Analysis of heat transfer characteristics of double-layered microchannel heat sink

https://doi.org/10.1016/j.ijheatmasstransfer.2012.02.038Get rights and content

Abstract

Numerical analysis is performed to examine the heat transfer characteristics of a double-layered microchannel heat sink. The three-dimensional governing equations are solved by the finite volume method. The effects of substrate materials, coolants, and geometric parameters such as channel number, channel width ratio, channel aspect ratio, substrate thickness, and pumping power on the temperature distribution, pressure drop, and thermal resistance are discussed. Predictions show that the heat transfer performance of the heat sink is improved for a system with substrate materials having a higher thermal conductivity ratio. A coolant with high thermal conductivity and low dynamic viscosity also enhances the heat transfer performance. The pressure drop decreases with the channel aspect ratio and channel width ratio. Further, the thermal resistance of the microchannel heat sink can be minimized by optimizing the geometric parameters. Finally, the results show that for the same geometric dimensions, the thermal performance of the double-layered microchannel heat sink is better than that of the single-layered one, by an average of 6.3%.

Introduction

The single-layered microchannel heat sink (SL-MCHS), firstly introduced by Tuckerman and Pease [1], is widely regarded as one of the most promising ways to improve the heat transport performance, owing to its small size, low volume per heat load, low coolant requirement, and low operational cost. In recent years, the SL-MCHS has been extensively used for cooling devices such as very-large-scale integrated (VLSI) circuits and microelectromechanical systems (MEMSs) [2], [3], [4], [5].

One disadvantage of the SL-MCHS is the relatively high and non-uniform temperature distribution along the channel as compared to that of other heat sinks. This high and non-uniform temperature distribution produces thermal stresses in IC chips or packages and then reduces the electrical performance via electrical-thermal instability, thermal breakdown, etc. The high temperature gradient is attributed to the fact that the large amount of heat generated by the chips cannot be dissipated because of the relatively low flow rate (i.e., low flow velocity) of the coolant. Previous studies indicated that the temperature distribution of the SL-MCHS is sensitive to variations in the channel’s geometric design parameters [6], [7], [8], [9], [10], [11], [12]. Thus, the high temperature gradient changes rapidly with variations in the channel’s geometric design parameters. Hence, to obtain a low temperature gradient and improve the heat transfer performance, an appropriate method is required that would decrease the variations in the temperature gradient.

One way to reduce variations in non-uniform temperature distribution is to allow more coolant to flow through the channels. This may be achieved by redesigning the double-layered channels for a specific geometric dimension, to increase the coolant flow rate. The channel can have two layers of heat sinks with the coolant in the top and bottom layers flowing in the opposite direction. However, to enhance the coolant flow rate, more pumping power and bulky packages may be required, and considerable noise could be generated. Therefore, further study on the heat transfer characteristics of double-layered MCHS (DL-MCHS) design is needed.

Several authors showed that the double-layered MCHS design exhibits better thermal performance than a single-layered one and, therefore, has great potential for use as a cooling device [13], [14], [15], [16], [17], [18]. Vafai and Zhu [13] first proposed the two-layered MCHS with a counter-current flow arrangement. Their results indicate that the two-layered MCHS design provides a substantial improvement in temperature rise and required pressure drop over the conventional one-layered MCHS. Chong et al. [14] constructed a numerical model of single-layered and double-layered counter-flow MCHSs by employing the thermal resistance network to evaluate the performance of the heat sinks. Khaled and Vafai [15] examined numerically the control of heat transfer and flow within a double-layered thin film channel standing on flexible complex seals and subjected to oscillatory disturbances. The results showed that the flow rate and heat transfer in the main thin film channel can be increased by increasing the softness of the seals, the thermal squeezing parameter, the thermal dispersion effect, and the total thickness of the double-layered thin film. Vafai and Khaled [16] also analyzed single-layered and double-layered flexible MCHSs. They showed that double-layered flexible MCHSs provide a higher rate of cooling at the lower range of pressure drops. Single-layered flexible MCHSs are preferred for large pressure drop applications, while double-layered flexible MCHSs are preferred for applications involving low pressure drops. Cheng [17] presented the numerical study of a stacked two-layer MCHS that had a mixing-enhanced passive microstructure. Results indicated that the performance improved by changing the ratio of embedded structure height to microchannel height from 0.13 to 0.26 but maintaining a microchannel Reynolds number of 14.8. The resulting optimized stacked microchannel outperformed smooth microchannels. Dixit et al. [18] designed an innovative multilayer water-cooled heat sink based on silicon micro-nanopillars. Their analysis showed that adding silicon pillars to a heat sink can improve the heat dissipation rate by 16% and that further gains in thermal efficiency and heat dissipation rate are possible. Although many investigations have compared the performance of single-layered and double-layered heat sink designs, the effects of the substrate materials, coolants, geometrical parameters (such as channel number, channel width ratio, channel aspect ratio, and base thickness), and pumping power on the thermal performance of heat sinks have not been comprehensively examined and compared. This motivates the present study.

The objectives of this work are to investigate numerically the heat transfer characteristics of the double-layered MCHS design. In the present analysis, four different types of substrate materials (copper, alumina, silicon, and steel) and three different coolants (namely water, ethylene glycol, and glycerol) were considered. The effects of geometric parameters such as channel number, channel width ratio, channel aspect ratio, base thickness, and pumping power on the thermal performance or thermal resistance were investigated and discussed in detail.

Section snippets

Problem formulation

The schematics of a double-layered MCHS (DL-MCHS) and the 3-D computational domain used in the present analysis are given in Fig. 1a and b, respectively. As shown in Fig. 1b, the double-layered heat sink has two layers of rectangular channels and ribs. Each layer of the channels are all of width Wc, and has height Hc,1 and Hc,2, respectively. In this model, it is assumed that Hc,1 = Hc,2. Each of the vertical ribs has a width Wr/2, and the horizontal ribs are of thickness δb, and δm,

Numerical method

The governing equations for the described problem are solved numerically using the finite volume method. For a given combination of parameters, the field solutions are calculated by an iterative scheme [19], [20]. Details of the solution procedure are not repeated herein.

To obtain enhanced accuracy in our numerical computations, the independence of grid points was examined first. In this work, the grids were arranged to be non-uniformly distributed both in the cross-sectional and streamwise

Results and discussion

The 3-D numerical simulations of the heat transfer characteristics of the DL-MCHS for different substrate materials (copper, aluminum, steel, and silicon), coolants (water, ethylene glycol, and glycerol), geometric parameters (channel number, channel width ratio, bottom channel aspect ratio, and substrate thickness), and pumping power on the DL-MCHS performance were examined in detail. In what follows, we are interested in investigating the effects of the above factors on temperature

Conclusion

The heat transfer characteristics of double-layered MCHSs were numerically analyzed in detail. The effects of substrate materials; coolants; geometric parameters such as the channel aspect ratio, channel width ratio, and channel base thickness; and pumping power on the temperature distributions and thermal resistance were comprehensively studied. Based on the results presented, the following conclusions are drawn:

  • 1.

    A lower temperature rise is experienced for a double-layer MCHS with substrate and

Acknowledgment

This research was financially supported by the National Science Council, R.O.C., through the Contract NSC 98-2221-E145-009; this support is highly appreciated.

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