Enhancement of Heat Transfer from Plate Fin Heat Sinks

Transcription

1 International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN ISSN Enhancement of Heat Transfer from Plate Fin Heat Sinks #1 S.D.Ratnakar, #2 D.D.PALANDE 1 swapsratnakar@yahoo.co.in 2 palande1@rediffmail.com #1 Mechanical Department, M.C.O.E&RC, Nasik, India #2 Mechanical Department, M.C.O.E&RC, Nasik, India ABSTRACT Heat Sinks are an extremely useful component used to lower the maximum temperature of various electronic devices during operation so as to increase their thermal efficiency and performance. Fins constitute an important and integral component of sinks. It is a passive cooling technique. Plate fin heat sinks are used in varied applications owing to its low manufacturing cost, ease of manufacture and its economical way to dissipate unwanted heat. Steady state natural convection is experimentally investigated for 6 sets of vertically mounted fin heat sinks. Aluminum is used because of its high conductivity. Length and fin thickness is kept constant at 2 mm and 5 mm respectively. Fin height is successively increased as 1mm, 3mm, 5mm.For the second set length is taken as 1mm. Height is varied in the range 5mm,15mm and 25mm.Aspect Ratio for above sets is thus.5,.15 and.25 respectively. Effect of varying height, heat input and aspect ratio, keeping length constant is investigated on heat transfer through the sinks. ARTICLE INFO Article History Received :18 th November 215 Received in revised form : 19 th November 215 Accepted : 21 st November, 215 Published online : 22 nd November 215 Keywords Heat Sinks, Fins, Natural Convection, Aspect Ratio. I. INTRODUCTION Heat sink is an object that absorbs and dissipates heat from another object by thermal contact. Heat sinks find use in wide range of applications, providing efficient heat dissipation. Some applications include cooling of electronic devices, lasers, heat engines and refrigeration. Thermal energy transfer between two objects rapidly brings first object into thermal equilibrium with second lowering temperature of first object thus accomplishing heat sinks role as a cooling device. If temperature limits exceed a certain value in above applications it may even lead to total system failure. Therefore different methods are used by engineering systems so as to minimize this overheating problem as much as possible. Fins form one of the easiest and cheapest ways to dissipate this heat. Owing to low production costs and high effectiveness, rectangular fins are the most popular fin type. Vertical orientation of fins is widely used from a combination of horizontal and vertical orientation as it is highly effective. Heat dissipation from fins to surroundings takes place by two modes convection and radiation. As aluminium is used for fins, it has low emissivity value hence radiation heat transfer value is low. So convection heat transfer is dominant mode of heat transfer from fins. Heat transfer coefficient and surface area of fins are two important parameters on which rate of heat dissipation depends. Fluid can be forced to flow over fins by fans and in this manner heat transfer coefficient h can be increased. But it is costly and requires more volume to include fan. Surface area can be increased by addition of more fins. But distance between adjacent fins reduces due to addition of more fins. This may result in offering resistance to air flow thereby reducing heat transfer coefficient. II. EXPERIMENTATON Main components include channel, concrete block, base plate, fin array, plate heater and power mains. Aerated concrete block 25x2x1 mm, is fixed on frame which ensures only one dimensional heat dissipation. A removable acrylic sheet is placed on front surface to replace fin arrays. Base thickness of arrays is kept as 5mm.The heater consists of nichrome wire wound around

2 International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN thin mica plate and mica sheet. Rating of heater plate is 25W and 23 V, AC.5 mm depth is provided on aerated concrete block to include heater plate. Extruded surface is kept over fin array for fitting to aerated concrete block. Concrete block has in-built 4 bolts to tighten fin array over heater plate Thus air gap between fin array and heater plate is considered to be negligible. A concrete block has high insulation quality and high temperature resistance. (K.15 W/m k) TABLE I SET-UP DIMENSIONS Sr.No Component Dimensions 1 Frame Channel 55x55x55 2 Heater Concrete 25x2x1 Fig.3 Set-up of indicator panel. Fin configurations are produced on D.R.O milling. The fin arrays are produced from rectangular bars with dimensions 18x2x6 mm. Fins are integral with base plate of thickness 5mm and fin thickness is kept constant at 5mm. Fig.1 erimental Set-up Fig.4 Photograph of Fin Arrays The fin array specifications are given in the following table Fig.2 The sides view of modeled set-up Block No. Fin Length TABLE II FIN SPECIFICATIONS Fin Fin Width Thickness Fin Height No. of Fins

