Experimental investigation of heat transfer on square pin fins.
In this study, we focus on thermal characteristics of pin fin arrays in a long and narrow rectangular channel that simulates a channel of fan-assisted, computer cooling heat sink. Generally, a heat sink possesses a large number of channels and the through-flow is generated by an external blower or suction fan. It is apparent that numerous micro pin fins positioned on the surfaces of the heat sink would increase heat transfer area dramatically and could readily enhance cooling capability. Totally, four different configurations of inline pin finned channels were fabricated by Wire Electro Discharge Machining on a thin copper plate. Heat transfer characteristics of such pin fin arrays at two different flow rates in an air-flow channel are documented. The data and conclusions based upon them provide further insight toward improved design of enhanced heat transfer in mini-channels for applications such as electronics cooling system.
Taiho yeom et al.  studied heat transfer and pressure drop of micro pin fin arrays in a rectangular channel. The copper micro pin fin arrays were fabricated using LIGA photolithography process. The maximum heat transfer enhancement of 79% is achieved with the arrays of micro pin fins of 250 pm length and 400 pm diameter. The main contribution to the enhancement is in fluid dynamics effects, rather than with the area increases. Sparrow et al.  compared the heat transfer between inline and staggered pin fin arrays and found that a staggered array provides higher heat transfer but induces a larger pressure drop compared to an inline array. O.N.Sara  presented the heat transfer and friction characteristics and performance analysis of convective heat transfer through a rectangular channel with square cross section pin fins attached over a flat surface. The experimental results showed that the use of square cross-section pin fins may lead to an advantage on the basis of heat transfer enhancement. Wan and Joshi  conducted parametric study considering diameter, height, spacing and Reynolds number on square and circular micro pin fins. The results showed that circular micro pin fins has better thermal performance with same pumping power.
Prasher et al.  conducted experimental study on staggered, silicon based micro pin fin array under water flow. The micro pin fins were both circular and square shaped. The results represented the variation of heat transfer per unit pumping power. Moores and Joshi  studied the effect of tip clearance over micro pin fin surfaces with pin fins of 400 pm in diameter and different height to diameter ratios. The working fluid was water. The pin fin tip clearance was up to 25% of the pin height. The results showed that heat transfer increased when a small tip clearance is introduced since the pin tips provide more heat transfer area. However, increases of tip clearance resulted in decreasing heat transfer due to bypassing of the flow through the tip clearance region rather than through the bank of pin fins.
2. Experimental set-up:
An overall schematic of the heat transfer experimental set-up is shown in Fig. 2.1.
[FIGURE 2.1 OMITTED]
The test section is a narrow rectangular channel with a pin-fin array housed in the middle of the channel. An air flow, created by a blower, enters from the left side of the channel. The test section was designed to simulate a portion of the heat sink channel. The Heater is connected to the power supply through the Auto-transformer to reduce the current and voltage. All thermocouples in the experimental facility are connected to indicator. A detailed illustration of the test section is featured in Fig. 2.2
[FIGURE 2.2 OMITTED]
A long, rectangular copper block is utilized to create a heat flow path from a heater to the convectivelycooled surface. A cartridge heater is inserted at the end of the copper block providing heat input to the system. The other end of the copper block is a thin copper plate (test plate) with micro pin fins. The assembly is made so that it is possible to easily replace the thin micro pin-fin plates with other configurations for continued experiments. The channel is created using the micro pin-fin surface and plastic plates for the all other surfaces. The channel has adiabatic extensions upstream and downstream of the test section to give fully developed flow in the channel upstream of the pin-fin surface. The unheated channel surfaces and the heater block are enclosed by thick Terylene insulation to eliminate heat loss. Temperatures are measured 1.0 mm below the contact surface between the micro pin-fin plate and the copper block. From these temperature measurements, surface temperatures of the micro pin-fin surface are found by extrapolation and heat flux is computed. These are taken as the base temperatures of the micro pin-fins plate. One thermocouple measure flow inlet temperature and four thermocouples measures the base temperature . All the thermocouples are connected to the temperature indicator to indicate the digital values.
