Liquid to Vapor Phase Change in Constant Cross-Section Silicon Microchannels

This paper presents research on the evaporation of methanol/water mixtures in uniformly heated, constant cross-section, serpentine silicon microchannels for potential application as a fuel injection system for a MEMS rotary engine. The phase change of a variety of mixtures of methanol and water was observed, characterized and compared to the phase change both of pure water and pure methanol. Seven different fluids were tested: pure water, pure methanol and five different molar fractions of methanol mixed with water. Flow rates were varied from Reynolds number five to ten. In the microscale system, it is shown that the flow boiling characteristics of the methanol/water mixtures are markedly different from those of pure liquids. Specifically, for the binary system there is a lack of a clear meniscus that spans the microchannel, which is seen in the pure fluid systems. Rather, the phase change of binary mixtures appears to occur over a much greater length of microchannel than for pure fluids. Unstable, intermittent evaporation fronts, for both pure fluids and binary mixtures, were also observed within the channels for moderate levels of superheat that are most likely dictated by the local temperature and pressure variations along the channel. Furthermore, at no time was bubble formation observed in either the pure fluids or the binary mixtures, despite the fact that several of the mixtures were subject to superheats as great as 20oC.