3]. The simulation result is shown in Figure 1. Wei Zhang, et al, used PIV (Particle Image Velocimetry) to measure the main area of flow past a cylinder with Re equal to 500 [4]. The measurement result is shown in Figure 2. Figure 1 and Figure 2 show that vortexes appear near two sides and backward position of the cylinder and mainly concentrate in a strip. The strip is about 3 D (D is the cylinder diameter) wide and about 10 D long. In order to make two sides of plate fins located in the turbulent area, the nearest distance between pins centers and plate fin walls should not be more than 1.5 D, and the center distance of two neighbor pins should not be more than 10 D in the flow direction. 3. Experimental study of heat transfer and flow performance to the three heat sinks According to above analysis, two rows of pins are staggered in the tunnel of the plate-fin heat sink, the nearest distance between pins centers and plate fins is 1.5 D, and the center distance of two neighbor pins is 10 D. The arrangement is shown in Figure 5 (c). Experiments in a wind tunnel are performed on the above three heat sinks to study duct flow performance and heat transfer performance of them. By comparing the thermal and flow resistance of them, the heat transfer and flow performance of the composite heat sink is obtained. 3.1 Experimental setup The wind tunnel is used to simulate forced convection through heat sinks. Figure 3 is a side view of the experimental equipment. The cross-section of the wind tunnel is rectangular (200 mm x 150 mm). The length of the wind tunnel is 1200 mm. The length of the heat sinks used in experiments is 86 mm. Compared with the wind tunnel, the volume of heat sinks is sufficiently small such that their influence on the flow field of the wind tunnel can be neglected. The heat sink is mounted at the bottom of the wind tunnel in such a way that the fin base stands out of the tunnel wall (Figure 4). The tested heat sinks are milled from solid aluminum (thermal conductivity 200 W/m- o C). Their configurations and structure parameters are shown in Figure 5 and Table 1. Heat sinks are heated by electrical heated foil. The heating power is 60W. The bottom of the heated foil is insulated with 80 mm of thick Polystyrene (thermal conductivity W/mo C). It is assumed that all heat is dissipated by the heat sink. The average temperature of the heat sink base is determined by averaging the readings of four symmetrically positioned thermocouples placed at each corner of the heat sink base, 10 mm from the edges. Two pressure taps (diameter 8 mm) are positioned 50 mm upstream and downstream from the heat sink respectively. The taps are connected to Autotran 700D pressure transducers. Three different transducers are used with a full scale of 25, 62 and 150 Pa depending on the pressure drop. The wind velocity of the fan is changed by altering the current through the fan motor and calculated from the flux measured by the orifice meter. 281

4 Figure 1. Streamline at the H/2 cross-section of the cylinder, Re = Y/D X/D Figure 2. Velocity field behind the cylinder, Re =

5 Heat Sink Fan Pressure Taps Orifice Meter Figure 3. Side view of experimental equipment Bottom of wind tunnel Electrically heated foil Heat sink Thermocouple Insulation Figure 4. Vertical cut view of heat sink, heated foil and insulation 283

6 (a) Plate-fin (b) Staggered-pin fin (c) Composite Figure 5. Configuration of the three heat sinks used in the experiments Table 1. Geometrical parameters of heat sinks (mm) Heat sink Fin length (L) Fin height (H) Fin thickness (t)/ Pin diameter (D) Fin space / transverse center distance of pins(s) Longitudinal center distance of pins (δ) Plate-fin Staggered pin Composite [2]

