Physics of Interfacial Heat Transfer Events in Flow Boiling of FC-72 Liquid in Microchannels

This work examines the microscale physics of heat transfer processes in flow boiling of FC-72 in a single microchannel. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events. The study is enabled through development of a device with a composite substrate that consists of a high thermal conductivity material coated by a thin layer of a low thermal conductivity material with embedded temperature sensors. This novel arrangement enables measurement of local heat flux with a spatial resolution of 40–65 μm and a temporal resolution of 50 μs. 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. The magnitude and time period of activation of these mechanisms of heat transfer are determined and their characteristics are discussed in details.