Numerical and experimental evaluation of flow boiling heat transfer in microchannels for R452B refrigerant

Energy management efficiency is crucial for the increasingly severe global energy challenges. Low global warming potential (GWP) refrigerant R452B used in air conditioning and refrigeration systems could reduce the global warming effect for applications. The objective of this paper is to conduct the numerical and experimental studies for examining the thermal fluid behaviors of flow boiling evolution of refrigerant R452B in the vertical microchannels. In the experimental approach, a gear pump and a programmable DC power supply are employed to regulate the mass flux and heat flux for determining the heat transfer and pressure drop outcomes across the microchannel at varied vapor qualities. A high-speed camera with a LED fiber optical light source is used to observe the close-up optical images over the complex flow boiling process. Theoretically, the volume-of- fluid (VOF) method built in the computational fluid dynamics (CFD) software ANSYS/Fluent® is employed to simulate the progression of bubble nucleation processes for resolving the distributions of velocity, pressure, temperature and liquid volume fraction in the microchannel. The predictions are compared against the measured heat transfer coefficients and pressure drops for the CFD model validation. The time sequences of vapor volume fraction contours are also simulated to characterize the evolving vapor-liquid interfaces for better grasping the detailed behaviors of nucleation, growth, departure, coalescence of bubbles and the transformations of dominant flow patterns. The measured results estimate the average heat transfer coefficients and pressure drops up to 14.1 kW/m²K and 76.5 kPa at a mass flux of 600 kg/m²s in the microchannel. This research further conducts the performance assessments by testing the popular correlations, and thereby reveals the effectiveness of Bertsch as well as Sun and Mishima correlations to reasonably calculate the heat transfer coefficients and pressure drops of R452B refrigerant, involving the mean absolute errors of 12.5 % and 22.6 %, respectively.