Experimental Analysis of Quasi-Local Non-Equilibrium Boiling Heat Transfer With R1234ze(E) in a Plate Heat Exchanger

Prediction methods play a critical role in the design and modeling of heat exchangers. The majority of the traditional prediction methods were developed based on the equilibrium theory. Despite the fact that two-phase flow exists in the subcooled boiling and superheated boiling regions, traditional prediction methods consider that single-phase heat transfer takes place in these regions. However, nucleate boiling occurs before the saturation liquid point, and droplets still exist after the saturation vapor point. The heat transfer coefficients in these regions, where latent heat transfer processes take place, are significantly higher than those predicted by single-phase prediction methods. This paper presents an experimental analysis of the boiling heat transfer in a gasketed plate heat exchanger with the low global warming potential working fluid R1234ze(E), considering the effects of the non-equilibrium phenomena. The quasi-local heat transfer coefficients were obtained in a test section consisting of 8 segments along the flow channels. On the boundary between the segments, two thermocouples are mounted at the same horizontal level to measure the local temperature of the plate wall and the secondary fluid. The experiments were conducted at different operating conditions. The mass flux ranged from 9.7 kg·m^-2·s^-1 to 15.6 kg·m^-2·s^-1. The boiling pressure varied between 1.2 MPa and 1.3 MPa. The inlet subcooling degree varied from 13 °C to 31 °C. The temperature profiles of the plate wall on the refrigerant side, and the refrigerant and oil along the flow channels are presented. The relationships between the heat transfer coefficient and critical parameters, including the equilibrium thermodynamic quality, subcooling degree, and mass flux were investigated. The experimental results indicate that the subcooled boiling occurs when the wall temperature reaches around 2.3 oC above the saturation temperature of the working fluid, and the wall temperature remains essentially constant in the subcooled boiling region. Moreover, the results suggest that more than 24 % of the heat transfer area in the evaporator is occupied by subcooled boiling at the operating conditions of high inlet subcooling degree and large mass flux. Therefore, it is essential to consider the subcooled boiling region at these operating conditions for an evaporation process including inlet subcooling.