scholarly journals Real Gas Models in Coupled Algorithms Numerical Recipes and Thermophysical Relations

2020 ◽  
Vol 5 (3) ◽  
pp. 20
Author(s):  
Lucian Hanimann ◽  
Luca Mangani ◽  
Ernesto Casartelli ◽  
Damian Vogt ◽  
Marwan Darwish
Keyword(s):  
Gas Flow ◽  
Measurement Data ◽  
Ideal Gas ◽  
Computational Time ◽  
State Equations ◽  
Cfd Simulations ◽  
Real Gas ◽  
Standard Ideal ◽  
Ideal Gas Model ◽  
The Ideal

In the majority of compressible flow CFD simulations, the standard ideal gas state equation is accurate enough. However, there is a range of applications where the deviations from the ideal gas behaviour is significant enough that performance predictions are no longer valid and more accurate models are needed. While a considerable amount of the literature has been written about the application of real gas state equations in CFD simulations, there is much less information on the numerical issues involved in the actual implementation of such models. The aim of this article is to present a robust implementation of real gas flow physics in an in-house, coupled, pressure-based solver, and highlight the main difference that arises as compared to standard ideal gas model. The consistency of the developed iterative procedures is demonstrated by first comparing against results obtained with a framework using perfect gas simplifications. The generality of the developed framework is tested by using the parameters from two different real gas state equations, namely the IAPWS-97 and the cubic state equations state equations. The highly polynomial IAPWS-97 formulation for water is applied to a transonic nozzle case where steam is expanded at transonic conditions until phase transition occurs. The cubic state equations are applied to a two stage radial compressor setup. Results are compared in terms of accuracy with a commercial code and measurement data. Results are also compared against simulations using the ideal gas model, highlighting the limitations of the later model. Finally, the effects of the real gas formulations on computational time are compared with results obtained using the ideal gas model.

Thermodynamic Property Models for Moist Air and Combustion Gases

2002 ◽  
Vol 125 (1) ◽  
pp. 374-384 ◽  
Author(s):  
D. Bu¨cker ◽  
R. Span ◽  
W. Wagner
Keyword(s):  
Ideal Gas ◽  
Moist Air ◽  
Combustion Gas ◽  
Real Gas ◽  
New Model ◽  
Combustion Gases ◽  
Dissociation Model ◽  
The Individual ◽  
Ideal Gas Model ◽  
The Ideal

A new model for the prediction of caloric properties of moist air and combustion gases has been developed. The model very accurately predicts ideal gas caloric properties of undissociated gas mixtures at temperatures from 200 K to 3300 K. In addition, a simple model has been developed to account for caloric effects of dissociation at temperatures up to 2000 K. As a part of the project, scientific equations for the ideal gas isobaric heat capacity of the individual combustion gas components have been established. Based on this reference, an assessment and comparison of the new model with the most common technical models have been carried out. Results of the simplified dissociation model are compared to the results of complex chemical equilibrium programs. To mark out the limits of the ideal gas hypothesis, some sample calculations are given, which compare results of the new ideal gas model to results from sophisticated real gas models.

AN APPLICATION OF NONEXTENSIVE PARAMETER: THE NONEXTENSIVE GAS AND REAL GAS

2007 ◽  
Vol 21 (06) ◽  
pp. 947-953 ◽  
Author(s):  
YAHUI ZHENG ◽  
JIULIN DU
Keyword(s):  
Heat Capacity ◽  
Second Virial Coefficient ◽  
Ideal Gas ◽  
Quasi Equilibrium ◽  
Equilibrium System ◽  
Real Gas ◽  
Nonextensive Statistics ◽  
Thomson Coefficient ◽  
Ideal Gas Model ◽  
The Ideal

By application of the nonextensive statistics to the ideal gas model, we establish a nonextensive gas model. If we regard the nonextensive gas as a real gas, we can use the nonextensive parameter q ∈ ℝ in Tsallis statistics to describe Joule coefficient, Joule–Thomson coefficient, second virial coefficient and etc. We also derive an expression, with a multiplier T1-q, of the heat capacity of the nonextensive gas. We can prove that in the quasi-equilibrium system there is 1 - q > 0, 2 so the heat capacity still vanishes if temperature tends to zero, just as that in Boltzmann-Gibbs statistics.

