The influence of real gas effects on thermally induced losses in reciprocating piston-cylinder systems

Abstract

The efficiency of expanders is of prime importance for various clean energy technologies. Once mechanical losses (e.g. through valves) are minimized, losses due to unsteady heat exchange between the working fluid and the solid walls of the containing device can become the dominant loss mechanism. In this device, gas spring devices are investigated numerically in order to focus explicitly on the thermodynamic losses that arise due to this unsteady heat transfer. The specific aim of this study is to investigate the behaviour of real gases in gas springs and compare this to that of ideal gases in order to attain a better understanding of the impact of real gas effects on the thermally losses in reciprocating piston expanders and compressors. A CFD-model of a gas spring is developed in OpenFOAM. Three different gas models are compared: an ideal gas model with constant thermodynamic and transport properties; an ideal gas model with temperature-dependent properties; and a real gas model using the Peng-Robinson equation of state with temperature and pressuredependent properties. Results indicate that, for simple, monoand diatomic gases like helium or nitrogen, there is a negligible difference in the pressure and temperature oscillations over a cycle between the ideal and real gas models. However, when looking at a heavier (organic) molecule such as propane, the ideal gas model tends to overestimate the temperature and pressure compared to the real gas model, especially if no temperature dependency of thermodynamic properties is taken into account. Additionally, the ideal gas model (both alternatives) underestimates the thermally induced loss compared to the real gas model for heavier gases. Real gas effects must be taken into account in order to predict accurately the thermally induced loss when using heavy molecules in such devices.