Comparative Study of Conjugate Heat Transfer in Uniform and Diverging Cross-Section Microchannels

Comparative Study of Conjugate Heat Transfer in Uniform and Diverging Cross-Section Microchannels

Yogesh K. Prajapati
Copyright: © 2018 |Pages: 20
DOI: 10.4018/978-1-5225-3722-9.ch005
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Abstract

This chapter covers single-phase heat transfer analysis in microchannel heat sink relevant to electronic cooling application. In order to estimate the correct heat transfer performance, it is required to consider both, conduction and convection. Hence, conjugate analysis of heat transfer has been considered where both conduction and convection heat transfer are calculated as a part of solution. Two different configurations of microchannels namely, uniform and diverging cross-section have been considered individually on different copper substrate. A copper substrate of dimension 25×0.9×4 mm has been used to generate microchannel. Inlet cross-section (0.4×0.75 mm) of both channels has been kept equal however; cross-section of diverging channel keeps on increasing as width is continuously increasing along the flow direction. A constant heat flux of 250 kW/m2 has been provided from the bottom. Comparative study has been done to analyse the heat transfer performance of both the configurations of microchannels.
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Introduction

Narrow flow channels with hydraulic diameters (Dh) in 1–200 µm range are termed as microchannels. Defining threshold diameter (macro to micro transition) of microchannels is still a matter of considerable discussion in the research community. Till date no consensus has been reached for defining the critical diameter below which a channel is termed as microchannel. Nonetheless, widely accepted bands for microchannel diameter have been proposed by few researchers. Mehendale (2000) defined the micro heat exchanger in range of 1–100 µm whereas Kandlikar (2002) defined this range between 10–200 µm. In addition to geometrical parameters, there are certain dimensionless numbers which are used to define the threshold limit for microchannels. Kew and Cornwell (1997) proposed confinement number (Co) as:

978-1-5225-3722-9.ch005.m01
(1) where 978-1-5225-3722-9.ch005.m02 and 978-1-5225-3722-9.ch005.m03 are densities of liquid and gas phases respectively and σ is the surface tension. Channels with Co > 0.5 are termed as microchannels. Based on this criterion, channels with hydraulic diameter less than 1000 µm can be treated as microchannels for flow boiling of water.

Microchannels have been established to dissipate high heat. These channels are gaining popularity due to fact that significant amount of heat dissipation could be accomplished using small area and less amount of coolant. Microchannels also have got important applications in other fields to access fluid flow from confined space. Channels of different geometry and shapes are essentially required for different applications in various fields. One of the important applications of microchannel is as heat sink especially for electronic cooling. Heat sink may be defined as a device that simultaneously absorb and dissipate undesirable heat form a component using thermal contact. Heat sink transfers thermal energy from a high temperature source to a low temperature coolant. In recent years, the development of compact electronic devices and high-speed processors has significantly increased power densities in the component. Therefore, heat generation per unit volume in these devices has increased drastically. To keep the component under working conditions, efforts are needed to design more efficient and stable heat sink. Miniaturization of electronic devices requires mini/micro cooling system capable of dissipating heat flux in the range of 300–500 W/cm2. Of course, the time is not far off when these requirements will shoot-up to 1000 W/cm2 krishnan (2007).

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