[FIGURE 2.3 OMITTED]
Fig.2.3 show the front view of the channel. The micro pin-fin plate is attached to the copper block using a thermal paste to minimize thermal contact resistance at the joint. The heated length (L), channel width (W), and channel height (Y) are 52 mm, 20 mm, and 6 mm, respectively. The plate has a base thickness of 3.0 mm
2.1. Fabrication of pin fin plates:
The copper plates are fabricated by using Wire cut - Electrical Discharge Machining. Even though other techniques like Photolithographic technique are available for high-volume batch production, Wire cut EDM is the simple technique for fabrication of pin fins. In this machining, the plane plate is machined in both longitudinal and transverse direction for a constant width of 2mm and giving a depth which is equal to the height of the pin fin. By this way fins having square cross-section is machined all over the surface with four (0.2mm, 0.4mm, 0.6mm, 0.8mm) different pin fin heights.
Four micro pin fin plates were fabricated and tested. All the micro pin fin arrays are of In-line configuration. The dimensions of the micro pin fin arrays are listed in Table 2.1.1, as well as their values of heat transfer area increase they offer. The pin fin plates having pin fin height of 0.2mm, 0.4mm, 0.6mm and 0.8mm are named as SQ 0.2, SQ 0.4, SQ 0.6 and SQ 0.8 respectively.
[FIGURE 2.1.1 OMITTED]
Four pin fin arrays each having different pin fin heights of 0.2mm, 0.4mm, 0.6mm, 0.8mm. All the cases of pin fin arrays have the same inter fin distance of 2mm. The area increase over the un-finned surface is calculated as follows:
Area increase (%) =
[(Area of finned surface - Area of plane surface)/Area of plane surface]*100
2.2. Formulae used:
Heat transfer experiments are done all the four pin finned copper plate along with the plane copper plate and the corresponding experimental heat transfer coefficients are calculated. Heat transfer coefficient (h) is computed as follows:
h = Q/[A.sup.s] ([T.sup.m] - [T.sup.a]) W/[m.sup.2]K
Where Q, [A.sup.s], ([T.sup.m] - [T.sup.a]) represents heat transfer from the cartridge heater, the convection surface area, mean temperature difference respectively.
Heat transfer the cartridge heater is calculated by measuring the voltage(V) and current(I) passing through the heater. The formula for the heat transfer from the heater is calculated as follows:
Q = V*I watts
The mean temperature difference is the temperature difference between the mean surface temperature and the ambient air temperature ([T.sup.a]). [T.sup.m] is calculated as the average temperature of four surface temperatures measured by the thermocouples inserted 1 mm below the surface of the plate at equal intervals. [T.sup.m] = ([T.sup.1] + [T.sup.2] + [T.sup.3] + [T.sup.4])/4
Where [T.sup.1], [T.sup.2], [T.sup.3], [T.sup.4] are all surface temperatures.
3. Experimental Results and Discussion:
Heat transfer coefficients for the pin fin arrays taken under two different flow rates at 10 LPM and 30 LPM are calculated for four groups of surfaces as classified according to pin fin height and a plane copper plate. The flow of air from the blower is controlled by the valve in the flow meter and it is maintained at the required flow rate (say 10 LPM or 30 LPM). Each plate is placed on the experimental set up with the help of heat sink paste and the air is passed at the required flow rate as well as the heater is switched ON. Then the temperature readings are taken with the help of temperature indicator. Thus the experiment is repeated with five different copper plates and two different flow rates and the readings are tabulated.
The following table represents the surface temperature of the five different copper plates and the corresponding heat transfer co-efficients at the flow rate of 10 LPM.
Input to heater: Voltage(V) = 70v, Current(I) = 0.08A
[FIGURE 3.1 OMITTED]
The following table represents the surface temperature of the five different copper plates and the corresponding heat transfer co-efficient at the flow rate of 30 LPM.
[FIGURE 3.2 OMITTED]
From the readings taken, for 10 LPM flow rate, the maximum heat transfer of 240.04 (W/[m.sup.2]K) was observed for the copper plate of SQ 0.8 and the minimum heat transfer of 187.18 (W/[m.sup.2]K) was observed for the plane copper plate.
For 30 LPM flow rate, the maximum heat transfer of 279.29 (W/[m.sup.2]K) was observed for the copper plate of SQ 0.6 and the minimum heat transfer of 205.07 (W/[m.sup.2]K) was observed for the plane copper plate.
In this paper, heat transfer characteristics of pin fin arrays in the rectangular channel were studied experimentally. The copper pin fin arrays were fabricated by Electrical Discharge Machining (EDM). Heat transfer experiments were conducted with two different flow rates of air (10LPM and 30LPM). In this set of experiments, the maximum heat transfer co-efficient of 279.29 (W/[m.sup.2]K) is achieved with the pin fin plate SQ 0. 6 at the flow rate of 30 LPM. Even though the heat transfer increases with the increase in surface area, the main contribution to the heat transfer is air flow rate. From this experiment we came to a conclusion that when the flow rate is increased, it is possible to achieve maximum heat transfer with the short pin fins.