3 International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN Fin Length TABLE III FIN SPECIFICATIONS Fin Width Fin Optimum Fin Height Spacing Aspect Ratio III. EQUTATIONS (1) Heat Loss by Convection, : = (heater input) (heat loss by radiation, Qr) = h A ΔT. Qr = ε σ A t (T avg 4 T amb 4 ) (2)Average Heat Transfer Coefficient. h avg = / (A ΔT) (3)Mean Film Temperature: Tf = (T avg + T amb )/2 (4)erimental Nusselt Number: Nu exp = (h avg L)/K Ra = {gβl 3 (T avg T amb )} / (αν). IV. EXPERIMENTAL PROCEDURE Heat inputs can be adjusted by a dimmer stat. The temperature of heat sink at different locations and ambient temperatures are recorded at time interval of 3 minutes till steady state is reached. Generally it takes around 2 hrs to attain steady state condition. Temperature variation of around.5 C is taken for steady state approximation. Six thermocouples are used. Five of them are attached to the base and one is kept suspended inside the channel to record ambient temperature. Parameters used for study are as follows: Heat Input (Qin) :2W, 4W, 6W, 8W, 1W Base Temperature : T 1, T 2, T 3, T 4, T 5 Ambient Temperature : T 6 OBSERVATIONS TABLE IV L=1MM, W=18MM, H=5MM Sr. Q T1 T2 T3 T4 T5 T6 No. watt TABLE V L=1MM, W=18MM, H=15MM Sr. Q T1 T2 T3 T4 T5 T6 No. watts TABLE VI L=1MM, W=18MM, H=25MM Sr.No. Q T1 T2 T3 T4 T5 T6 watts TABLE VII L=2MM, W=18MM, H=1MM Sr.No. Q T1 T2 T3 T4 T5 T6 watts TABLE VIII L=2MM, W=18MM, H=3MM Sr.No. Q T1 T2 T3 T4 T5 T6 watts TABLE IX L=2MM, W=18MM, H=5MM Sr.No. Q T1 T2 T3 T4 T5 T6 watts V.EXPERICAL CORELATIONS Empirical relations are used to validate the vertical orientation model. Following correlations are considered: (1)Mc Adam s correlation: (2)Churchill and Chu s first correlation:

4 Nusselt No. Nusselt No. Nusselt No. Nusselt No. Nusselt No. International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN [ [ ( ) ] ] 4 2 McAdams Chur&Chu I (3)Churchill and Chu s Second correlation: Chur&Chu II Chur&Usagi [ ( ) ] (4)Churchill and Usagi s correlation: Fig.7Set C (2, 5) 5 [ ( ) ] VI.RESULT ANALYSIS A.Variation of Nusselt Number with Different Figure 5-1 shows variation of Nusselt No. with different power input for fin arrays. Value of Nu number from experimentation is close to that of from existing equations. Three sets are plotted for 1, 3, 5 mm height and length of 2mm.Also remaining three sets are for 1 mm length and 5mm, 15mm, 25mm height. We observe that as the power input increases the Nusselt number increases Fig.8 Set A (1, 5) McAdams Chur&Chu I Chur&Chu II Chur&Usagi McAdams Chur &Chu I Chur &Chu II Chur &Usagi Fig.9 Set B (1, 15) Fig.5 Set A (2, 1) Fig.6 Set B (2, 3) McAdams Chur&Chu I Chur&Chu II Chur&Usagi Mc Adams Chur&Chu I Chur &Chu II Chur&Usagi Fig.1 Set C (1, 25) Variation of Nu with input power B. Variation of Heat Transfer Coefficient with Different for Fin arrays Figure shows variation of heat transfer co efficient with different power input for various fin arrays. As power input increases h increases for all arrays. The convective heat transfer coefficient is more for a smaller fin array i.e. smaller height fin array with constant length for given power input. This is because h not only depends on

5 Convective Heat Transfer Heat Transfer Coeficient Heat Transfer Coefficient Convective Heat Transfer International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN area but temperature difference between fin and surrounding air Fig.11 L=2mm and H=1, 3,5mm Fig.12 L=1mm and H=5, 15,25mm Variation of heat transfer coefficient with input power 2,1 2,3 2,5 1,5 1,15 1,25 C. Variation of Convective Heat Transfer Rate with Fin Height for Different. From the figure it can be observed that for every power input and fin length combination, the convection heat transfer rate from the fin array increases with the increase in the fin height. With an increase in fin height, the total heat dissipation area also increases. Since the convection heat transfer rate directly related to the surface area in contact with air, increasing fin height increases the total heat dissipation Fig.14 L=1mm and H=5, 15,25mm Variation of Convective Heat transfer with power input TABLE X CONVECTIVE HEAT TRANSFER FOR 1mm LENGTH Sr.No (15) (115) (125) TABLE XI CONVECTIVE HEAT TRANSFER FOR 2mm LENGTH P (W) (21) (23) (25) VII.FLOW VISULAZATATION 1,5 1,15 1,25 Computational fluid dynamics CFD is the useful to visualize the flow. In this section the variation of temperature and velocity of the flow with different parameters of heat sink is displayed. Since there are many fin arrays and their combinations with different heat inputs, it is not possible to show variation of flow speed and temperature for all. Therefore, only one fin configuration is selected to represent every visualization figure Fig.13 L=2mm and H=1, 3,5mm 2,1 2,3 2,5 Fig.15 Fin Arrays Before Meshing

