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.
Description:
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.