Effect of gas equation of state on CFD predictions for ignition characteristics of hydrogen escaping from a tank

This work involves the investigation of the sensitivity of computational fluid dynamics based models of auto ignition of hydrogen gas escaping into the surroundings to the use of an ideal gas and a real gas Noble–Abel equation of state. Ensuring consistent modeling techniques when the real gas equation of state is implemented, real gas based thermodynamic properties, real gas based property mixture models, and real gas based chemical equilibrium constant formulations are utilized. Within the standard computational fluid dynamics models, a customized chemical kinetic equation integrator is employed. An LES based turbulence model is implemented. For tank pressures of 40, 80, and 120 MPa, differences in the gas conditions, including gas pressures, temperatures, velocities, flow rates, energy, and chemical species mass fractions, are compared. The relationships between the local and time varying gas conditions, chemical reaction indicators, the tank pressure, and the equation of state captured in the simulations are described in detail. The results clearly show the increasing deviation between the ideal gas and Noble–Abel based results as the tank pressure increases, indicating the importance of the use of the proper material model and chemical equilibrium formulation for the conditions of interest.     

Thermodynamic modeling based on a generalized cubic equation of state for kerosene/LOx rocket combustion

Based on a generalized cubic equation of state proposed by Cismondi and Mollerup [Fluid Phase Equilib. 232 (2005) 74–89], this study derived formulations for the thermodynamic properties of mixtures that are needed for combustion simulations of liquid rocket engines. The present model was validated thoroughly against reference data provided by NIST in order to assess its validity over a wide range of critical compressibility factors, pressures, and temperatures. The numerical results clearly indicate that the present model is superior in its handling of fluid mixtures with quite different critical compressibility factors, while maintaining the advantages of the cubic equation of state compared to those of the Soave–Redlich–Kwong and Peng–Robinson equations of state. In addition, steady flamelet analysis has been performed to investigate the effects of detailed chemical kinetics, real fluid behavior, and increased pressure on the local flame structures of the kerosene surrogate and liquid oxygen relevant to liquid rocket engines.     

Impacts of equations of state (EOS) and impurities on the volume calculation of CO2 mixtures in the applications of CO2 capture and storage (CCS) processes

Volume property is the necessary thermodynamic property in the design and operation of the CO₂ capture and storage system (CCS). Because of their simple structures, cubic equations of state (EOS) are preferable to be applied in predicting volumes for engineering applications. This paper evaluates the reliabilities of seven cubic EOS, including PR, PT, RK, SRK, MPR, MSRK and ISRK for predicting volumes of binary CO₂ mixtures containing CH₄, H₂S, SO₂, Ar and N₂, based on the comparisons with the collected experimental data. Results show that for calculations on the volume properties of binary CO₂ mixtures, PR and PT are generally superior to others for all of the studied mixtures. In addition, it was found that the binary interaction parameter has clear effects on the calculating accuracy of an EOS in the volume calculations of CO₂ mixtures. In order to improve the accuracy, k_ij was calibrated for all of the EOS regarding the gas and liquid phases of all the studied binary CO₂ mixtures, respectively.     

Isentropic analysis and numerical investigation on high-pressure hydrogen jets with real gas effects

The present work performs the isentropic analysis and numerical simulation of high-pressure hydrogen jets to study the hydrogen leakage. The exit parameters and the flow characteristics are studied with the ideal gas assumption and real gas effects. The jet exit parameters calculated by the real gas thermodynamic model are different from the results obtained by the ideal gas assumption at high initial pressure based on the isentropic analysis. The ideal gas and the real gas equation of state results in the differences of Mach disk parameters at high initial pressures. The ideal gas assumption underestimates the Mach disk distance by 8% and overestimates the Mach disk diameter by 15% at the initial pressure of 50 MPa. The exit mass flow rates computed from the isentropic expansion assumption agree well with the numerical simulations. The results show that it is reasonable to evaluate mass flow rates of high-pressure hydrogen jets by the isentropic expansion assumption.     

Thermodynamic equation of state approach for the choice of working fluids of absorption cooling cycles

A methodology is developed for the application of thermodynamic equations of state of fluids and fluid mixtures in evaluating working fluid combinations of absorption cooling cycles. Thermodynamic phase equilibrium formulation of this methodology is presented. In the application of this approach for the comparitive study and choice of working fluids, the Redlich-Kwong equation of state is used for a number of possible working fluid combinations for solar absorption cooling cycles. It is demonstrated that when limited experimental data are at hand this approach could be a useful screening technique for potential working fluid combinations.     