CFD-Simulation of Centrifugal Fan Performance Characteristics Using Ideal and Real Gas Models for Air and Organic Fluids

2019 ◽  
Author(s):  
Manuel Fritsche ◽  
Philipp Epple ◽  
Karsten Hasselmann ◽  
Felix Reinker ◽  
Robert Wagner ◽  
...  
Keyword(s):  
Cfd Simulation ◽  
Ideal Gas ◽  
High Tech ◽  
Test Case ◽  
Centrifugal Fan ◽  
Real Gas ◽  
The Real ◽  
Organic Fluids ◽  
Ideal Gas Model ◽  
The Ideal

Abstract Efficient processes with organic fluids are becoming increasingly important. The high tech fluid Novec™ is such an organic fluid and is used, for example, as a coolant for highperformance electronics, low-temperature heat transfer applications, cooling of automotive batteries, just to mention a few. Thus, efficient designed fans for the transport of organic fluids are becoming more and more important in the process engineering. CFD-simulations are nowadays integral part of the design and optimization process of fans. For air at the most usual application conditions, i.e. no extreme temperatures or pressures, the ideal gas model is in good agreement with the real gas approach. In the present study, this real gas approach for organic fluids have been investigated with CFD methods and, the deviation from the ideal gas model has been analyzed. For this purpose, a simulation model of a centrifugal fan with volute has been designed as a test case. First, the ideal gas model approach has been compared with the real gas approach model of Peng-Robinson for air using the commercial solver ANSYS CFX. Thereafter, the same comparison has been performed using the organic fluid Novec™. After a detailed grid study, the entire fan characteristics, i.e. the design point and the off-design points, have been simulated and evaluated for each fluid (air and Novec™) and gas model (ideal gas and Peng-Robinson real gas). The steady state simulations of the centrifugal fan have been performed using the Frozen Rotor model. The simulation results have been compared, discussed and presented in detail.

The Effect of Gas Models on Compressor Efficiency Including Uncertainty

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
Fangyuan Lou ◽  
John Fabian ◽  
Nicole L. Key
Keyword(s):  
Pressure Ratio ◽  
Ideal Gas ◽  
Experimental Measurements ◽  
Perfect Gas ◽  
Real Gas ◽  
The Real ◽  
Compressor Efficiency ◽  
Ideal Gas Model ◽  
Isentropic Efficiency ◽  
The Ideal

Since isentropic efficiency is widely used in evaluating the performance of compressors, it is essential to accurately calculate this parameter from experimental measurements. Quantifying realistic bounds of uncertainty in experimental measurements are necessary to make meaningful comparisons to computational fluid dynamics simulations. This paper explores how the gas model utilized for air can impact not only the efficiency calculated in an experiment, but also the uncertainty associated with that calculation. In this paper, three different gas models are utilized: the perfect gas model, the ideal gas model, and the real gas model. A commonly employed assumption in calculating compressor efficiency is the perfect gas assumption, in which the specific heat, is treated as a constant and is independent of temperature and pressure. Results show significant differences in both calculated efficiency and the resulting uncertainty in efficiency between the perfect gas model and the real gas model. The calculated compressor efficiency from the perfect gas model is overestimated, while the resulting uncertainties from the perfect gas model are underestimated. The ideal gas model agrees well with the real gas model, however. Including the effect of uncertainty in gas properties results in very large uncertainties in isentropic efficiency, on the order of ten points, for low pressure ratio machines.

Computational fluid dynamics simulation of the supersonic steam ejector. Part 1: Comparative study of different equations of state

2011 ◽  
Vol 226 (3) ◽  
pp. 709-714 ◽  
Author(s):  
H T Zheng ◽  
L Cai ◽  
Y J Li ◽  
Z M Li
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Flow Field ◽  
Equations Of State ◽  
Ideal Gas ◽  
Dynamics Simulation ◽  
Real Gas ◽  
Steam Ejector ◽  
Ideal Gas Model ◽  
The Ideal

