On dynamics of flash boiling bubbles within liquid ammonia systems

It is crucially important to understand the dynamics of flash boiling bubbles of liquid fuels in order to achieve optimal fuel spray and mixing performance in realistic combustion engines. In this study, a modified pseudo-potential lattice Boltzmann method was adopted to capture the dynamics of flash boiling bubbles of liquid ammonia. The critical conditions for the flash boiling state of liquid ammonia were determined using isothermal depressurization techniques. Meanwhile, the evolutions of bubble clusters were investigated, allowing for bubble deformation, coalescence, and collapse in different flash boiling stages. The results show that the evolution of flash boiling bubble clusters presents two main stages, i.e., the early rapid expansion stage and the later slow expansion stage, and both stages are highly sensitive to temperature variations. Low-temperature environments can intensify the competition between bubble coalescence and collapse events, thereby changing the density, pressure, and velocity distribution and delaying the transition of bubbles to the eventual equilibrium state. In addition, the evolution of bubble clusters conforms to the Ostwald ripening mechanism, where large bubbles absorb smaller bubbles, promoting the formation and propagation of pressure waves in liquid ammonia. These pressure waves not only disturb the surrounding fluid to form vortices but also cause significant deformation of adjacent bubbles, thereby affecting the overall efficiency of flash boiling processes.