Flow boiling heat transfer, dewetting-rewetting, and dryout visualization of HFOs in an asymmetrically heated rectangular plain channel

This paper presents the experimental measurements conducted during liquid and flow boiling heat transfer of R1234ze(E) and R1234yf in an asymmetrically heated rectangular plain channel. The tests were run by varying the refrigerant mass velocity from 50 to 200 kg m-2 s-1, the vapour quality from 0.2 to 1, and the heat flux between 50 and 100 kW m-2, at a constant saturation temperature of 30 °C. The results show that the nucleate boiling dominates the phase change process; furthermore, R1234yf shows higher two-phase heat transfer coefficients as compared to R1234ze(E), at the same operating test conditions. The developed measurement technique also permitted to estimate the vapour quality at the onset of the dryout. Furthermore, the direct visual observation of the boiling process allowed for a description of the phase-change and thermal crisis phenomena, including a detailed analysis of the dewetting rewetting phenomenon.