The efficiency of expanders is of prime importance in determining the overall
performance of a variety of thermodynamic power systems, with reciprocating-piston expanders
favoured at intermediate-scales of application (typically 10–100 kW). Once the mechanical losses
in reciprocating machines are minimized (e.g. through careful valve design and operation), losses
due to the unsteady thermal-energy exchange between the working fluid and the solid walls of
the containing device can become the dominant loss mechanism. In this work, 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 the study is to investigate the behaviour
of real gases in gas springs and to compare this to that of ideal gases in order to attain a better
understanding of the impact of real-gas effects on the thermally induced losses in reciprocating
expanders and compressors. A CFD-model of a gas spring is developed in OpenFOAM. Three
different fluid models are compared: (1) an ideal-gas model with constant thermodynamic
and transport properties; (2) an ideal-gas model with temperature-dependent properties; and
(3) a real-gas model using the Peng-Robinson equation-of-state with temperature and pressure-
dependent properties. Results indicate that, for simple, mono- and 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 considering heavier (organic) molecules,
such as propane, the ideal-gas model tends to overestimate the pressure compared to the real-gas
model, especially if the temperature and pressure dependency of the thermodynamic properties
is not taken into account. In fact, the ideal-gas model predicts higher pressures by as much as
25% (compared to the real-gas model). Additionally, both ideal-gas models underestimate the
thermally induced loss compared to the real-gas model for heavier gases. This discrepancy is
most pronounced at rotational speeds where the losses are highest. The real-gas model predicts
a peak loss of 8.9% of the compression work, while the ideal-gas model predicts a peak loss of
5.7%. These differences in the work loss are due to the fact that the gas behaves less ideally
during expansion than during compression, with the compressibility factor being lower during
compression. This behaviour cannot be captured with the ideal-gas law. It is concluded that
real-gas effects must be taken into account in order to predict accurately the thermally induced
loss mechanism when using heavy fluid molecules in such devices.