Flashing evaporation under different pressure levels

Flashing evaporation of water inside a vertical pipe under four different system pressures is investigated by mono-dispersed CFD simulations. Phase change is assumed to be caused only by thermal heat transfer. The available thermal energy decreases rapidly with the increase of pressure level. Under low-pressure conditions, the maximum steam volume fraction is over-predicted, while an overestimation is observed in high-pressure cases. The possible explanation for the deviation is that bubble growth during the evaporation is especially significant in low-pressure cases, which reduces the evaporation rate but is not considered in the simulation. Furthermore, as the system pressure increases the contribution of nucleation to the total steam generation rate becomes large, which is confirmed by the experimental data. In addition, the effect of pressure on the critical bubble diameter that succeeds in starting flashing as well as the nucleation mechanism is discussed.