Nivesh Agrawal et al. / IJAIR ISSN: 78-7844 Comparison of heat transfer characteristics of liquid coolants in forced convection cooling in a micro heat sink Mr.Nivesh Agrawal #1 Mr.Mahesh Dewangan * #1 M.E. Student * Deptt. Of Mechanical Engineering, Shri Shankaracharya Group of Institutions, Bhilai Chhattisgarh, Bhilai - India #1 nivesh369@gmail.com * mahesh_k_d@yahoo.com Abstract The paper presents the comparison of heat transfer characteristics of liquid coolants in forced convection cooling in a micro-heat sink. For a given heat sink and pressure difference existing within the heat sink, the heat transfer characteristics of water and Propylene Glycol (PG) water are obtained. Numerical simulation on the conjugate heat transfer and fluid flow in the micro-heat sink is obtained using commercial CFD software ANSYS-CFX. The numerical results are validated against available results in the open literature. Within the range of operating parameters, the heat transfer characteristics of PG water has been found better than water. Keywords micro-channel heat sink, CFD, heat transfer, fluid flow, forced convection, liquid coolant I. INTRODUCTION The high power density and compactness of next generation electronic component require efficient cooling method for heat dissipation in order to maintain the component at acceptable temperature level, e.g. below1 C micro-channel heat sink have emerged as one of the effective cooling techniques proposed by Tuckerman and Pease [1,] for electronic cooling. They built a water cooled integral heat sink with microscopic flow channels. C.Y.Zhao and T.J.LU [3] investigated the heat transfer performance of micro-channel heat sink by using the fin approach and the porous medium approach. The result shows that fin approach significantly overestimates the heat transfer due to the assumption of constant fluid temperature in the direction perpendicular to coolant flow. Weilin Qu and Issam Mudawar [4] analysed the threedimensional fluid flow and heat transfer processes in a rectangular silicon micro-channel heat sink numerically and a detailed description of the local and average heat transfer characteristics, i.e. temperature, heat flux and Nusselt number was obtained. Ryu et al. [5] analysed the thermal performance of a microchannel heat sink and obtained the optimal fin channel shape that minimized the thermal resistance. Dorin Lelea et al. [6] established experimental setup to investigate a micro-channel heat transfer and fluid flow on tube of inner diameter D =.1,.3 and.5 mm. having water as a working fluid. J.Li et al. [7] gave a detailed simulation of the heat transfer occurring in silicon based micro channel heat sink was conducted using a simplified three dimensional conjugate heat transfer model. The influence of the geometric parameters of the channel and the thermo physical properties of the fluid on the flow and the heat transfer are investigated using a temperature dependent thermo physical property method. The results indicate that the thermo physical properties of the liquid can significantly influence both the flow and heat transfer in the micro channel heat sink. 1 IJAIR. ALL RIGHTS RESERVED 183
Nivesh Agrawal et al. / IJAIR ISSN: 78-7844 G.Hetsroni et al. [8] compared the heat transfer from different micro channel section such as circular, rectangular, triangular and trapezoidal. D.Klein et al. [9] studied the effect of alkyl poly glycosides (APG) surfactants on heat transfer from a single phase and boiling flow in micro channels. The result was compared with heat transfer in water flow under similar conditions. For single-phase flow, no significant difference was observed between heat transfer in water and surfactant solutions at various mass concentrations. J. Li and G.P.Peterson [1] developed a full three dimensional numerical simulation for fluid flow and heat transfer in parallel micro-channel heat sink and evaluated the optimal geometric conditions. N.Amanifard et al. [11] investigated