Non equilibrium two phase expansion in a piston expander plays a key role in the future development of a.o. Organic Rankine Cycles. The present  project aims to predict the expansion work based on the initial quality, the expansion rate and the setup geometry. A secondary goal is to correctly model the boiling mechanism and link this to the lower order homogeneous relaxation model. A volume of fluid (VOF) approach can be used when boiling on the interface is dominant, however the approach is ill suited when boiling in the liquid volume occurs. Previous experimental two-phase expansion experiments suggests that boiling on the free surface is dominant for ethanol while boiling in the liquid volume is dominant for water expansion. This project aims at developing the methods required to simulate two phase expansion over a broad range of operating conditions thus combining evaporation on the liquid surface and in the liquid bulk.