7 3.2 Results and discussion The heat transfer coefficient, h, is calculated as [5]: P h = Ahs ( t hs t in ) W/(m2 ok) (1) P = Heat dissipation rate, W t in = Temperature at the wind tunnel inlet, o K A hs = Area of the heat sink, m 2 t = Average temperature of the heat sink hs base, o K The thermal resistance, R th, can be defined by: 1 Rth = o h A K/W (2) hs The thermal resistance and pressure drop curves of the three heat sinks are shown in Figure 6 and Figure 7 respectively. Although the pin placement is not optimum, Figure 6 and Figure 7 indicate that the composite heat sink has high heat transfer and flow performance. Figure 6 indicates that, at the same velocity, the thermal resistance of the plate-fin heat sink is the largest while that of the composite heat sink is the smallest and is about 10% - 20% lower than that of the plate-fin heat sink. Figure 6 also shows that the thermal resistance of all the three heat sinks will decrease as the wind velocity increases, and they decrease more sharply at low wind velocity (velocity lower than 2m/s). When the wind velocity increases, the reducing trend of the thermal resistance becomes smoother. Figure 7 shows that pressure drops of all the three heat sinks are rising as the wind velocity increases. The pressure drop of the staggered-pin fin heat sink is sharpest with respect to the wind velocity increment. It also can be found in Figure 7 that the pressure drop of the plate-fin heat sink is least while that of the staggered-pin fin heat sink is the largest. The pressure drop of the composite heat sink is about 10% higher than that of the plate-fin heat sink when the wind velocity is 2 m/s. As the wind velocity increases, the disturbance caused by the pins is more violent, so the pressure drop of the composite heat sink becomes even larger than that of the plate-fin heat sink, reaching to a factor of two larger at 6m/s, but is still much lower than that of the staggered-pin fin heat sink. In the case of air-cooling of small electrical equipments, the wind velocity is usually lower than 5 m/s. In this region, compared to the plate-fin and staggered-pin heat sinks, the composite heat sink has a much better heat transfer performance. Therefore, this new kind of heat sink has high practical value in the future. 4. Conclusions 1. A new plate-fin heat sink with staggered pins is introduced in this paper. Compared to the traditional plate-fin and staggered-pin fin heat sinks, the composite heat sink has the advantages of good heat transfer effect and low flow resistance. 2.The nearest distance between pins centers and plate fin walls should not be more than 1.5 D. The distance between centers of neighboring pins should not be more than 10 D in the flow direction. 3.At the same wind velocity and heat dissipation rate, the thermal resistance of the plate-fin heat sink is the largest, while that of the composite heat sink is the smallest, about 10% 20 % lower than that of the plate-fin heat sink; The pressure drop of the plate-fin heat sink is the smallest while that of the 285

8 Thermal resistance stance (K/ ( o K/W) Plate-fi n St aggered- pi n Compos i t e Wind d velocity l ((m/s) / ) Figure 6. Thermal resistance of three heat sinks at different wind velocities Pressure drop (N/m 2 ) Pr e s s ur e dr op Pa Plate-fin St agger ed-pi n Compo s i t e Wind velocity (m/s) Figure 7. Pressure drop of three heat sinks at different wind velocities 286

9 staggered-pin fin heat sink is the largest. When the wind velocity is 2 m/s, the pressure drop of the composite heat sink is about 10% higher than that of the plate-fin heat sink. With the wind velocity is increasing, the pressure drop of the composite heat sink reaches to twice that of the pressure drop of the plate-fin heat sink, but it is still much lower than that of the staggered-pin fin heat sink. References [1] Trevor Zapach, Todd Newhouse, Jeff Taylor, Peter Thomasing, Experimental Verification of Pin Fin Heat Sinks, Proceedings of the Seventh Intersociety Conference on Thermal and Thermo-mechanical Phenomena in Electronic Systems, [2] Hans Jonsson, Bahram Moshfegh, Modeling of the Thermal and Hydraulic Performance of Plate fin, Strip Fin, and Pin Fin heat Sinks- Influence of Flow Bypass, Proceedings of the Seventh Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, Biography Yu Xiaoling was born in She received her bachelor s degree in fluid mechanics from Xi an Jiaotong University, China, in She is now pursuing a doctorate in fluid mechanics from the same University. Her research interest is power and electronic device cooling. [3] Yaling Wang, Yingzhong Liu, Guoping Miao, Three-dimensional Numerical Simulation of Viscous Flow around Circular Cylinder. Journal of Shanghai Jiaotong University,Vol. 35 No. 10, Oct [4] Wei Zhang-Yuan Wang, Zhong Xu, Wen Jin, Application of Particle Image Velocimetry to Measurement of Poisoeullie Flow and Flow around a Circular Cylinder. Journal of Xi an Jiaotong University, Vol. 36, No. 3, Mar [5] Jerry E. Sergent, Al Krum, Thermal Management Handbook for Electronic Assemblies, McGraw-Hill,