Experimental investigation of R-245fa flow boiling in minichannels at high saturation temperatures: Flow patterns and flow pattern maps

•Image processing method and analysis of the heat transfer coefficient for two-phase flow pattern characterization.•Four flow patterns (intermittent flow, annular flow, dryout flow and mist flow) and their transitions were recognized.•The influence of the high saturation temperature (up to 120 °C) on flow pattern with R-245fa was investigated.•Comparisons with available transition lines in the literature were made.

In this work, flow visualizations and measurements are carried out to identify flow regime transitions during the flow boiling of R-245fa in a 3.00 mm inner diameter minichannel at high saturation temperatures, i.e. saturation temperature ranging from 60 °C to 120 °C. Flow boiling experiments at such high temperatures have so to say almost not been reported in the open literature so far. The mass velocity is ranging from 100 to 1500 kg/m2 s, the heat flux is varying from 10 to 50 kW/m2 and the inlet vapor quality from 0 to 1. Based on an image processing method and with the help of an adequate analysis of the heat transfer coefficient, four main flow regimes can be identified: intermittent flow, annular flow, dryout and mist flow. Their transitions were detected and plotted on flow pattern maps. The influence of the heat flux and the saturation temperature are highlighted and discussed. Comparisons with available transition lines in the literature have been made. Whereas the flow regime would suggest a macroscale type of flow, the flow patterns transitions are closer to those observed in microscale.