The present study is aimed at consideration of forced boiling heat transfer in a channel whose width is comparable and less than the capillary constant. When boiling occurs inside very small and non-circular passages the constrain phenomena become important in determining the aspects of flow phenomena such as the flow pattern and microscale heat transfer. We consider the experimental data on flow patterns and boiling heat transfer for two refrigerants (R318C and R21), which were obtained for rectangular and annular channels in a range of flow rates from 50 kg/m2s up to 900 kg/m2s and heat fluxes density of up to 90 kW/m2. The temperature variation over the channel perimeter and in time was measured for the rectangular channel also. Different heat transfer mechanisms were revealed depending of flow patterns. We observed dominant nucleation boiling and the joint mode of nucleation boiling and convective evaporation. We have also found the mode when evaporation of thin liquid films makes the main contribution to heat transfer. The modified model of Liu and Winterton and Kandlikar model describe well the experimental data for the flow patterns when nucleation boiling and convective heat transfer make the most contribution to heat transfer. The approach to predict the suppression of flow boiling in the annular flow is proposed based on comparison of the turbulent boundary layer thickness and cavity radius based on the tangency criterion for boiling incipience.

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