Two-phase flow pattern and frictional performance across small rectangular channels

This study presents flow visualizations and two-phase frictional pressure drop data for three rectangular channels with channel height of 3, 6 and 9 mm, and a fixed width of 3 mm. It is found that the stratified flow pattern still exists for an aspect ratio of unity at a low mass flux of 100 kg/m2 s but it completely vanishes when G > 200 kg/m2s. For the same plug flow of intermittent flow pattern, the number of plug increases whereas its length decreases when the aspect ratio is increased. This is especially pronounced when the mass flux is further increased over 500 kg/m2 s. The major departure of the observed flow pattern relative to the conventional Mandhane flow map is the transition boundary for slug/annular had been moved to a much lower superficial vapor velocity. The two-phase frictional pressure drop data are compared to homogeneous and Chisholm method, Wambsganss and Ide-Fukano correlations. It is found that none of the existing methods or correlations can satisfactorily predict the two-phase pressure gradient in rectangular channels. A modified C factor of Chisholm method considering the effect of aspect ratio was proposed from the empirical fit with the data sets of Wambsganss et al., Ide-Fukano, and this study. The corresponding mean deviations of the proposed correlation against the datasets are 24.99%, 10.83% and 10.73%, respectively. This correlation is applicable in wide rages of mass flux (50 < G < 700 kg/m2 s), vapor quality (0.001 < x < 0.95), Martinelli parameter (0.05 < X < 20) and aspect ratio (0.1 < A < 1.0).