The present study investigates experimentally the traits of air-steam two-phase flow patterns in an air-feed rectangular minichannel with steam through a porous bottom wall. Steam may condense in the porous layer as well as on the channel wall. Two-phase flow patterns in the minichannel are visualized and reported with different combinations of steam and air flowrates. The porous layer may become impermeable at high air flowrates due to steam condensation.
In addition, steam condensation in rectangular microchannels with uniform, converging, and diverging cross-sections with a mean hydraulic diameter of 117 μm are studied. The steam flow in the microchannel is cooled by directly immersing its bottom and side surfaces in a still, cool water bath. The flow patterns, bubble emission frequency and bubble velocity, two-phase flow pressure drop, and temperature drop through the microchannel with three different cross-section designs are presented. Bubble coalescence is observed and results show that the channel cross-section has a significant effect on the two-phase flow pressure drop and channel outlet temperature. In addition, the study concludes that microchannels with a converging cross-section design work best for draining two-phase fluids composed of uncondensed steam and liquid water.
Finally, steam condensation in rectangular microchannels with uniform and converging cross-sections and a mean hydraulic diameter of 135 μm are explored. The steam flow in the microchannels is cooled by water cross-flowing along its bottom surface, which is different from other methods reported in the literature. The flow patterns, two-phase flow pressure drop, and condensation heat transfer coefficient are determined. The microchannels with a uniform cross-section design has a higher heat transfer coefficient than those with a converging cross-section during condensation in the mist/annular flow regimes, although the latter work best for draining two-phase fluids composed of uncondensed steam and liquid water, which is consistent with the result of our previous study. From the experimental results, dimensionless correlations of condensation heat transfer for the mist and annular flow regions and a two-phase frictional multiplier are developed for the microchannels with both types of cross-section designs. The experimental data agree well with the obtained correlations, with the maximum mean absolute errors of 6.4% for the two-phase frictional multiplier and 6.0% for the condensation heat transfer.

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