Numerical and experimental investigation on thermal-hydraulic characteristics of a scroll expander for organic Rankine cycle

A new numerical model of a scroll expander is developed applying the deterministic method to analyze its thermal-hydraulic characteristics for the application to the organic Rankine cycle. Several geometrical models are refined as a subsection function of the orbiting angle and area models regarding suction and tangential gas force are newly developed. The geometrical models match well with the actual values extracted from the 3D computer-aided design model. In addition, mathematical models that reflect the effects of mass leakage and heat transfer are integrated in a closed-loop iteration process. Two critical unknown parameters of overall leakage clearance and wall-to-gas temperature ratio are identified via the coupling of the simulation model and experimental data. Experiments were conducted on a 1 kW scale organic Rankine cycle test rig using R245fa as the working fluid. The identified parameters are expressed as functions of the operating conditions and the simulation model is validated within 5% error range. The best isentropic efficiency appeared to be 62.8% according to the test results of the scroll expander. The simulation model allows the analysis of various aspects of the internal expansion process that could not be examined through empirical or semi-empirical models. Mechanisms regarding under and over-expansion and heat transfer are thoroughly investigated for profound understanding of the scroll expander transient behavior and energy level degradation.