Flow pattern characterization for R-245fa in minichannels: Optical measurement technique and experimental results

• Optical measurement method using image processing for two-phase flow pattern characterization.
• Four flow patterns (bubbly flow, bubbly–slug flow, slug flow and annular flow) were recognized.
• The bubble frequency, the percentage of small bubbles as well as their velocity were measured.
• The influence of the flow pattern on the heat transfer coefficient with R-245fa was investigated.

An optical measurement method using image processing for two-phase flow pattern characterization in minichannel is developed. The bubble frequency, the percentage of small bubbles as well as their velocity are measured. A high-speed high-definition video camera is used to measure these parameters and to identify the flow regimes and their transitions. The tests are performed in a 3.0 mm glass channel using saturated R-245fa at 60 °C (4.6 bar). The mass velocity is ranging from 100 to 1500 kg/m2 s, the heat flux is varying from 10 to 90 kW/m2 and the inlet vapor quality from 0 to 1. Four flow patterns (bubbly flow, bubbly–slug flow, slug flow and annular flow) are recognized. The comparison between the present experimental intermittent/annular transition lines and five transition lines from macroscale and microscale flow pattern maps available in the literature is presented. Finally, the influence of the flow pattern on the heat transfer coefficient is highlighted.