Hydrodynamics, heat transfer and flow boiling instabilities in microchannels

Abstract

Boiling in microchannels is a very efficient mode of heat transfer with high heat and mass transfer coefficients achieved. Less pumping power is required for two-phase flows than for single-phase liquid flows to achieve a given heat removal. Applications include electronics cooling such as cooling microchips in laptop computers, and process intensification with compact evaporators and heat exchangers. Evaporation of the liquid meniscus is the main contributor to the high heat fluxes achieved due to phase change at thin liquid films in a microchannel. The microscale hydrodynamic motion at the meniscus and the flow boiling heat transfer mechanisms in microchannels are not fully understood and are very different from those in macroscale flows. Flow instability phenomena are noted as the bubble diameter approaches the channel diameter. These instabilities need to be well understood and predicted due to their adverse effects on the heat transfer. A fundamental approach to the study of two-phase flow boiling in microchannels has been carried out. Simultaneous visualisation and hydrodynamic measurements were carried out investigating flow boiling instabilities in microchannels using two different working fluids (n-Pentane and FC-72). Rectangular, borosilicate microchannels of hydraulic diameter range 700-800 μm were used. The novel heating method, via electrical resistance through a transparent, metallic deposit on the microchannel walls, has enabled simultaneous heating and visualisation to be achieved. Images and video sequences have been recorded with both a high-speed camera and an IR camera. Bubble dynamics, bubble confinement and elongated bubble growth have been shown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodic instabilities have been observed during flow boiling in the microchannel. Analysis of the IR images in conjunction with pressure drop readings, have allowed the correlation of the microchannel pressure drop to the wall temperature profile, during flow instabilities. Bubble size is an important parameter when understanding boiling characteristics and the dynamic bubble phenomena. In this thesis it has been demonstrated that the flow passage geometry and microchannel confinement effects have a significant impact on boiling, bubble generation and bubble growth during flow boiling in microchannels.