Numerical investigations on flow boiling heat transfer of ammonia water binary solution (NH3/H2O) in a horizontal microchannel

The flow boiling heat transfer characteristics of NH3/H2O mixture in a 2D single horizontal microchannel (0.4 mm width x 6 mm length) were investigated by Computational Fluid Dynamics (CFD) method. The multiphase VOF model and modified phase change Lee method were adopted to address the nonisothermal phase change process of the flowing zeotropic NH3/H2O mixture, while the variations of the binary mixture thermophysical properties were also taken into account. The effects of mass flux (46 similar to 552 kg/(m(2).K)), inlet NH3 concentration (0-35% by mole) and heating wall temperature (20.5 similar to 70 degrees C) on the overall and local flow boiling heat transfer performance have been comparatively evaluated under constant heating wall temperatures. According to the numerical results, the heat dissipation rate of NH3/H2O mixture flow boiling could reach up to 1.41 MW/m(2) at a mass flux of 552 kg/(m(2).s), which was 2.05 times of water single-phase flow cooling under a same constant heating wall temperature of 50 degrees C. It was also revealed that, for NH3/H2O mixture flow boiling in the microchannel, there was a threshold of inlet NH3 concentration to maintain a certain level of heat dissipation rate at a given mass flow rate and further increasing the inlet NH3 concentration would no longer benefit the heat dissipation process. Furthermore, there were no local dry-outs found throughout the whole microchannel length under all the simulation conditions in this study, which could be attributed to the unique flow boiling behaviors of zeotropic NH3/H2O mixture. Therefore, it should be noticed that NH3/H2O mixture, under certain conditions, could be a good alternative coolant for preventing local dry-outs and maintaining a certain functional temperature of electronic components. (C) 2021 Elsevier Ltd. All rights reserved.