Numerical analysis of gaseous hydrogen/liquid oxygen flamelet at supercritical pressures

Supercritical conditions are typically encountered in high-pressure combustion devices such as liquid propellant rockets and gas turbine engines. Significant real fluid behaviors including steep property variations occur when the fluid mixtures pass through the thermodynamic transcritical regime. The laminar flamelet concept is a robust and reliable approach that correctly accounts for real fluid effects, the large variation in thermophysical properties, and the detailed chemical kinetics for turbulent flames at transcritical and supercritical conditions. In the present study, the flamelet equations in the mixture fraction space are extended to treat the flame field of general fluids over transcritical and supercritical states. Flamelet computations are carried out for gaseous hydrogen and cryogenic liquid oxygen flames under a wide range of thermodynamic conditions. Based on numerical results, the detailed discussions are made for the effects of real fluid, pressure, and differential diffusion on the local flame structure and the characteristics encountered in liquid propellant rocket engines.     

Performance analysis of open cycle gas turbines

In this study, the performance of ideal open cycle gas turbine system was examined based on its thermodynamic analysis. The effects of some parameters, such as compressor inlet temperature (CIT), pressure ratio (PR) and the turbine inlet temperature (TIT), on the performance parameters of open cycle gas turbine were discussed. The turbine net power output, the thermal efficiency and the fuel consumption of the turbine were taken as the performance parameters. The values of these parameters were calculated using some basic cycle equations and variables values of thermodynamic properties. Other variables such as lower heating value, combustion efficiency and isentropic efficiencies of compressor and turbine were assumed to be constant. The result showed that the net power output and the thermal efficiency increased by a decrease in the CIT and increase in the TIT and PR values. If it is aimed to have a high net power output and the thermal efficiency for the turbine, the CIT should be chosen as low as possible and the TIT should be chosen as high as possible. Copyright © 2008 John Wiley & Sons, Ltd.     

Energy and exergy analyses of a two‐stage vapour compression refrigeration system

In this paper exergy analysis of two‐stage vapour compression refrigeration (VCR) system has been carried out with an objective to evaluate optimum inter‐stage temperature (pressure) for refrigerants HCFC22, R410A and R717. A thermodynamic model based on the principles of mass, energy and exergy balances is developed for this purpose. The computed results illustrate the effects of evaporation and condensation temperatures, isentropic efficiencies of compressors, sub‐cooling of refrigerant and superheating of suction vapour on optimum inter‐stage saturation temperature (pressure). The optimum inter‐stage saturation temperatures (pressures) for HCFC22 and R410A are proximate to arithmetic mean of evaporation and condensation temperatures (AMT) when assuming superheating of suction vapour and non‐isentropic compression processes in low‐pressure and high‐pressure compressors. The optimum inter‐stage saturation temperatures (pressures) for HCFC22 and R410A are near to geometric mean of evaporation and condensation temperatures (GMT) when it is assumed that cycle involves the effects of sub‐cooling, superheating of suction vapour and non‐isentropic compression of the suction vapour. The optimum inter‐stage saturation temperature (pressure) for R717 is close to GMT irrespective of sub‐cooling, superheating of suction vapour and non‐isentropic compression in the cycle. The efficiency defects, computed corresponding to optimum inter‐stage temperature in condenser is higher in comparison to the other components. Finally, it is deduced that R717 is a better alternative refrigerant to HCFC22 than R410A in two‐stage VCR system. Copyright © 2009 John Wiley & Sons, Ltd.     

Calculation of thermodynamic properties of lubricant + refrigerant mixtures using GMA equation of state

In this work, we have used a simple equation of state (EoS) to calculate the density of five lubricant/refrigerant mixtures including octane/dimethyl carbonate, TriEGDME/HFC-134a, TEGDME/HFC-134a, heptane/TEGDME, and decane/dimethyl carbonate at different temperatures, pressures, and compositions. The excess molar volumes of these mixtures have been calculated using this equation of state. Also, we have computed other thermodynamic properties such as isobaric expansion coefficient, isothermal compressibility, and internal pressure, for octane/dimethyl carbonate system for which the corresponding experimental values are available. A wide comparison with experimental data has been made for each thermodynamic property. The values of statistical parameters between experimental and calculated properties show the ability of this equation of state in reproducing and calculating of different thermodynamic properties for studied mixtures.     

