Flow Boiling in Micro-Passages: Developments in Fundamental Aspects and Applications

Abstract

Flow boiling in mini to micro passages located at the heat source, and as part of a thermal management system, has been identified as a possible way to remove the increasing high heat fluxes generated by high power electronic devices due to their capability of high heat transfer rates with small surface temperature variations. However, some still unresolved fundamental issues hinder the possible full adoption of this technology. These relate to the prevailing flow patterns, heat transfer rates and pressure drop in such geometries, and their dependence on key parameters. The possible major applications of flow boiling in microchannels are first mentioned in this paper, highlighting the requirements and the challenges of the thermal management of each application. The paper then presents new experimental research by the present authors as well as research reported in the literature on flow boiling in single tubes and rectangular multi microchannels to help elucidate the following fundamental issues: the definition of a microchannel, prevailing flow patterns, heat transfer mechanisms, flow instability and reversal and their effect on heat transfer rates, effect of channel material and surface characteristics (including latest research in coatings), effect of different fluid properties, and its relation to channel material, effect of channel length and aspect ratio. An appreciation of the above can help explain the interpretation of the prevailing fluid flow and heat transfer phenomena and the data scatter and discrepancies observed in past studies. In addition, models and correlations predicting flow patterns and heat transfer rates are presented.

Wettability effect on flow boiling characteristics within micro-passages

“…Interface sharpening is very important in simulating two-phase flows of two immiscible fluids. In OpenFOAM the sharpening of the interface is achieved artificially by introducing the extra compression term ∇ • (α(1 − α)U ⃗ ⃗ r ) in Equation (4). U ⃗ ⃗ r is an artificial compression velocity.…”

“…The main reason for this, is the lack of accurate design correlations as well as the lack of understanding of several underpinned principles and mechanisms. Karayiannis and Mahmoud [4] recently reported that there are still open fundamental research aspects and issues. One of these is the effect of the solid surface characteristics on the underpinned bubble dynamics and heat transfer.…”