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How Air Velocity Affects the Thermal Performance of Heat Sinks: a Comparison of Straight Fin, Pin Fin and Maxiflowtm Architectures

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How Air Velocity Affects the Thermal Performance of Heat Sinks: a Comparison of Straight Fin, Pin Fin and Maxiflowtm Architectures How Air Velocity Affects The Thermal Performance of Heat Sinks: A Comparison of Straight Fin, Pin Fin and maxiFLOWTM Architectures Introduction A device’s temperature affects its op- erational performance and lifetime. To Air, Ta achieve a desired device temperature, Heat sink the heat dissipated by the device must be transferred along some path to the Rhs environment [1]. The most common method for transferring this heat is by finned metal devices, otherwise known Rsp as heat sinks. Base Resistance to heat transfer is called TIM thermal resistance. The thermal resis- RTIM tance of a heat sink decreases with . Component case, Tc Q Component more heat transfer area. However, be- cause device and equipment sizes are decreasing, heat sink sizes are also Figure 1. Exploded View of a Straight Fin Heat Sink with Corresponding Thermal Resis- tance Diagram. growing smaller. On the other hand, . device heat dissipation is increasing. heat sink selection criteria. Q RTIM, as shown in Equation 2. Therefore, designing a heat transfer The heat transfer rate of a heat sink, , . path in a limited space that minimizes depends on the difference between the Q = (Thot - Tcold)/Rt = (Tc - Ta)/Rt (1) thermal resistance is critical to the ef- component case temperature, Tc, and fective design of electronic equipment. the air temperature, Ta, along with the where total thermal resistance, Rt. This rela- This article discusses the effects of air tionship is shown in Equation 1. For Rt = RTIM + Rsp + Rhs (2) flow velocity on the experimentally de- a basic heat sink design, as shown in termined thermal resistance of different Figure 1, the total thermal resistance Therefore, to compare different heat heat sink designs. To be able to com- depends on the sum of the heat sink sink designs, the thermal interface re- pare these designs, we need to first resistance, Rhs, the spreading resis- sistance, RTIM, and the spreading resis- review basic heat transfer theory as tance in the heat sink base, Rsp, and tance, Rsp, was similar among the heat applied to heat sinks. Previously pub- the thermal interface resistance from sinks tested. lished work is discussed, along with the component to the heat sink base, 12 For this study, the same thermal in- on the steady state temperature of the tests. terface material (TIM) was used with heat sinks, unlike Lasance and Egg- 2. The base thickness, width, and all heat sinks. This minimized the dif- ink, whose test are based on transient length were the same for all the ference in the thermal interface resis- data. The maxiFLOWTM heat sinks heat sinks tested. tance, RTIM, between heat sink tests. As were found to have the lowest thermal 3. The same size heat source area is normal, the spreading resistance of a resistance, especially for air flow veloc- was used for the different heat sink heat sink’s base, Rsp, increased with de- ity below 2 m/s [4]. tests. creasing base thickness and conductiv- 4. The heat sinks had the same fin ity. It also increased with an increasing The heat sinks tests in this article were height. difference in the heat sink base area conducted in a wind tunnel and the and the heat dissipation area [2]. data points were taken at steady state About the Heat Sinks Tested values. The heat sinks were selected Heat sinks with 42 x 42 mm and with Lasance and Eggink have provided a based on the following heat sink selec- 27 x 27 mm base nominal dimensions method to rank a heat sink for certain tion criteria: were selected, as they are common applications [3]. This measurement is sizes used in electronics cooling. Fin based on extracting the average heat 1. The same thermal interface mate- types included straight fin, pin fin, transfer coefficients from time-depen- rial was used for all the heat sink elliptical fin and ATS’ maxiFLOW, as dant temperature curves as a function of velocity and bypass. The measured Table 1. Types of Heat Sinks Tested, With Their Dimensions and Mass. effective heat transfer coefficient is L w H Mass, then scaled by mass, volume, weight # Heat Sink Type Part No. [mm] [mm] [mm] m (g) or height. This provides several per- formance metrics to give designers a 1 Cylindrical Pin Fin (CPF) 42 42 24 CPF4224 32 novel way to rank heat sinks in con- ditions that resemble an application. 