Hydrodynamics, heat transfer and flow boiling instabilities in microchannels

摘要

Boiling in microchannels is a very efficient mode of heat transfer with high heat andudmass transfer coefficients achieved. Less pumping power is required for two-phaseudflows than for single-phase liquid flows to achieve a given heat removal.udApplications include electronics cooling such as cooling microchips in laptopudcomputers, and process intensification with compact evaporators and heatudexchangers.udEvaporation of the liquid meniscus is the main contributor to the high heat fluxesudachieved due to phase change at thin liquid films in a microchannel. The microscaleudhydrodynamic motion at the meniscus and the flow boiling heat transfer mechanismsudin microchannels are not fully understood and are very different from those inudmacroscale flows. Flow instability phenomena are noted as the bubble diameterudapproaches the channel diameter. These instabilities need to be well understood andudpredicted due to their adverse effects on the heat transfer.udA fundamental approach to the study of two-phase flow boiling in microchannels hasudbeen carried out. Simultaneous visualisation and hydrodynamic measurements wereudcarried out investigating flow boiling instabilities in microchannels using twouddifferent working fluids (n-Pentane and FC-72). Rectangular, borosilicateudmicrochannels of hydraulic diameter range 700-800 μm were used. The noveludheating method, via electrical resistance through a transparent, metallic deposit onudthe microchannel walls, has enabled simultaneous heating and visualisation to beudachieved. Images and video sequences have been recorded with both a high-speedudcamera and an IR camera.udBubble dynamics, bubble confinement and elongated bubble growth have beenudshown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodicudinstabilities have been observed during flow boiling in the microchannel.udAnalysis of the IR images in conjunction with pressure drop readings, have allowedudthe correlation of the microchannel pressure drop to the wall temperature profile,udduring flow instabilities.udBubble size is an important parameter when understanding boiling characteristicsudand the dynamic bubble phenomena. In this thesis it has been demonstrated that theudflow passage geometry and microchannel confinement effects have a significantudimpact on boiling, bubble generation and bubble growth during flow boiling inudmicrochannels.