The aim of this study is to investigate the use of computational fluid dynamics in predicting the performance and geometry of the optimal design of a steam ejector used in a steam turbine. Many scholars have analysed the steam ejector using the ideal gas model, which lacks accuracy in terms of calculating the flow field of the ejector. This study is reported in a series of two papers. The first part covers the validation of CFX 11.0 results using different equations of state (EOS) on the converging–diverging nozzle flow field carried out with the experimental value. The IAPWS IF97 real gas model works well with the experimental value. The flow field of the ejector was analysed using different EOS after grid-dependent learning. The results show that the performance of the ejector was underestimated under the ideal gas model; the entrainment ratio was 20–40 per cent lower than when using the real gas model. The effect of the optimal geometrical design and operating conditions will be discussed in Part 2.

scholarly journals High-intensity air-explosion-induced shock loading of structures: consideration of a real gas in modelling a nonlinear compressible medium

2015 ◽  
Vol 471 (2176) ◽  
pp. 20140825 ◽  
Author(s):  
R. Ghoshal ◽  
N. Mitra
Keyword(s):  
High Intensity ◽  
Intermolecular Forces ◽  
Ideal Gas ◽  
Compressible Medium ◽  
Light Weight ◽  
Air Blast ◽  
Real Gas ◽  
The Real ◽  
Ideal Gas Model ◽  
The Ideal

The existing practice of designing air-blast-resistant structures relies on the ideal gas model. But this model predicts the maximum value of the reflection coefficient (ratio of the reflected to the incident pressure) to be 8, whereas it can go up to 20 or more as reported in the literature. To address this discrepancy, air medium is modelled as a real gas instead of an ideal gas, where the effect of intermolecular forces, vibration, dissociation, electronic excitation and ionization are included. Ranges of peak over-pressure are identified where the ideal gas assumption cannot be used. Differences in impulse transmitted to the free-standing plates of different mass owing to relaxing of the ideal gas assumption and consideration of the real gas model are evaluated. Impulse transmitted to the structures for constant and variable back pressure (VBP) is also compared considering the real gas model. The result shows that for high-intensity shock, the ideal gas model under-predicts impulse transmitted to heavy plates but over-predicts the same for light-weight plates. Impulse transmitted to light-weight plates is also overestimated if VBP is neglected. The implications of this research are substantial for designing high-intensity air-blast mitigating structures, which if not considered properly, may lead to compromise in structural performance.

The Use of Departure Functions to Estimate Deviation of a Real Gas From the Ideal Gas Model

2019 ◽  
Author(s):  
Matt Taher
Keyword(s):  
Isothermal Compressibility ◽  
Compressibility Factor ◽  
Ideal Gas ◽  
Screening Criteria ◽  
Real Gas ◽  
Practical Applications ◽  
Thermodynamic Conditions ◽  
Isobaric Expansivity ◽  
Ideal Gas Model ◽  
The Ideal

Abstract In many practical applications of thermodynamics, the use of simplified relationships of the ideal-gas model over a more accurate but more complex real gas model, is a critical decision to make. Thermodynamic departure functions provide screening criteria to evaluate whether the ideal-gas model can accurately represent a gas behavior. This paper reports several departure functions to evaluate deviation of a real gas from the ideal-gas model. Included in this paper is the derivation of departure functions based on isothermal compressibility, isobaric expansivity, isochoric change of pressure with temperature, isochoric change of internal energy with pressure, sonic speed, and heat capacities difference. The description of each of these departure functions is accompanied by a numerical example. Departure functions defined in this paper have led to improved representation of deviation from the ideal-gas model across a range of ±2% deviation of the specific volume departure (also known as the compressibility factor, Z) for a typical gas mixture encountered in natural gas processing. The limitations involved in using the compressibility factor, Z, to evaluate departure from the ideal-gas model is highlighted. It is shown that even as the compressibility factor, Z, approaches unity at certain thermodynamic conditions, other departure functions exhibit considerable deviations from the ideal-gas model. It is concluded that the compressibility factor, Z, should not be used as “the only criterion” to evaluate conformance to the ideal-gas model. This paper also explains the physical significance of Schultz compressibility functions X, Y, and L [3] by introducing departure functions based on isothermal compressibility and isobaric expansivity.

