I have been told that for the case of two phase/single phase mixed flow there is always problem with the numerical treatment of the fluxes at the interface. I have used the upwind scheme and have problems for the two phase/single phase interface (where we have phase change from single phase to two phase, such as using the flash calculation). Did anyone had the same time of problem in the past and if so how did you managed to solve it?

have you thought of solving Riemann problem from which you can use upwind schemes !