Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results

Reciprocating-piston devices can be used as high-efficiency compressors and/or expanders. With an optimal valve design and by carefully adjusting valve timing, pressure losses during intake and exhaust can be largely reduced. The main loss mechanism in reciprocating devices is then the thermal irreversibility due to the unsteady heat transfer between the compressed/expanded gas and the surrounding cylinder walls. In this paper, pressure, volume and temperature measurements in a piston-cylinder crankshaft driven gas spring are compared to numerical results. The experimental apparatus experiences mass leakage while the CFD code predicts heat transfer in an ideal closed gas spring. Comparison of experimental and numerical results allows one to better understand the loss mechanisms in play. Heat and mass losses in the experiment are decoupled and the system losses are calculated over a range of frequencies. As expected, compression and expansion approach adiabatic processes for higher frequencies, resulting in higher efficiency. The objective of this study is to observe and explain the discrepancies obtained between the computational and experimental results and to propose further steps to improve the analysis of the loss mechanisms

Simulation of thermally induced thermodynamic losses in reciprocating compressors and expanders: Influence of real-gas effects

The efficiency of positive-displacement components is of prime importance in determining the overall performance of a variety of thermodynamic systems. Losses due to the unsteady thermal-energy exchange between the working fluid and the solid walls of the device are an important loss mechanism. In this work, heat transfer in gas-spring devices is investigated numerically in order to focus explicitly on these thermodynamic losses. The specific aim of the study is to investigate the behaviour of real gases in gas springs and compare this to that of ideal gases in order to understand the impact of real-gas effects on the thermally induced losses in reciprocating expanders and compressors. This work relates these losses to the fluid properties and quantifies the influence of the thermophysical models applied. A CFD-model of a gas spring is developed in OpenFOAM. Four different fluid models are compared: (i) a perfect-gas model (i.e., an ideal-gas model with constant thermodynamic and transport properties); (ii) an ideal-gas model with temperature-dependent properties; (iii) a real-gas model using the Peng-Robinson equation-of-state with temperature and density-dependent properties; and (iv) a real-gas model using gas-property tables to interpolate values of thermodynamic and transport properties as functions of temperature and pressure. 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 temperature and pressure (by as much as 20%) compared to the real-gas model. A real-gas model that uses the Peng-Robinson equation of state underestimates the pressure relative to the more accurate model based on look-up tables by as much as 10%. Furthermore, both ideal-gas and Peng-Robinson models underestimate the thermally induced loss compared to the table-based model for heavier gases. Different alkanes and alkane mixtures are also compared. It is found that, for a fixed volume ratio, pure and mixed alkanes that exhibit a higher heat capacity incur lower losses due to the lower temperature amplitudes, and thus, lower heat transfer occurring in the gas spring. For example, propane, which has a heat capacity only half of hexane, exhibits a loss of 5.1% (defined as the ratio of the net cyclic heat loss to the compression work), while the loss with hexane amounts to 3.6% (in both cases for a volume ratio of 6.63). Real-gas effects play an increasing role for heavier alkanes because the critical temperature and pressure are lower. The thermodynamic state of the gas is close to the critical point where real-gas effects are very prevalent. Finally, mixtures exhibit losses which lie between the value of their respective pure fluids, whereby increasing the proportion of the pure substance with the higher loss also leads to a higher loss for the mixture

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

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

The Influence of Real Gases Effects on Thermal Losses in Reciprocating Piston-Cylinder Systems

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

Augmented Reality Based on Fast Deformable 2D-3D Registration for Image Guided Surgery

Augmented reality systems (ARS) allow the transparent projection of preoperative CT images onto the physicians view. A significant problem in this context is the registration between the patient and the tomographic images, especially in the case of soft tissue deformation. The basis of our ARS is a volume rendering component on standard PC platform, which allows interactive volumetric deformation as a supplement to the 3D-texture based approaches. The volume is adaptively subdivided into a hierarchy of sub-cubes, each of which is deformed linearly. In order to approximate the Phong illumination model, our system allows pre-calculated gradients to be deformed efficiently. The registration is realized by the introduction of a two-stage procedure. Firstly, we compute a rigid pre-registration by the use of fiducial markers in combination with an electro-magnetic navigation system. The second step accounts for the non-linear deformation. For this purpose, several views of an object are captured and compared with its corresponding synthetic renderings in an optimization method using mutual information as metric. Throughout the experiments with our approach, several tests of the rigid registration has been carried out in a real laparoscopic intervention setup as a supplement to the actual clinical routine. In order to evaluate the non-linear part of the registration, up until now several dummy objects (synthetically deformed datasets) have been successfully examined