the heat transfer in circular, rectangular, triangular and trapezoidal section. The numerical result is obtained for the thermal resistance and the Nusselt number. Liu Yanping and Yang Yuliangwas [1] simulated the water cooled heat sink in three dimension flow field and achieved the unique three dimension graphic display of flow velocity, pressure, temperature and other parameters. Fig.1 Structure of a rectangle micro-channel heat sink and the unit With a constant heat flux and at the top surface is well insulated. The physical dimensions of the micro-channel are presented in Table 1, whereas the thermo-physical properties of the fluid (water or PG ) and solid are presented in Tables and 3, respectively. Table 1 Governing dimensions of the single micro-channel: H h W w (µm) (µm) (µm) (µm) S t S b t L (µm) (µm) (µm) (mm) 9 18 1 57 45 7 1.5 1 II. PHYSICAL MODEL The micro heat sink model consists of a 1 mm long silicon substrate with silicon cover. The rectangular micro-channels have a width of 57µm and a depth of 18µm. A schematic of the structure of a rectangular micro-channel heat sink is shown in fig.1, where a unit of cell consisting of one channel was selected because of symmetry of the structure. The bottom surface of heat sink is uniformly heated. Fluid Table Thermo physical Properties of fluid: K Kg/m 3 W/m-K Kg/m-s J/Kg-K K 998..6.13 418 93 PG 16.36.64 34 93 Table 3 Thermo physical Properties of solid Solid K C p C p Kg/m 3 W/m-K J/Kg-K T Silicon 33 148 71 III. GOVERNING EQUATIONS For a fully developed laminar flow in a micro channel the entrance length =.575Re D h (1) 1 IJAIR. ALL RIGHTS RESERVED 184
havg. (w/mk) h avg (w/m k) Nivesh Agrawal et al. / IJAIR ISSN: 78-7844 For a hydraulic diameter D h = 86.58 and Reynolds number R e = 16, the entrance length = 57.75 µm which is less than 1 mm. So fully developed laminar flow is valid. 1. Continuity equation + u v + x y w = () z. Momentum equation (Navier- stokes equations) X-momentum equation u u u p u u u u v w x y z x x y z (3) Y-momentum equation v v v p v v v u v w x y z y x y z (4) Z- Momentum equation w w w p w w w u v w x y z z x y z (5) 3. Energy equation T u x T v y T w z 1 T x T y T z The hydrodynamic boundary condition can be stated as: at the inner bottom wall surface of channel (no-slip condition) u =, v =, w =, at the inlet, (6) (7a) z =, P l = P in, u =, v = (7b) at the outlet, z = L z, P l = P out, u =, v = the following thermal boundary conditions at bottom wall y =, -k at inlet z =, T = T in (7c) T = q (8a) y (8b) IV. RESULT AND DISCUSSION The present numerical simulation has been done using ANSYS-CFX and meshing is done by ICEM-CFD with tetra mesh with 159468 nodes. The results have been partially validated with grid independence tests, and there upon the lower mesh size is selected for rest of the simulations. The model of present study is validated against the numerical results of [5]. Fig. shows the validated results of average heat transfer coefficient inside the channel for constant pressure drop of 5 kpa and heat flux 9 W/cm. 14 1 1 8 6 4 Fig.. Model validation using average heat transfer coefficient inside the channel with the numerical result for Δp = 5kPa at q = 9W/cm Fig.3. Comparison of average Wall heat transfer coefficient between water and PG of micro channel heat sink for Δp = 5 kpa and q"=9 W/cm. The average wall heat transfer coefficient of PG and water are compared in fig.3. It is interesting to note that the average wall heat transfer coefficient sharply decreases due to growing boundary layer thickness. The heat transfer coefficient decreases along the flow direction and it is extremely high in the entrance region due to very thin boundary layer. 1 3 4 5 6 7 8 14 1 1 8 6 4 1 3 4 5 6 7 8 9 Li et al Present Solution PG 1 IJAIR. ALL RIGHTS RESERVED 185
