Micro-Channel Heat Sinks

Chapter 4 is devoted to single-phase heat transfer. Data on heat transfer in circular micro-tubes and in rectangular, trapezoidal and triangular ducts are presented. Attention is drawn to the effect of energy dissipation, axial conduction and wall roughness on the thermal characteristics of flow. Specific problems connected with electro-osmotic heat transfer in micro-channels, three-dimensional heat transfer in micro-channel heat sinks and optimization of micro-heat exchangers are also discussed. 

We attempt here to describe the fundamental equations of fluid mechanics and heat transfer. The main emphasis, however, is on understanding the physical principles and on application of the theory to realistic problems. The state of the art in high-heat flux management schemes, pressure and temperature measurement, pressure drop and heat transfer in single-phase and two-phase micro-channels, design and fabrication of micro-channel heat sinks are discussed. 

Fig. 2.10 High-performance micro-channel heat sink. Reprinted from Tuckerman and Pease (1981) with permission...

The overall performance of micro-channel heat sinks... 

A micro-channel heat sink can be classified as single-phase or two-phase according to the state of the coolant inside it. For single-phase fluid flow in smooth... 

Experimental and numerical study of the pressure drop and heat transfer in a single-phase micro-channel heat sink by Qu and Mudawar (2002a,b) demonstrated that the conventional Navier-Stokes and energy equations can adequately predict the fluid flow and heat transfer characteristics. 

As boiling in micro-channel heat sinks is an attractive method for cooling computer CPUs and other high-heat flux devices (such as laser diodes), it is of crucial importance to accurately predict the critical heat flux (CHF) in the small-diameter channels. Critical heat flux or burnout is a limiting value for safe operation of heat dis-... 

The CHF correlation (2.18) was developed by Qu and Mudawar (2004) for water in a rectangular micro-channel heat sink, as well as Bowers and Mudawar s CHF data (1994) for R-113 in a circular mini/micro-channel heat sink. 

Since the CHF for both mini/micro-channel heat sink databases shows no dependence on inlet subcooling, these databases were correlated without the subcooling multiplier. 

Figure 2.48 compares the predictions of this correlation with the flow boiling CHF data for water both in the rectangular micro-channel heat sink (Qu and Mudawar 2004) and in the circular mini/micro-channel heat sinks (Bowers and Mudawar 1994). The overall mean absolute error of 4% demonstrates its predictive capability for different fluids, circumferential heating conditions, channel geometries, channel sizes, and length-to-diameter ratios. 

Fig. 2.48 Comparison of CHF data for water and R-113 in mini/micro-channel heat sinks correlation (2.18). Reprinted from Qu and Mudawar (2004) with permission...

Design and Fabrication of Micro-Channel Heat Sinks... 

One drawback of a micro-channel heat sink is a relatively high temperature rise along the micro-channel compared to that for the traditional heat sink designs. In the direction of the flow, the wall temperature rises in a single-phase flow even when the wall heat flux is uniform. In a micro-channel heat sink, the large amount... 

It was demonstrated that for a fixed pumping power, the overall thermal resistance for a two-layered micro-channel stack is 30% less than for a one-layered micro-channel due to doubling of the heat transfer area. Methods for optimizing the thermal performance of micro-channel heat sinks were discussed by Kim (2004). 

In Proceedings of 21st SemiTherm Symposium, San Jose, 15-17 March 2005, pp 1-7 Copeland D, Behnia M, Nakayama W (1997) Manifold micro-channel heat sinks isothermal analysis. IEEE Trans Comp Packag Manuf Technol A 20 96-102 Dupont V, Thome JR, Jacobi AM (2004) Heat transfer model for evaporation in microchannels. 

PhUlips RJ (1990) Micro-channel heat sinks. In Bar-Cohen A, Kraus AD (eds) Advances in thermal modeling of electronic components and systems, vol 2, pp 109-184 Plosl A, Krauter G (1999) Wafer direct bonding tailoring adhesion between brittle materials. Mater Sci Eng R25 92-98... 

Qu W, Mudawar 1 (2002a) Experimental and numerical study of pressure drop and heat transfer in a single-phase micro-channel heat sink. Int J Heat Mass Transfer 45 2549-2565 Qu W, Mudawar 1 (2004) Measurement and correlation of critical heat flux in two-phase micro-channel heat sinks. Int J Heat Mass Transfer 47 2045-2059 Qu W, Mudawar 1 (2002b) Prediction and measurement of incipient boiUng heat flux in micro-channel heat sinks. Int J Heat Mass Transfer 45 3933-3945... 

Wei XJ, Joshi Y (2002) Optimization study of stacked micro-channel heat sinks for microelectronic cooling. IEEE Trans Comp Packag Technol 26(1) 55-61... 

Yang H, Lee F, Chein R (2006) Micro-channel heat sink fabrication with roughened bottom walls. Microsyst Technol 12 760-765... 

Zhang L, Goodson KE, Kenny TW (2004). Silicon micro-channel heat sinks. Springer-Verlag Berlin Heidelberg... 

Because most applications for micro-channel heat sinks deal with liquids, most of the former studies were focused on micro-channel laminar flows. Several investigators obtained friction factors that were greater than those predicted by the standard theory for conventional size channels, and, as the diameter of the channels decreased, the deviation of the friction factor measurements from theory increased. The early transition to turbulence was also reported. These observations may have been due to the fact that the entrance effects were not appropriately accounted for. Losses from change in tube diameter, bends and tees must be determined and must be considered for any piping between the channel plenums and the pressure transducers. It is necessary to account for the loss coefficients associated with singlephase flow in micro-channels, which are comparable to those for large channels with the same area ratio. 

One particular characteristic of conduction heat transfer in micro-channel heat sinks is the strong three-dimensional character of the phenomenon. The smaller the hydraulic diameter, the more important the coupling between wall and bulk fluid temperatures, because the heat transfer coefficient becomes high. Even though the thermal wall boundary conditions at the inlet and outlet of the solid wall are adiabatic, for small Reynolds numbers the heat flux can become strongly non-uniform most of the flux is transferred to the fluid at the entrance of the micro-channel. Maranzana et al. (2004) analyzed this type of problem and proposed the model of channel flow heat transfer between parallel plates. The geometry shown in Fig. 4.15 corresponds to a flow between parallel plates, the uniform heat flux is imposed on the upper face of block 1 the lower face of block 0 and the side faces of both blocks... 

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