The Parameter Variation in Pressure Vessel during Loading Operation by Considering Heat Transfer Impact Based on Real Gas Model

2012 ◽  
Vol 516-517 ◽  
pp. 467-470
Author(s):  
Wei Qing Wang ◽  
Li Yang ◽  
Shi Gui Lv
Keyword(s):  
Heat Transfer ◽  
High Pressure ◽  
Pressure Vessel ◽  
Simulation Method ◽  
Ideal Gas ◽  
Molecular Volume ◽  
Real Gas ◽  
The Real ◽  
Ideal Gas Model ◽  
The Ideal

Since the molecular force and the molecular volume were ignored in the ideal gas model, and it was less accurate when the ideal gas model was used to depict characteristics of real gas under high pressure, so the real gas model was adopted and the heat transfer was considered, the dynamic variation model was set up for internal gas in the pressure vessel during loading operation. The model was solved by using the numerical simulation method of Runge-Kutta. Comparison was made between the ideal gas model and the real gas model under adiabatic and non-adiabatic conditions, it showed that under low pressure the results obtained by the two models were in good agreement, but under high pressure the deviation was enlarged, the real gas model with considering the heat transfer influenced would be more coincident with the reality.

scholarly journals Analysis of Pressure Pulsation Influence on Compressed Natural Gas (CNG) Compressor Performance for Ideal and Real Gas Models

2019 ◽  
Vol 9 (5) ◽  
pp. 946 ◽  
Author(s):  
Zhan Liu ◽  
Wenguang Jia ◽  
Longhui Liang ◽  
Zhenya Duan
Keyword(s):  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Ideal Gas ◽  
Specific Work ◽  
Real Gas ◽  
The Real ◽  
Compressor Performance ◽  
Ideal Gas Model ◽  
The Ideal

This work investigates the effects of pressure pulsations on reciprocating natural gas compressor performance thermodynamically. A nonlinear hybrid numerical model is thus developed to consider the interaction between the compressor and the pipeline system. The suction chamber, compressor cylinder and discharge chamber are modelled integrally based on the first law of thermodynamics and mass balance, and the pipeline flow is described by using the gas dynamic model. Methane is considered as the working fluid and its properties are computed based on ideal and real gas assumptions. For the real gas model, the methane properties are obtained by means of calling the NIST REFPROP database. The validity of numerical results is confirmed by previous experimental values. Results from the examinations of pressure pulsation influence demonstrate that discharge resonance requires more specific work than suction resonance in the same harmonic; in the suction system, the first harmonic response reduces the mass flow rate but significantly increases specific work, and the second harmonic response has a strong supercharging effect but the specific work is increased slightly; in the discharge system, the mass flow rate is changed little by pressure pulsations, but the indicated power and specific work are increased significantly; for the real gas model, the in-cylinder temperature during the compression and discharge phases, mass flow rate and indicated power are higher than those for the ideal gas model, whereas the specific work is less for the real gas model than for the ideal gas model.

Isentropic Expansion and the Three Isentropic Exponents of R152a

1999 ◽  
Author(s):  
D. A. Kouremenos ◽  
X. K. Kakatsios ◽  
O. E. Floratos ◽  
G. Fountis
Keyword(s):  
Specific Heat ◽  
Vapor Phase ◽  
Constant Pressure ◽  
Initial Pressure ◽  
Ideal Gas ◽  
Constant Volume ◽  
State Equations ◽  
Real Gas ◽  
Isentropic Expansion ◽  
The Ideal

Abstract The isentropic change of an ideal gas is described by the well known relations pvk = const., Tv(k-1) = const. and p(1-k)Tk = const., where the exponent k is defined as the ratio of the constant pressure to the constant volume specific heat, k = Cp/Cv. The same relations can be used for real gases only if the differential isentropic changes under consideration are small. A better examination of the differential isentropic change shows that for p, v, T systems, there are three different isentropic exponents corresponding to each pair formed out of the variables p, v, T. These three exponents noted kT,p, kT,v, kp,v after the corresponding pair of variables used, are interconnected by one relation, and accordingly only two out of the three are independent. The analysis of the present paper shows the numerical values of these exponents as well as the isentropic expansion ratios for R152a in the vapor phase, presented in diagram form. It can be seen that the deviations of the three isentropic exponents relative to the conventional k = Cp/Cv values are considerable and depend upon the initial pressure and the stage of the expansion. Additionally, the effect of the three isentropic exponents on the ideal gas relations describing the isentropic expansion ratios is examined, in order to develop simple yet more accurate relations without having to resort to the complex real gas state equations.

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