The influence of the thermodynamic model of equilibrium-hydrogen on the simulation of its liquefaction

In view of a possible hydrogen infrastructure a number of studies on large-scale hydrogen liquefiers state that conventional plant efficiencies may be substantially increased. Generally those studies consider different (i) working conditions, (ii) operational unit performances and (iii) thermodynamic models, making it difficult to compare their results. The present work focuses specifically on the third issue by assessing the influence of the thermodynamic modeling of the fluid on the simulation outcomes. Numerical approaches to compute the heat capacities as well as the equations of state of hydrogen forms (orthohydrogen and parahydrogen) and their mixtures (equilibrium-hydrogen and normal-hydrogen) are described here. The attention is on equilibrium-hydrogen because the ortho-to-para conversion, which is naturally slow and largely exothermic, is to be catalytically promoted during liquefaction in order to reduce the liquefaction work compared to batch conversion. Because to the knowledge of the authors no commercial code comprises equilibrium-hydrogen, a fluid file for REFPROP is developed and used as a reference case for comparison against four other modeling alternatives that can be readily executed with commercial codes. The comparison is based on the analysis of the cooling curves, which are temperature profiles as functions of specific enthalpies, employed to calculate the ideal liquefaction work and estimate the real one. The results indicate that the choice of the heat capacity model is crucial for the accurate simulation of the overall process, whereas the choice of the equation of state is of minor importance for the overall process, but turns crucial for analyzing local processes and for the dimensioning of the operational units, like turbomachines, that are influenced by volume flows.     

Evaluating cubic equations of state for calculation of vapor–liquid equilibrium of CO2 and CO2-mixtures for CO2 capture and storage processes

Proper solution of vapor liquid equilibrium (VLE) is essential to the design and operation of CO₂ capture and storage system (CCS). According to the requirements of engineering applications, cubic equations of state (EOS) are preferable to predict VLE properties. This paper evaluates the reliabilities of five cubic EOSs, including PR, PT, RK, SRK and 3P1T for predicting VLE of CO₂ and binary CO₂-mixtures containing CH₄, H₂S, SO₂, Ar, N₂ or O₂, based on the comparisons with the collected experimental data. Results show that SRK is superior in the calculations about the saturated pressure of pure CO₂; while for the VLE properties of binary CO₂-mixtures, PR, PT and SRK are generally superior to RK and 3P1T. The impacts of binary interaction parameter k_ij were also analyzed. k_ij has very clear effects on the calculating accuracy of an EOS in the property calculations of CO₂-mixtures. In order to improve the calculation accuracy, the binary interaction parameter was calibrated for all of the studied EOSs regarding every binary CO₂-mixture.     

Thermodynamic property formulations and heat transfer aspects for replacement refrigerants: R-123 and R-134a

This paper reports analytical relations for the thermodynamic properties enthalpy, entropy, heat capacities at constant pressure and temperature of the replacement refrigerants R-123 and R-134a. These refrigerants are considered promising as substitutes for the fluids R-11 and R-12, respectively, which are two of the most widely used CFC refrigerants. In addition to the properties, the three real gas isentropic exponents k_p,v,k_v,T, k_p,T are calculated, which may be used instead of the classical exponent k=c_p/c_v in the ideal gas isentropic change equations to describe with good accuracy the real gas behaviour. A systematic study to research the influence of various parameters on heat transfer during condensation of R-123 and R-134a on horizontal integral-fin tubes is also carried out. The results are useful in refrigeration applications to improve the basic design, as a significant concern about new refrigerants to replace the CFCs has increased very rapidly due to the destruction of stratospheric ozone and global warming. © 1997 John Wiley & Sons, Ltd.     

太阳能驱动的有机朗肯-喷气增焓（带二次吸气的增效）蒸汽压缩制冷系统性能分析

为有效利用太阳能,以有机朗肯−喷气增焓（带二次吸气的增效）蒸汽压缩式制冷系统为研究对象,建立了系统的热力学模型,分别选取R236fa、R245fa、RC318和R141b作为系统工质,研究了发生温度、凝结温度、冷凝温度、蒸发温度、膨胀机等熵膨胀效率及压缩机等熵压缩效率对系统性能的影响,并以系统性能最佳为目标对工质进行了优选。计算结果表明：对整个系统而言,R141b是最合适的工质,凝结温度和冷凝温度对系统性能有重要影响。以R141b为例,当发生温度在85℃、凝结温度为40℃、冷凝温度为40℃、蒸发温度为 −15℃时,系统COP_s达到0.2528,采用喷气增焓技术对于环境温度很低、太阳能资源丰富的北方地区具有很大的优势。     

Numerical analysis on the effects of refrigerant injection on the scroll compressor

Refrigerant injection is an effective method to improve the performance of the scroll compressor and its system under high compression ratio working conditions. This paper intends to find the exhaustive relationship between the injection parameters and the compressor’s performance. Based on a thermodynamic model, the effects of various parameters of refrigerant injection on general performance and inner compression process of scroll compressor have been investigated. As a result, it is found that the injected scroll compressor will get the maximum indicated efficiency when the ratio of inner compression ratio and outer compression ratio is a right value. The right value is 1 for the isentropic compression process, and smaller than 1 for a real compression process. Finally, the effects of all the injection factors on the compression work, refrigerant mass flow rate, p–h diagram, volumetric efficiency, and indicated efficiency are investigated detailedly.     

