Two-phase flow heat transfer in micro-channels

The requirement of modern industrial society is to continuously improve the performance of manufactured products and most notably increase performance density. At the same time, this has caused the micro-electronics industry to be faced with increasingly high heat fluxes which need to be dissipated. It is expected in few years that advanced microprocessors will be dissipating heat fluxes as high as 300 W/cm2 and require cooling to maintain device temperature below a limit that is set by reliability and material concerns. This limit varies, from 85°C for commercial microprocessors to 125°C for defence electronics applications. Flow boiling in micro-channels is gaining significant attention in recent years due to its capability to dissipate very high heat fluxes. The major advantage of flow boiling systems is the ability of the fluid to carry larger amounts of thermal energy through the latent heat of vaporisation. For the performance assessment and design of a micro-channel cooling device, it is very important to be able to define accurately the pressure drop and flow boiling heat transfer for a given operating condition for a particular micro-channel geometry. The present study aims to add to the knowledge of the fundamentals of two-phase flow heat transfer in a micro-channel heat sink with parallel small passages, through analysis of the effect of different fluid properties, operational conditions and channel sizes. The database includes test results for two different fluids, deionised water and refrigerant R134a, for a total of over 1400 data points. The experimental data was compared to several correlations from literature. An observation of the two-phase flow was conducted with and without an orifice (porous insert) positioned at the inlet of micro-channels. Visualisation confirmed the existence of the back flow, flow instability and non-uniform flow distribution among the channels (maldistribution) when the porous insert was removed. Flow patterns in the micro- channels and their evolution with increasing heat flux were observed. Keywords: Two-phase flows, micro-channel, heat transfer, pressure drop, flow pattern.EThOS - Electronic Theses Online ServiceGBUnited Kingdo