Two-phase flow characteristics across sudden expansion in small rectangular channels

This study examines the two-phase flow pattern change and frictional pressure loss pertaining to the sudden flow area expansion from a small diameter tube into small rectangular channels. It is found that a unique flow pattern “liquid jet-like flow pattern” occurs at a very low quality region of G=100kg/m2s in a test section of 3mm×9mm. This flow pattern is almost inconceivable for the 3mm×6mm test section. The pressure difference usually increases with vapor quality but a setback is encountered provided that the liquid jet flow pattern prevails. A phenomenal observation suggests that a conceivable reduction of the pressure difference is related to the liquid jet-like flow pattern, yet a flat or level-off of the pressure difference is seen when the liquid jet-like flow is less pronounced. By contrast, an appreciable increase of pressure difference is seen when the liquid jet-like flow pattern is completely gone. The measured pressure differences are compared with existing correlations/models, whereas none of them gives satisfactory predictions against the present data. It is found that a slight modification to the empirical constant K of Wadle’s correlation may give rise to a fair improvement of the predictive ability.