Two-phase expansion can be defined by the expansion of a working fluid from a high to a low pressure while during this process vapour and liquid coexist (i.e. pass the two-phase region). Two-phase expansion is seen in various applications and in many cases it allows increasing the overall efficiency of the system. For example, the use of a two-phase expander instead of an expansion valve allows increasing the efficiency of vapour compression cycles (e.g. refrigeration or heat pump units). Another application is found in LNG liquefaction plants where Joule Thompson valves are replaced with two-phase expanders that generate work and increase the overall process efficiency. Among several other examples, also low-temperature heat-to-power cycles benefit from two-phase expansion. By omitting full evaporation of the working fluid in an (organic) Rankine cycle (ORC), a higher power output can be achieved. The key challenge in this type of cycles, frequently called trilateral cycles (TLC), is the availability of two-phase expanders. The adiabatic efficiency of these expanders should be kept high in order to achieve the improved cycle performance. Only in the last decade, the research community has started its effort in investigating two-phase expanders. Currently, all existing numerical models for predicting two-phase expander performance are lumped 0D models. These models are only valid under strict constraints like identical geometry, identical working fluids and identical operating conditions. The various assumptions made in these models and the fundamental physics of the flashing process are never verified on actual experiments. This project has as aim investigating the fundamentals driving the two phase-expansion process. First, a phenomenological approach is pursued on a novel test-rig to be constructed. The data is subsequently used in the development of a new two-phase expansion model.