CFD Analysis of Thermally Induced Thermodynamic Loses in the Reciprocating Compression and Expansion of Real Gases

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Title: CFD Analysis of Thermally Induced Thermodynamic Loses in the Reciprocating Compression and Expansion of Real Gases
Authors: Taleb, AI
Sapin, PMC
Barfuß, C
Fabris, D
Markides, CN
Item Type: Conference Paper
Abstract: 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.
Issue Date: 7-Apr-2017
Date of Acceptance: 20-Oct-2016
ISSN: 1742-6588
Publisher: IOP Publishing: Conference Series
Journal / Book Title: Journal of Physics: Conference Series
Volume: 821
Issue: 012016
Copyright Statement: © 2017 The Authors. Published under licence by IOP Publishing Ltd. Content from this work may be used under the terms of theCreative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Sponsor/Funder: Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/J006041/1
Conference Name: 1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion and Power
Keywords: 02 Physical Sciences
09 Engineering
Publication Status: Published
Start Date: 2016-10-20
Finish Date: 2016-10-21
Conference Place: Vareena, Italy
Appears in Collections:Faculty of Engineering
Chemical Engineering

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