Influence Of Real Gas Effects Research Articles

Numerical simulation of shock attenuation with real gas effects and a turbulent boundary layer in the expansion tube

AbstractThe influence of real gas effects and a turbulent boundary layer on shock wave attenuation in the expansion tube is studied by numerically solving the axisymmetric compressible Navier–Stokes equations with an adaptive mesh refinement technique. Numerical simulation results reveal that the ideal gas assumption is not applicable to the expansion tube, and the turbulent boundary layer plays a major role in decreasing the shock wave speed in the acceleration tube of the expansion tube. Shock wave attenuation is attributed to the turbulent boundary layer decreasing the pressure behind the shock wave. The numerical simulations that include the real gas effects and the development of turbulent boundary layers qualitatively agree with analytical solutions in the shock tube, and they show good agreement with the experimental results, especially for the shock speed in the acceleration tube of the expansion tube. Both effects should be considered in the numerical simulation model aimed to support experiments in expansion tubes.

Influence of real gas effects on shockwaves in shock tube

Influence of real gas effects on shockwaves in shock tube

Real Gas Corrections Based on the Interaction of One-Dimensional Unsteady Waves in a Shock Tube

There are multiple unsteady wave system interactions such as shock waves, expansion waves, and reflection, transmission, and intersection of the contact surface inside a shock tube. Studying the law of unsteady wave system interaction has important guiding significance for the design of gas wave equipment. This paper proposes a solution method for the interaction law of unsteady wave systems based on real gas equation of state (EoS) and applies it to shock tubes. The results show that the temperature maximum error between the code based on ideal gas EoS and simulation results is 3.64%, and the error of wave velocity results is between 2.99% and 18.27%. While the error of temperature results between code based on real gas EoS and simulation is not exceed 3%. More importantly, the wave velocity calculated by the code based on real gas EoS is pretty consistent with the simulation results. The code based on the real gas model can help to solve the problem of offset design point. Research has also shown that as pressure and temperature gradually increase, the deviation between ideal and real gases increases, which also proves that the influence of real gas effects is becoming increasingly significant.

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 lookup 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.

The Effect of Gas Nonideality on the Interface Reflectivity When Interacting With a Shock Wave

The phenomenon of shock wave refraction on a gaseous interface with plasma was analyzed for nonideal gas effects. The developed model was applied to nondissociating diatomic nitrogen gas at 3000 K. Comparatively to ideal gas, the interface reflectivity was found noticeably decreased resulting in 12% more intense shock transmission. The transition point determining the character of reflected wave was also found shifted with its coordinate being strongly dependent on incident shock strength and sort of gas. It was found that a delay in gas pressure drop and a sharp reduction of vorticity generated in the postshock flow are possible. The influence of real-gas effects on the shock instability locus, interface distortion, and wave drag reduction is discussed.

Numerical study of chemically reacting flow in a shock tube using a high-order point-implicit scheme

The shock wave/boundary-layer interaction of chemically reacting flow in a shock tube is studied using a high-order point-implicit solver. The solver employs a high-resolution weighted essentially non-oscillatory (WENO) scheme to capture the complex shock structures, together with a point-implicit method to overcome the stiffness of the chemical production term in the multicomponent Navier–Stokes equations. The numerical code is carefully validated with three benchmark tests, which demonstrates the robustness and good performance of the combined numerical methods. The unsteady interaction process between the shock wave and boundary layer in a two-dimensional shock tube is clearly captured with detailed flow patterns in the simulation. Simulation results show that regular vortex arrangements appear in the flow field for the case of Mach number 2.37, while for the case of Mach number 3.15, the vortex structures break up and chemical nonequilibrium effects become apparent. The influence of real gas effects on shock wave/boundary-layer interaction is further identified on the temperature field and triple point trajectory.

Solutions of supercritical CO2 flow through a convergent-divergent nozzle with real gas effects

Supercritical CO2 (S-CO2) has a density as high as that of its liquid phase while the viscosity remains closer to its gaseous phase. S-CO2 has the potential to be used as a working fluid in compressor as it requires much less work due to its low compressibility as well as relatively small flow resistance. Besides the material properties and design calculations, the thermodynamic properties of working gases play a vital role in the design and efficiency of various machinery such as compressors, turbines, etc. Equation of state (EOS) which accounts the real gas effects is used in computational analysis to estimates the thermodynamic properties of the working fluid. Each EOS gives priority to real gas effects phenomena differently and for the analysis of the fluid dynamic machinery, as of today, there is no standard procedure for selecting a suitable equation of state (EOS). To understand the influence of real gas effects on the formation of a shock wave, S-CO2 flow through a convergent-divergent nozzle, is theoretically analyzed. The thermophysical and transport properties calculated with the six different equation of state (EOS) are used to estimate the flow characteristics.An in-house code based on the real gas approach which solves gas dynamics equations with variable gas properties is used to analytically analyze the compressible flow of S-CO2 through nozzle. The solution for the shock strength, total pressure loss, and location of the normal shock wave at different back pressure conditions is obtained. Using Becker’s solutions with varying viscosity and thermal conductivity estimated from each EOS, the properties inside the normal shock wave are calculated. Numerical results of supersonic S-CO2 flow varies with different EOS candidates and the formation of shock wave is found to be significantly influenced by the real gas effects.