2 Elliptical Pin Fin (EPF) 42.5 42.5 33 EPF4233 36 Heat sinks with heights between 5 and 20 mm were tested [3]. In addition to 3 Cylindrical Pin Fin (CPF) 25 25 25 CPF2525 12 varying heights, the heat sink base TM thickness varied from 1.2 to 3 mm. The 4 maxiFLOW 42.5 42.5 17.5 MF4217 28 base width and length varied from 42.2 TM to 49.8 mm. The ratio between the width 5 maxiFLOW 25 25 17.5 MF2517 12 and length of the heat sink also varied, i.e. both square and rectangular base 6 Elliptical Pin Fin (EPF) 25 25 33 EPF2533 12 shapes were used. ATS maxiFLOWTM and pin fin heat sinks were tested, with 7 Straight Fin (SF) 42.5 42.5 17.5 SF4217 18 the maxiFLOWTM series showing the 8 Straight Fin (SF) 27 27 17.5 SF2517 10 lowest thermal resistance [3]. 9 Elliptical Pin Fin (EPF) 42 42 17 EPF4217 24 Forghan, et.al. also discuss the thermal resistance of various heat sink designs 10 Elliptical Pin Fin (EPF) 25 25 17 EPF2517 18 [4]. They conducted their tests based October 2008 |Qpedia 13 THERMAL ANALYSIS (a) (b) (c) (d) Figure 2. Various Heat Sinks Types Tested: Straight Fin (a), Elliptical Pin Fin (b), Cylindrical Pin Fin (c), and ATS’ maxiFLOW™ (d). shown in Table 1 along with dimensions, part numbers and mass. Figure 2 shows some of the selected heat sinks. Wind tunnel fans For testing purposes, a thermocouple hole was drilled into the center of the base of each heat sink, on the side per- pendicular to the air flow direction in which each was tested. The position of a thermocouple hole can be seen in Figure 3. Power supplies Air flow Wind tunnel direction test section (CWT-106) H Data w logger Thermocouple L hole Figure 3. Illustration of a Straight Fin Heat Sink. Experimental Set-Up of the Heat Sinks in a Wind Tunnel WTC-100 The natural and forced convection thermal resistances of Wind Tunnel Controlller each heat sink were determined in an ATS CWT-106™ open PC loop wind tunnel managed by a WTC-100 wind tunnel con- Figure 4. The ATS CWT-106 Open Loop Wind Tunnel and WTC- Inlet troller (Figures 4, 5, 6). Heater pads of 25 x 25 mm were 100 Wind Tunnel Controller. placed in the center of the heat sinks to provide the heat sources. The heat sinks were mounted in the wind tunnel upstream of the heat sink. Thermal paste was applied before test section. Air flow and temperature were measured 15 cm the thermocouple was inserted into the hole. Test Procedure After being set-up for testing as de- Air flow velocity direction Wind tunnel test scribed in Section 2, heat dissipated by section back wall the heater pad was transferred to the Wind tunnel test Thermocouple wires air by natural convection. The heat sink section lid base temperature and inlet air temper- Selotape ature were monitored until steady tem- Heat sink Chomerics T405 perature state occurred. At this time, the heat sink base and inlet air temper- Gravity Heater pad atures were recorded, along with the Insulating board voltage supplied to each heater pad. Heater pad wires PCB test piece For forced convection, steady state val- 152 52 ues were recorded at a range of air flow Figure 5. Illustration of the Experiment Set-up Inside the Wind Tunnel Test Section. velocities. Power input for the 42 mm heat sinks was 12 W, while for the 27 mm heat sinks the power ranged from 6 to 9 W. Processed Results The heat sinks were compared based on their thermal resistances. The heat transfer rate from heat sink base to air is given by Equation 3 (below). From here, the heat sink’s thermal resistance is calculated using Equation 4. This as- sumes that the heat transferred by the sink to the air is equal to the power in- put to the heat source. After applying Equation 4 to the experimental data, the results are plotted in Figures 7 and 8. Q = (Tb - Ta)/R (3) R = (Tb - Ta)/P (4) Figure 6. The Wind Tunnel Test Section. Heat Sinks EPF4233 and CPF4225 are Mounted Figure 7 shows that heat sink thermal Side by Side. resistance decreases for natural con- vection. The maxiFLOWTM heat sinks had a higher thermal resistance than drical pin fin heat sinks had about the had the lowest thermal resistance for the maxiFLOW heat sink, and the 17 same thermal resistance as the straight natural convection of the sinks tested. mm elliptical pin fins had the highest fin heat sinks. If it had been possible to The 33 mm elliptical pin fin heat sinks thermal resistance. The 25 mm cylin- cut the cylindrical pin fin heat sinks to October 2008 |Qpedia 15 THERMAL ANALYSIS 30 16 ] 14 [K/W] 25 R 12 20 CPF2525 10 CPF4224 EPF2533 EPF4233 15 MF2517 8 MF4217 SF2517 SF4217 EPF2517 6 EPF4217 10 4 5 2 Heat sink thermal resistance, R [K/W Heat sink thermal resistance, 0 0 0 0.5 1 1.5 2 2.5 3 3.5 0 1 2 3 4 5 6 7 8 9 Input power, P [W] Input power, P [W] Figure 7.
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