T (k) Nu Nivesh Agrawal et al. / IJAIR ISSN: 78-7844 Fig4 show the comparison of Nusselt number of water and PG water. the average Nusselt number decreases along the channel due to formation of boundary layer at p= 5kpa at q = 9W/cm. 16 1 8 4 PG 1 3 4 5 6 7 8 9 Fig.4. Comparison of Nusselt number between water and PG of micro channel heat sink for Δp = 5 kpa and q"=9 W/cm. Fig. 5 represents the comparison of the temperature variation of PG water and water. It should be noted that the exit temperature of PG water is more than that of water which indicate that more heat can be transferred from the heat sink when using PG. 343 333 33 313 liquid bulk temp of PG liquid bulk temp of 33 93 83 73 1 3 4 5 6 7 8 9 1 bottom wall temp of PG bottom wall temp of Fig.6. Temperature contour of water in x-y plane of micro-channel heat sink at 1/3, /3, and outlet of the channel at p = 5kpa at q = 9 W/cm Fig.5. Comparison of temperature difference between water and PG of micro channel heat sink for Δp = 5 kpa and q"=9 W/cm. Fig. 6 and 7 show the temperature contours at one-third, two-third and outlet section of the micro-channel for and PG respectively. 1 IJAIR. ALL RIGHTS RESERVED 186
Nivesh Agrawal et al. / IJAIR ISSN: 78-7844 micro-channel, the outlet temperature of PG water is more which implies that more heat can be transferred from the heat sink when using pg-water. REFERENCES Fig.7. Temperature contour of PG water in x-y plane of micro-channel heat sink at 1/3, /3, and outlet of the channel p = 5kpa at q = 9W/cm The laminar heat transfer in microchannel is highly strange and complicated, compared with conventionally sized situation. The heat transfer characteristics of laminar flow are highly affected by liquid temperature and velocity [13]. V. CONCLUSION Heat transfer characteristics in forced convection cooling in a micro-heat sink using two liquid coolants, namely, water and PG has been compared. It has been found that the heat transfer characteristics of PG water are better compared to only water. For a given pressure difference existing in the [1] D.B.Tuckerman,R.F.W.Pease, High performance heat sinking for VLSI,IEEE Electron Dev. Lelt.(1981) 16-19. [] D.B.Tuckerman,R.F.W.Pease, ultra high thermal conductance microstructures for integral circuit, in:ieee Proc.3 electronics conference,198 pp.145-149. [3] C.Y.Zhao,T.J.Lu, Analysis of micro-channel heat sink for electronics cooling, Int.J.Heat and Mass Transfer 45 () 4857-4869. [4] Weilin Qu, Issam Mudawar, Analysis of three-dimensional heat transfer in Micro-channel heat sink, Int.J.Heat and Mass Transfer 45 () 3973-3985. [5] J.H.Ryu,D.H.Choi,S.J.kim,Numerical optimization of the thermal performance of a micro-channel heat sink, Int.J.Heat and Mass Transfer 45 () 83-87. [6] Dorin Lelea, Shigefumi Nishio, Kiyoshi Takano, the experiment research on micro tube heat transfer and fluid flow of distilled water. Int.J.Heat and Mass Transfer 47 (4) 817-83. [7] J.Li,G.P.Peterson,P.Cheng, three dimensional analysis of heat transfer in a micro heat sink with single phase flow, Int.J.Heat and Mass Transfer 47 (4) 415-431. [8] G.Hetsroni, A. Mosyak, E.Pogrebnyak, L.P.Yarin, heat transfer in Micro-channel: Comparison of experiments with theory and numerical results. Int.J.Heat and Mass Transfer 48 (5) 558-561. [9] D.Klein, G.Hetsroni, A.Mosyak, heat transfer characteristics of water and APG surfactant solution in a micro channel heat sink, Int.J.of multiphase flow 31 (5) 393-415. [1] J.Li,G.P.Peterson, 3-dimensional numerical optimization of silicon based high performance parallel micro-channel heat sink with liquid flow, Int.J.Heat and Mass Transfer 5 (7) 895-94. [11] N.Amanifard, M.Borji, A.K.Haghi, heat transfer and fluid flow through Micro-channels-a theoretical approach,iasme/wseas Int. Conf. On heat transfer,7. [1] Liu Yanping, Yang Yuliang, Numerical simulation on the water cooled heat sink for high power semiconducter,int. Conf. On computer application and system modelling (1). [13] B.X.Wang and X.F.Peng, experimental investigation on liquid forced convection heat transfer through micro channel,int.j.heat and Mass Transfer 37(1994)pp 73-8. 1 IJAIR. ALL RIGHTS RESERVED 187