The influence of real gas effects on the performance of supercritical CO2 dry gas seals

The current work investigates dry gas seals operation with supercritical CO2 (sCO2) at two operating conditions using CFD. One close and one far from the critical point. At the operating condition far from the critical point a maximum change of 1.7% in pressure, 0.4% in temperature and 1.1% in density are observed. Contrary, closer to the critical point a maximum change of 6.5% in pressure, 6.7% in temperature and 39.5% in density are observed. These changes also influence opening force and leakage rate. Far from the critical point the maximum changes are 0.7% and 3.1%, whereas close to the critical point maximum changes of 3.4% and 10.3% are observed. The centrifugal effect plays an important role when operating with dense gases.

Numerical Study of Shock/Boundary Layer Interaction of Chemically Reacting Flow in Shock Tube

In this paper, a high-order Navier-Stokes solver is developed for chemically reacting flow and is applied to investigate the unsteady, viscous interaction between shock wave and boundary layer in an air filled shock tube. The solver employs a high- resolution weighted essentially non-oscillatory (WENO) scheme to capture the complex flow pattern induced by shock/boundary- layer interaction, together with a point-implicit method to overcome the stiffness of chemical production term. The results show the influence of real gas effects and give insights into the mechanism of shock/boundary-layer interaction process.

High-enthalpy flow over a rearward-facing step – a computational study

AbstractHypersonic, high-enthalpy flow over a rearward-facing step has been numerically investigated using computational fluid dynamics (CFD). Two conditions relevant to suborbital and superorbital flow with total specific enthalpies of$26$and$50~\mathrm{MJ} ~{\mathrm{kg} }^{\ensuremath{-} 1} $, are considered. The Mach number and unit Reynolds number per metre were 7.6, 11.0 and$1. 82\ensuremath{\times} 1{0}^{6} $,$6. 23\ensuremath{\times} 1{0}^{5} $respectively. The Reynolds number based on the step height was correspondingly$3. 64\ensuremath{\times} 1{0}^{3} $and$12. 5\ensuremath{\times} 1{0}^{2} $. The computations were carried out assuming the flow to be laminar throughout and the real gas effects such as thermal and chemical non-equilibrium are studied using Park’s two-temperature model with finite-rate chemistry and Gupta’s finite-rate chemistry models. In the close vicinity of the step, detailed quantification of flow features is emphasised. In particular, the presence of the Goldstein singularity at the lip and separation on the face of the step have been elucidated. Within the separated region and downstream of reattachment, the influence of real gas effects has been identified and shown to be negligible. The numerical results are compared with the available experimental data of surface heat flux downstream of the step and reasonable agreement is shown up to 30 step heights downstream.

Influence of real gas effects on ablative Rayleigh-Taylor instability in plastic target

In this research, real gas effects on ablative Rayleigh-Taylor instability are investigated in a plastic target. The real gas effects are included by adopting the quotidian equation of state (QEOS) model. Theoretical solutions for both QEOS and ideal gas EOS are obtained and compared, based on a same set of ablation parameters. It is found that when real gas effects are considered, the density gradient becomes less steep than that of ideal gas assumption, even though this cannot be used directly to draw a stabilization conclusion for the real gas effects. Further analysis shows that when real gas effects are considered, lower ∂p/∂T in the dense shell region has the effect of stabilization, whereas the dependence of the internal energy on the density, lower specific heat (at constant volume) in the dense shell region, and higher specific heat in the low-density ablation region contribute to stronger destabilization effects. Overall, when real gas effects are considered, the destabilization effects are dominant for long wavelength perturbations, and the growth rates become much higher than the results of ideal gas assumption. In our specific case, the maximum relative error reaches 18%.

Boundary Layers on a Flat Plate at Sub- and Superorbital Speeds

The heat flux and boundary layers over a flat plate at suborbital and superorbital speeds were measured in a freepiston expansion tube at enthalpies of 26.0 and 53.4 MJ/kg, respectively. Estimates of the gas-phase and surface Damkohler numbers were made that indicate that although the boundary layer might be frozen, there is a possibility of surface reactions occurring. The heat-flux results were compared with theoretical predictions of heat flux for both frozen and equilibrium chemistries. The results indicated that the influence of real gas effects, such as recombination and surface catalycity, were minimal for the present flow conditions. Interferograms were obtained using resonance enhancement of the thermal boundary layer over a flat plate. Comparison of the measured boundary-layer thickness with several expressions used to predict the boundary-layer thickness allowed an evaluation of their effectiveness at high enthalpies.

