Cfd simulation of water evaporation under pressure release transients

Water evaporation (flashing) inside a vertical pipe under pressure release transients is investigated by CFD simulations based on the Eulerian two-fluid model. Important assumptions made in the current work are: a) constant bubble diameter; b) no nucleation and c) mass transfer caused only by thermal heat transfer between phases. The predicted evolution of steam volume fraction and water temperature is compared with the measured one, respectively. The results show that the applied model setup in principle can reproduce the flashing process. Whether the evaporation of superheated water can start depends on the initial guess of steam volume fraction and especially the presumed bubble size. A large initial steam volume fraction and a small bubble size will favor the inception of evaporation. Once the flashing is successfully triggered, the maximum evaporated steam is determined by the pressure drop ultimately. Furthermore, the increase of steam volume fraction per unit evaporated mass reduces drastically at high pressure levels due to a large steam to water density ratio. As a result, a relatively small value has to be presumed for bubble size under high pressure conditions in order to launch the evaporation. In addition, the predicted maximum steam volume fraction at a pressure of 65bar is much lower than the measured one. In this case besides the interphase thermal heat transfer consideration of nucleation and bubble growth due to mechanical non-equilibrium across the interface might be helpful.