Lumped dynamic analysis and design of a high-performance reciprocating-piston expander

© 2017 IMEKO A spatially-lumped dynamic model of a reciprocating-piston expander is presented in this paper. The model accounts for the three main loss mechanisms in realistic piston machines, namely: pressure losses through the intake and exhaust valves, heat transfer between the gas and the surrounding cylinder walls, and the mass leakage between the compression/expansion chamber and the crankcase through the piston rings. The model also accounts for real-gas effects with the fluid properties calculated from the NIST database using REFPROP. The numerical calculations are first compared with experimental pressure-volume-temperature data obtained on a custom reciprocating-piston gas spring over a range of oscillation frequencies. The comparison between numerical and experimental results shows good agreement. It also allows the most accurate heat transfer correlation to be selected for calculating the gas-to-wall in-cylinder heat transfer. The semi-heuristic modelling tool is then used to design an expander for specific pressure ratios and mass flowrate, and to predict the thermodynamic performance of the piston device over a range of part-load conditions.