The influence of non-equilibrium dissociation on the flow produced by shock impingement on a blunt body

We describe an investigation of the effects of non-equilibrium thermochemistry on the interaction between a weak oblique shock and the strong bow shock formed by a blunt body in hypersonic flow. This type of shock-on-shock interaction, also known as an Edney type IV interaction, causes locally intense enhancement of the surface heat transfer rate. A supersonic jet is formed by the nonlinear interaction that occurs between the two shock waves and elevated heat transfer rates and surface pressures are produced by the impingement of the supersonic jet on the body. The current paper is motivated by previous studies suggesting that real gas effects would significantly increase the severity of the phenomenon. Experiments are described in which a free-piston shock tunnel is used to produce shock interaction flows with significant gas dissociation. Surprisingly, the data that are obtained show no significant stagnation enthalpy dependence of the ratio of the peak heat transfer rates with and without shock interaction, in contrast to existing belief. The geometry investigated is the nominally two-dimensional flow about a cylinder with coplanar impinging shock wave. Holographic interferometry is used to visualize the flow field and to quantify increases in the stagnation density caused by shock interaction. Time-resolved heat transfer measurements are obtained from surface junction thermocouples about the model forebody. An improved model is developed to elucidate the finite-rate thermochemical processes occurring in the interaction region. It is shown that severe heat transfer intensification is a result of a jet shock structure that minimizes the entropy rise of the supersonic jet fluid whereas strong thermochemical effects are promoted by conditions that maximize the entropy rise (and hence temperature). This dichotomy underlies the smaller than anticipated influence of real gas effects on the heat transfer intensification. The model accurately predicts the measured heat transfer rates. Improved understanding of the influence of real gas effects on the shock interaction phenomenon reduces a significant element of risk in the design of hypersonic vehicles. The peak heat transfer rate for the Edney type IV interaction is shown to be well-correlated, in the weak impinging shock regime, by an expression of the form [equation] for use in practical design calculations.

Numerical simulation of fuel sprays at high ambient pressure: the influence of real gas effects and gas solubility on droplet vaporisation

This paper deals with the numerical simulation of the vaporisation of an unsteady fuel spray at high ambient temperature and pressure solving the appropriate conservation equations. The extended droplet vaporisation model accounts for the effects of non-ideal droplet evaporation and gas solubility including the diffusion of heat and species within fuel droplets. To account for high-temperature and high-pressure conditions, the fuel properties and the phase boundary conditions are calculated by an equation of state and the liquid/vapour equilibrium is estimated from fugacities. Calculations for an unsteady diesel-like spray were performed for a gas temperature of 800 K and a pressure of 5 MPa and compared to experimental results for droplet velocities and diameter distribution. The spray model is based on an Eulerian/Lagrangian approach. The comparison shows that the differences between the various spray models are pronounced for single droplets. For droplet sprays the droplet diameter distribution is more influenced by secondary break-up and droplet coagulation.

Nonequilibrium Effects in Near-Wake Ionizing Flows

An analysis of thermal and chemical nonequilibrium effects in near-wake ionizing flows is presented. The influence of real-gas effects on the establishment of scaling laws of the near-wake properties is analyzed, and a simplified model, which relies on an idealized flow solver still capable of accounting for dissociation and ionization effects, is developed.

Real Gas Corrections in Shock Tube Studies at High Pressures

AbstractReaction kinetics studies at high pressure in shock tubes can be significantly affected by the influence of real gas effects on state variables. The effect on the temperature, pressure, and density can be calculated by solving the shock wave equations using real gas equations of state (EOS). We present a method to solve the shock wave equations in a straightforward way by taking advantage of the recently‐developed real gas properties package by Schmitt et al. (U. of Iowa Rep. UIME PPB 93–006, 1994). The solutions of these equations for shock waves in pure argon are presented. We find that the reflected shock temperatures and densities can be significantly different from those predicted by using an ideal gas EOS. Using the Peng‐Robinson EOS, for example, the calculated real gas reflected shock temperature is less than the ideal gas temperature by 83 K per 1000 atm. An illustrative example of the effect of real gas corrections on a rate coefficient determination is also presented.

Influence of real gas effects on the hypersonic rarefied flow near the sharp leading edge of a thin plate

In this paper are presented the results of an investigation of the induced pressure distribution on a thin plate with a sharp leading edge placed in supersonic rarefaction flow. The research was carried out in the low-density wind tunnel under the following conditions in the incident flow 5·7 ⩽ M ⩽ 14, 30 ⩽ Re ∞/mm ⩽ 600. The results obtained show that as one approaches sufficiently closely to the leading edge, the strong interaction theory no longer agrees with the experimental data and as χ = [(μ 3 ∞√C ∞)/√Re x∞ increases, the induced pressure on the plate approaches a constant value starting with χ> χ lim , where χ lim depends upon the Mach number. For example, χ lim is similar to 8 at the Mach number 5·7. The most characteristic of the induced pressure distribution is the deviation of the induced pressure from the strong interaction theory as rarefaction increases. There is a strong decrease in Δ P/ χP ∞ for [( μ ∞√ C ∞)/√ Re x∞ ] > 0·2.