DEVELOPMENT OF AN EXPERIMENTAL METHOD FOR FLOW-BOILING HEAT TRANSFER IN MICROCHANNELS

In this paper, we report on an experimental method, supported by a numerical conduction model, to determine the fundamental mechanisms controlling flow-boiling heat transfer processes, such as during bubble ebullition. This is achieved by synchronizing high-speed visualization with surface temperature measurement and using the numerical model to infer the heat transfer coefficient from the surface temperature measurements. A slip coefficient, S, is defined and provides a quantitative measure of the effect of conduction heat transfer in typical flow-boiling experiments in microchannels. To demonstrate the method, three high-speed experimental measurements are detailed. Surface temperatures at high frequencies (0(10 kHz)) are obtained with micron-sized thermistors; boiling events are simultaneously visualized and used in conjunction with transient temperature measurements and the S coefficient to infer processes controlling heat transfer in a microchannel. The results demonstrate that microdomains formed by high-thermal-conductivity substrates, including silicon and copper, cannot be used to reveal transient processes at the microscale. Even results obtained using low-thermal-conductivity materials such as Pyrex and Benzocyclobutene (BCB) require conduction numerical analysis in the solid structure to decouple the convection and conduction processes.