6 International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN Fig.16 Mesh Distribution on Fin Surface Fig.19 Fin Length 2 mm Fin Height 5 mm (Velocity Contours) Fig.17 Boundary Conditions Fig.2 Velocity Boundary Layer Effects Fin Length 2 mm, Height 5 mm The boundary layer effects had been captured in the simulation as evident from the velocity gradients near the fin surfaces in the above image. Also, the channel width between the fins results in high velocity also can be observed in the above image. Fig.18 Fin Length 2 mm Fin Height 5 mm (Contours of Static Temperature)

7 International Engineering Research Journal (IERJ) Special Issue 2 Page , 215, ISSN Fig.21 Thermal Boundary Layer Effects Fin Length 2 mm, Height 5 mm The thermal boundary layers surrounding the fins can be observed in the above image. Also, the thermal gradient was observed only the zones near the fins and the remaining locations have no change in temperatures. This was expected as the heat was supplied from the base plate and to the fins only. Fig.21 Contours of Surface Nusselt Number. High Nusselt number at the bottom section of the Fins indicates the higher heat transfer rate at the bottom than the top surfaces. The air that approaches the fin bottom is at the ambient temperature of 25 C. Due to the higher temperature on Fin surface, the heat transfer takes place and the air temperature increases. As the air moves upwards due to the density difference, the chances of heat removal from Fin top surfaces reduces since the air temperature is already high. This had been observed in all configurations. VIII. CONCLUSION Natural convective heat transfer depends on fin height. Convective heat transfer rate from fins increases with an increase in height of fin arrays. erimental Nusselt Number is quite close to the value obtained from correlation With an increase in heat input and temperature difference, natural convective heat transfer also increases. With an increase in heat input, heat transfer by radiation mode also correspondingly increases. Convective heat transfer increases with aspect ratio for given power input. ACKNOWLEDGMENT The report is outcome of guidance, moral support and devotion bestowed on me throughout my work. For this I acknowledge and express my profound sense of gratitude and thanks to everybody who have been a source of inspiration during the experimentation. First and foremost I offer my sincere phrases of thanks with innate humility to Prof.J.H.Bhangale, (H.O.D) Mechanical Engineering Department, MCOE&RC, Nasik for providing help whenever needed. The consistent guidance and support provided by Prof. D.D.Palande is very thankfully acknowledged and appreciated for the key role played by him in providing me with his precious ideas, suggestions, help and moral support that enabled me in shaping the experimental work. REFERENCES [1] Kamal-Eldin Hassan and Salah A. Mohamed, Natural Convection from Isothermal Flat Surfaces, ht. J. Heat Mass 7kznsjer. Vol. 13, pp Pergamon Press 197. [2] E. M. Sparrow and L. F. A. Azevedo, Vertical-channel natural convection spanning between the fullydeveloped limit and the single-plate boundary-layer limit, Int. Journal Heat mass transfer. Vol. 28, No. 1, pp , 1985 [3]Rong-hua Yeh, Shih-Pin Liaw and Ming Chang, optimum Spacings of Longitudinal convective fin arrays, Journal of marine science and technology. Vol.5, No.1, pp (1997) [4] Witold M. Lewandowski, Ewa Radziemska, Heat transfer by free convection from an isothermal vertical round plate in unlimited space, Applied Energy 68 (21) [5] A. Ozsunar, S. Baskaya and M. Sivrioglu, Numerical analysis of Grashof number, Reynolds number and inclination effects on mixed convection heat transfer in rectangular channels, Int. Comm. Heat Mass Transfer, Vol. 28, No. 7, pp , 21 [6] J.J.Wei, B.Yu, H.S. Wang, W.Q. Tao, Numerical study of simultaneous natural convection heat transfer from both surfaces of a uniformly heated thin plate with arbitrary inclination, Heat and Mass Transfer 38 (22) [7] A. Giri 1, G.S.V.L. Narasimham, M.V. Krishna Murthy, Combined natural convection heat and mass transfer from vertical fin arrays, International Journal of Heat and Fluid Flow 24 (23) [8] A.S. Krishnan, B. Premachandran, C. Balaji, S.P. Venkateshan, Combined experimental and numerical approaches to multi-mode heat transfer between vertical parallel plates, erimental Thermal and Fluid Science 29 (24) [9] Xiaoling Yu, Jianmei Feng, Quanke Feng, Qiuwang Wang, Development of a plate-pin fin heat sink and its performance comparisons with a plate fin heat sink, Applied Thermal Engineering 25 (25) [1] C.J. Kobus,T. Ohio, 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 (25) [11] Barak Yazicioğlu., Performance of Rectangular Fins on a Vertical Base in Free Convection Heat Transfer, M.S. Thesis in Mechanical Engineering, Middle East Technical University, Ankara (25). [12] Moghtada Mobedi, Bengt Sunden, Natural convection heat transfer from a thermal heat source located in a

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