Adiabatic two-phase flow experiment have been conducted to investigate the effects of channel diameter and liquid property on void fraction in horizontal microchannels. Water/nitrogen gas and ethanol-water/nitrogen gas mixtures were pumped through circular microchannels of 50, 75, 100 and 251 μm diameter. The concentration of ethanol in water was varied to change the surface tension and liquid viscosity. The void fraction data were obtained by an image analysis technique and correlated as a function of homogeneous void fraction. The void fraction data in channels with a diameter between 50 and 100 μm conformed well to Kawahara et al.’s (2002) correlation, but the data for a 251 μm diameter channel agreed with the Armand correlation (1946) suitable for minichannels. There was no significant effect of liquid properties on void fraction for all the channel sizes. Thus, these results suggest that the boundary between microchannels and minichannels would lie between 100 and 251 μm, and not be sensitive to the fluid property.

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