| Fluid flow and heat transfer in microchannels are very important in various fields, including miniature thermal systems, compact heat exchangers and electronics cooling. The purpose of this investigation is to contribute to this new research area through carefully designed and well controlled experimental work. Experiments were performed to characterize single-phase and two-phase flow pressure drop in rectangular microchannels with Dh = 69.5--304.7 mum. In general, the measured single-phase friction factors agree with the analytical solution based on the Navier-Stokes equations, and the critical Reynolds numbers approach the conventional values. Based on the adiabatic two-phase flow pressure drop data, a new correlation was developed, in which the parameter C in Lockhart-Martinelli type correlation was correlated as a function of three nondimensional parameters in two flow regimes. Adiabatic flow was visualized in a microchannel, and the result suggests a wider range of surface tension dominated flow in microchannels than in minichannels. R134a liquid superheats before the onset of nucleate boiling and the local evaporation heat transfer coefficient were measured in 75 x 811.94 mum microchannels. No indication of deviation from the classic nucleation theory was observed, but the analysis showed that the lack of large range of various sized 'active' nucleation sites in microchannels resulted in higher than usual liquid superheats. The heat transfer data suggest a combined effect of nucleation and convection, as well as an earlier dry-out in microchannels. The comparison of the data with existing evaporation heat transfer correlations identified a need for developing new correlations in microchannels. Based on the present data, a semi-empirical correlation was proposed. Microscale orifice-tubes with inner diameter D = 31--52 mum were tested with R134a flowing with and without a phase change. No indication of choking was observed for a wide range of experimental conditions, and this phenomenon was attributed to the strengthening of metastable effect in microscale tubes. A semi-empirical correlation was proposed based on the experimental data. |
