Two-phase flow patterns inside inclined microfin tubes: Experimental investigation and empirical modelling

The paper studies the effect of inclination angle on condensation flow patterns inside helical micro-fin tubes. Since the open literature is completely lacking in studies concerning such topics with particular reference to low GWP refrigerants, R515B was selected and its flow patterns were investigated utilizing a high-speed camera. Three micro-fin tubes with different outer diameters (4, 5, and 7 mm) were explored at three different upward flow inclinations: 30°, 45°, and 60° with respect to the horizontal. Tests were conducted at constant saturation temperature of 30 °C, by varying the mass flux in the range 50–400 kg m−2 s−1 and the vapor quality from 0.02 to 0.96. Among the over than 500 experimental points, five types of flow patterns were detected: annular, transitional, churn, wavy-stratified, and intermittent. At increasing inclination angles, stratification tends to phase out and be replaced by intermittent flow and churn flow. Regarding the occurrence of an annular flow pattern, which is of high importance as it infers a shear-stress controlled heat transfer mechanism, the inclination did not impose a significant discrepancy, which was surmised to stem from the turbulence-inducing effect of microfin tubes boosting the inertia forces. The flow pattern map of Irannezhad and Diani provides fairly accurate predictions regarding the transition to annular flow pattern.