Two-phase expansion is seen in various applications and in many cases, it allows increasing the overall efficiency of the system. For example organic Rankine cycles (ORC), a mature technology to convert low temperature heat to electricity, can benefit from this. The modified cycle, also called a Trilateral cycle (TLC), can boost the power output up to 30%. The main challenge however is the development of high efficiency expanders that can cope with liquid-vapour mixtures (i.e two-phase mixtures) at the inlet and during expansion. Volumetric expanders are considered suitable, yet few results are presented up to now. Experimental results are scarce and there is no model that can predict the performance of the expansion process. This is mainly due to the knowledge gap on the fundamental aspects of two-phase expansion and the non-equilibrium effects. In this research proposal, the expansion process in an optical accessible variable volume chamber will be experimentally measured. A model of the two-phase expansion is proposed, specifically focussing on the interfacial effects between the liquid-vapour phase. Different working fluids and velocity profiles of the expander are considered. The new model will be implemented in full cycle simulations to better assess the performance and optimization potential.