[1.] Taiho Yeom, Terrence Simon, Tao Zhang, Min Zhang, Mark North, Tianhong Cui, 2016. Enhanced heat transfer of heat sink channels with micro pin fin roughened walls, International Journal of Heat and Mass Transfer, 92: 617-627.
[2.] Sparrow, E.M., J.W. Ramsey, 1978. Heat transfer and pressure drop for a staggered wall-attached array of cylinders with tip clearance, International Journal of Heat Mass Transfer, 21(11): 1369-1378.
[3.] Sara, O.N., 2003. Performance analysis of rectangular ducts with staggered square pin fins, Journal of Energy Conversion and Management, 44(11): 1787-1803.
[4.] Wan, Z., Y. Joshi, 2014. Pressure drop and heat transfer characteristics of square pin fin enhanced microgaps in single phase microfluidic cooling, 14th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITHERM), pp: 649-657.
[5.] Prasher, R.S., J. Dirner, J.-Y. Chang, A. Myers, D. Chau, D. He, S. Prstic, 2007. Nusselt number and friction factor of staggered arrays of low aspect ratio micropinfins under cross flow for water as fluid, Journal of Heat Transfer, 129(2): 141-153.
[6.] Moores, K.A., Y.K. Joshi, 2003. Effect of tip clearance on the thermal and hydrodynamic performance of a shrouded pin fin array, Journal of Heat Transfer, 125(6): 999-1006.
(1) T. Nithyanandham, (2) S. Srigopalakrishnan, (3) T. Thiyagarajaperumal, (4) S. Vadivel and 5R. Shibi
(1) Asst. Professor, Department of Mechanical Engineering, M.Kumarasamy College of Engineering, India.
(2,3,4,5) U.G.Students, Department of Mechanical Engineering, M.Kumarasamy College of Engineering, India.
Received 25 April 2016; Accepted 28 May 2016; Available 5 June 2016
Address For Correspondence: T. Nithyanandham, Asst. Professor, Department of Mechanical Engineering, M.Kumarasamy College of Engineering, India.
Table 2.1.1: Dimensions of pin fins Test plates Height of each Distance Surface area Fin (mm) between each increase in Fin (mm) percentage (%) SQ 0.2 0.2 2 6.15 SQ 0.4 0.4 2 12.30 SQ 0.6 0.6 2 18.46 SQ 0.8 0.8 2 24.61 Plane plate Table 3.1: Heat transfer co-efficient at the flow rate of 10 LPM Test plates Ambient Surface temperature ([degrees]C) temperature ([degrees]C) [T.sub.a] [T.sub.1] [T.sub.2] Plane plate 32.8 51.4 51.3 SQ 0.2 32.8 55.3 56.6 SQ 0.4 32.9 54.4 54 SQ 0.6 33 50.7 49.4 SQ 0.8 32.7 55.3 53.1 Test plates Surface temperature ([degrees]C) Heat transfer co-efficient [T.sub.3] [T.sub.4] (W/[m.sup.2]K) Plane plate 50.5 48.6 205.07 SQ 0.2 54.9 53 229 SQ 0.4 52 52.8 235.025 SQ 0.6 48 49 279.29 SQ 0.8 50.7 52.6 213.65 Table 3.2: Heat transfer co-efficient at the flow rate of 30 LPM Surface temperature ([degrees]C) Ambient temperature ([degrees]C) Test plates [T.sub.a] [T.sub.1] [T.sub.2] Plane plate 32.8 51.4 51.3 SQ 0.2 32.8 55.3 56.6 SQ 0.4 32.9 54.4 54 SQ 0.6 33 50.7 49.4 SQ 0.8 32.7 55.3 53.1 Surface temperature ([degrees]C) Heat transfer co-efficient Test plates [T.sub.3] [T.sub.4] (W/[m.sup.2]K) Plane plate 50.5 48.6 205.07 SQ 0.2 54.9 53 229 SQ 0.4 52 52.8 235.025 SQ 0.6 48 49 279.29 SQ 0.8 50.7 52.6 213.65 Input to heater: Voltage(V) = 70v, Current(I) = 0.08A