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 × 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 non-isothermal 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 (46~552 kg/(m²∙K)), inlet NH₃ concentration (0-35% by mole) and heating wall temperature (20.5~70 ^∘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/m² at a mass flux of 552 kg/(m²∙s), which was 2.05 times of water single-phase flow cooling under a same constant heating wall temperature of 50 ^∘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.