Microscale boiling heat transfer in a micro-timescale at high heat fluxes

Microscale boiling heat transfer experiments were performed using acetone as the working fluid in ten parallel silicon microchannels with hydraulic diameters of 155.4 mum. An infrared radiator image system is used for the chip temperature measurements, while an optical system combining a microscope and a high-speed camera is used for transient flow pattern identification. By covering the present data range it is found that all microchannels repeat the transient flow patterns in a timescale of milliseconds while the fluid pressures/temperatures are stable. A full cycle can be subdivided into three substages: liquid refilling stage, bubble nucleation, growth and coalescence stage, and transient annular flow stage. Correspondingly four flow patterns are identified. Paired or triplet bubbles are observed to be nucleated and grow up simultaneously in, or very close to, the channel corners at the same cross section. The nucleated bubbles experience several milliseconds of growth until coalescence takes place. Then a single liquid plug is separated into two parts, which are pushed out of the flow field view in less than 1 ms. In the transient annular flow stage, the liquid films that are drawn into the comers of the channel become less and less versus time. Once a partially or fully dried-out state is reached, all the microchannels are refilled with fresh liquid and a new cycle begins. The probabilities of each flow pattern occurring and the liquid refilling follow the statistical principle well. The measured chip temperatures are not uniform across the whole heating area, attributed to the uneven liquid refilling probabilities for different channels and the uneven possibilities that are immersed in the liquid for different heating regions. The chip temperatures display spatial variation behavior in the majority of the heating area, due to the liquid and vapor alternatively passing through the microchannels. To the authors' knowledge, some of the above experimental findings have not been previously reported.