Microscale study of mechanisms of heat transfer during flow boiling in a microchannel

This study examines the microscale physics of heat transfer events in flow boiling of FC-72 in a microchannel. Experimental results presented here provide new physical insight on the nature of heat transfer processes during bubbles growth and flow through the microchannel. The study is enabled through development of a device with a composite heated wall that consists of a high thermal conductivity substrate coated by a thin layer of a low thermal conductivity material with embedded temperature sensors. This novel arrangement enables calculation of the local heat flux with a spatial resolution of 40-65 mu m and a temporal resolution of 50 mu s. The device generates isolated bubbles from a 300 nm in diameter artificial cavity fabricated at the center of a pulsed function micro-heater. Analysis of the temperature and heat flux data along with synchronized images of bubbles show that four mechanisms of heat transfer are active as a bubble grows and flows through the channel. These mechanisms of heat transfer are (1) microlayer evaporation, (2) interline evaporation, (3) transient conduction, and (4) micro-convection. Details of these mechanisms including their time period of activation and corresponding surface heat flux and heat transfer coefficient are extensively discussed. Published by Elsevier Ltd.