Evaluation of a spark ignition engine cycle using first and second law analysis techniques

In the present work, a simulation model of the actual processes occurring during the thermodynamic cycle of a real spark ignition engine is developed. The model incorporates such important features as heat exchange of the cylinder gases with the chamber walls (during all phases), real spark ignition timings, real valve opening and closing timings, accurate simulation of the spherical flame front movement issuing from the spark plug and calculation of eight chemical species concentration during combustion, at every engine degree crank angle. The results from this first law analysis of the real cycle (for example pressure indicator diagrams, efficiencies) are compared favourably with the relevant experimental results obtained from a flexible, variable compression ratio, Ricardo E-6 spark ignition engine, located at the author's laboratory, forming thus a sound basis for moving towards a second law evaluation of this cycle. The thermodynamic state points, determined from the first law analysis, are used to determine the availability (second law analysis) at each engine crank angle and so lead to the effectiveness computation, as well as to the revelation of the magnitude of the work-potential lost during the various processes in a much more realistic way than the first law analysis can. The second law analysis results, for the actual engine in hand, are compared with the up-to-now existing ideal cycle Otto engine results. Also, a second law parametric investigation is performed over a wide range of design and operation conditions (compression ratio, fuel-air ratio, ignition advance), providing useful information for the cycle processes performance assessment by bringing state degradations and thermodynamic losses into perspective.     

Thermodynamic model of viscosity of hydrocarbons and their mixtures

Activation hypothesis suggested by Eyring for modeling viscosity of liquids is generalized for calculation coefficient of dynamic viscosity of pure hydrocarbons and their mixtures in wide region of temperature, pressure and concentrations. Energy of vacancy activation is modeled as difference between enthalpy of ideal gas and enthalpy of real substance. Enthalpy and other thermodynamic parameters for pure substances and mixtures are calculated on the base of well-known Lee–Kesler equation of state. Thermodynamics of mixture calculated in the frame of pseudofluid hypothesis with pseudocritical thermodynamic constants. Pseudocritical thermodynamic constants are estimated with the help of mixing rules offered also by Lee–Kesler. Two additional constants are including in the suggested model of viscosity. For normal paraffin these constant have universal value. For other substances, for example, oxygen containing hydrocarbons values of the constant are installed in accordance to the experimental data. The model with sufficiently accuracy reproduces viscosity experimental data as pure substances in vapor and liquid phases and also solutions in the wide regions of thermodynamical parameters and concentrations. Calculation results are compared with the literature experimental data.     

PVTxy properties of CO2 mixtures relevant for CO2 capture,transport and storage: Review of available experimental data and theoretical models

The knowledge about pressure–volume–temperature–composition (PVTxy) properties plays an important role in the design and operation of many processes involved in CO₂ capture and storage (CCS) systems. A literature survey was conducted on both the available experimental data and the theoretical models associated with the thermodynamic properties of CO₂ mixtures within the operation window of CCS. Some gaps were identified between available experimental data and requirements of the system design and operation. The major concerns are: for the vapour–liquid equilibrium, there are no data about CO₂/COS and few data about the CO₂/N₂O₄ mixture. For the volume property, there are no published experimental data for CO₂/O₂, CO₂/CO, CO₂/N₂O₄, CO₂/COS and CO₂/NH₃ and the liquid volume of CO₂/H₂. The experimental data available for multi-component CO₂ mixtures are also scarce. Many equations of state are available for thermodynamic calculations of CO₂ mixtures. The cubic equations of state have the simplest structure and are capable of giving reasonable results for the PVTxy properties. More complex equations of state such as Lee–Kesler, SAFT and GERG typically give better results for the volume property, but not necessarily for the vapour–liquid equilibrium. None of the equations of state evaluated in the literature show any clear advantage in CCS applications for the calculation of all PVTxy properties. A reference equation of state for CCS should, thus, be a future goal.     

Theoretical performances of various refrigerant–absorbent pairs in a vapor-absorption refrigeration cycle by the use of equations of state

The vapor-absorption refrigeration cycle is an old and well-established technique, particularly with ammonia/water and water/LiBr systems. New types of refrigerant–absorbent pairs are also being actively studied. Modeling the cycle performance requires thermodynamic properties, which have been largely based on empirical correlation equations fitted to a large amount of experimental data such as solubility at various temperatures, pressures, and compositions. In this report, we have demonstrated, for the first time, a thermodynamically consistent model based on the equations of state for refrigerant–absorbent mixtures. Various commonly known binary-pairs for the absorption cycle are used as examples. Cycle performances and some new insights on understanding the cycle process are shown.