Effect of inclination angle on two-phase flow patterns inside microscale enhanced tubes

The ambitious goal of mitigating global warming calls for increasingly sophisticated steps in the field of refrigeration. In this scenario, the combination of low GWP refrigerants with enhanced heat transfer geometries deserves proper attention in the scientific community. This thesis work specifically addresses the effect of the inclination angle on two-phase flow patterns inside small diameter helical micro-fin tubes. The experimental work was carried out in the Micro-Geometries Heat Transfer Laboratory of the Department of Industrial Engineering at the University of Padua. Given that the open literature is completely lacking in studies concerning the aforementioned topic with particular reference to low GWP refrigerants, the azeotropic mixture R515B (91.1% HFO-1234ze(E) and 8.9% HFC-227ea, by mass) was selected and its flow patterns at the exit of diagonally cut micro-fin tubes were investigated by means of a high-speed camera. Three tubes with different outer diameters (4 mm, 5 mm and 7 mm) were explored at three different upward flow inclinations: 30°, 45° and 60° with respect to the horizontal. Tests were conducted at a constant saturation temperature of 30 °C, by varying the mass flux in the range 50-400 kg m-2 s-1 and outgoing vapor quality ranging from 0.02 to 0.96. Five types of flow pattern were detected, namely annular, transitional, churn, wavy-stratified and intermittent. The flow regimes were compared with those previously observed in the same three tubes in horizontal arrangement. At increasing inclination angles, stratified and transitional flow patterns tend to phase out and be replaced by intermittent and churn flow patterns. Conversely, the occurrence of annular flow pattern does not appear to be significantly affected by the inclination angle and this could stem from the beneficial effect of the micro-fins in boosting the inertia forces. The 2-zone based predictive models of Mohseni and Akhavan-Behabadi, Doretti et al., Jige et al., and Irannezhad and Diani were adopted to plot the experimental datapoints and their thoroughness in the prediction of the transition to annular flow was assessed. While the former three show a high level of inaccuracy, the fourth model displays an excellent accuracy in the whole set of inclinations and tube diameters. Finally, an alternative 3-zone based predictive model was built in order to account for both the transition to annular flow (i.e., to a shear stress-controlled heat transfer) and the departure from stratified/intermittent flow (i.e., from a gravity-controlled heat transfer), thus including an intermediate zone of transitional/churn flow where a mixed heat transfer mechanism is surmised to occur.