Pressure drop and void fraction during flow boiling in rectangular minichannels in weightlessness

An experimental investigation has been carried out on flow boiling in a minichannel to explain heat transfer enhancement observed experimentally under the conditions of weightlessness. The analysis is based on the local void fraction and frictional pressure loss measurements. Frictional pressure loss in two-phase flows in minichannels under terrestrial gravity is described by several well-known correlations. In weightlessness, however, few experimental results are available on the void fraction and the frictional pressure loss. The experiments for this study have been performed at constant heat flux supplied to the minichannel with inlet liquid mass velocity ranging between 30 and 248 kg s−1 m−2. The influence of hypergravity (gravity level of 1.8g) and microgravity (gravity level of ±0.05g) on the frictional pressure loss is observed and explained using the flow patterns visualization and experimental void fraction determination through image treatment. Pressure drops for two-phase flow in microgravity are found to be significantly higher than for single-phase flow under similar conditions; possible explanations for the difference are discussed. The experimental thermal measurements have been previously analyzed using inverse techniques which led to evaluation of the local heat transfer coefficient. The heat transfer enhancement observed during weightlessness is explained in the present work by investigating the differences in flow patterns and void fraction under different levels of gravity.