Experimental and Numerical Analysis of the Valve Timing Effects on the Performances of a Small Volumetric Rotary Expansion Device

Single stage expansion devices are currently studied for small scale size power plant, often in combination with Organic Rankine Cycles for the employment of solar, geothermal, biomass or waste heat energies. A volumetric rotary single-stage expander was chosen in this study as expansion device for such type of plants. Its main characteristics and performances are discussed as a function of both the working conditions (fluid type, inlet temperature) and the working parameters (rotating speed, admission and recompression grades, valves advance). These analyses were carried out with numerical and experimental techniques. The analysis of the effects of the working condi- tions on the expander performances was carried out through a numerical model created with the simulation tool AMESim. At the same time, a prototype was built and experimented with compressed air to validate the model used by means of air mass flow rate, torque and indicated cycle. This way the isentropic and mechanical efficiency are discussed. The validation of the model was carried out by comparison with the experimental data collected at the engine test bench by operating the engine with compressed air. The indicated cycle, the air mass flow rate and the delivered torque were used as parameters of comparison. Moreover an experimental analysis at the fluid dynamic bench was carried out to validate the numerical 3D CFD model of the valves. The part load performances of this expansion device were studied by hypothesizing different control strategies and comparing them in terms of efficiency reduction respect to the design point. The influence of valves advance was also discussed.