View Issue Details
| ID | Project | Category | View Status | Date Submitted | Last Update |
|---|---|---|---|---|---|
| 0001411 | OpenFOAM | Bug | public | 2014-10-07 19:24 | 2016-02-25 10:27 |
| Reporter | dkxls | Assigned To | henry | ||
| Priority | normal | Severity | minor | Reproducibility | N/A |
| Status | closed | Resolution | fixed | ||
| Platform | Linux x86_64 | OS | openSUSE | OS Version | 13.1 |
| Summary | 0001411: [janafThermo]: Incorrect expression for entropy | ||||
| Description | The entropy values obtained by evaluating the NASA polynomials account for the temperature dependency of the entropy only. Unlike enthalpy, the entropy of an ideal gas varies with pressure as well as the temperature. The variation of entropy with pressure needs to be accounted for by an additional term. To my understanding, the correct expression would hence be: s(T,p) = s(T) - R*log(p/p_ref), where, R is the gas constant, p pressure and s(T) obtained from the NASA polynomials as implemented in the janafThermo class. References for further reading are listed below. | ||||
| Additional Information | References: Cengel, Yunus A., and M. A. Boles. Thermodynamics: An Engineering Approach. 5th ed. McGraw-Hill Science/Engineering/Math, 2004. Poling, B.E., J.M. Prausnitz, and J.P. O’Connell. The Properties of Gases and Liquids. McGraw-Hill, 2001. | ||||
| Tags | entropy, thermo | ||||
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The janafThermo package provides only the temperature dependency for entropy, the pressure dependency is a function of the equation of state which is selected independently of the thermodynamics. One option would be to provide an entropy function for each equation of state to handle the pressure dependency and add that contribution to the thermal contribution: template<class EquationOfState> inline Foam::scalar Foam::janafThermo<EquationOfState>::s ( const scalar p, const scalar T ) const { const coeffArray& a = coeffs(T); return RR* ( (((a[4]/4.0*T + a[3]/3.0)*T + a[2]/2.0)*T + a[1])*T + a[0]*::log(T) + a[6] ) + EquationOfState::s(p, T); } for which we need the entropy functions corresponding to each equation of state. If you are happy to dig-up the appropriate entropy expression for each of adiabaticPerfectFluid incompressiblePerfectGas PengRobinsonGas perfectGas icoPolynomial linear perfectFluid rhoConst I will implement them all. |
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This work is nearly complete in OpenFOAM-dev: commit cb6f22edcf78b1db274d0ade0d1aab6eaa002056 Author: Henry <Henry> Date: Sat Mar 7 21:47:04 2015 +0000 thermophysicalModels/specie: Added entropy function to equations of state Added the pressure dependent part of the entropy from the equation of state to that returned by the thermo entropy function. TODO: Add entropy defect to the PengRobinsonGas::s(p, T) function. |
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I have been a bit short-sighted with my initial report and didn't consider all the different implementations for an equation of state. However, I see you have fixed already the perfect gas equation and it's variants. For the real gas equation (i.e. Peng-Robinson), the situation is unfortunately a bit more involved as there not only the entropy, but also the enthalpy and hence heat capacity require correction terms. In literature these correction terms are often referred to as departure functions: http://en.wikipedia.org/wiki/Departure_function I can have a look at the code/literature and see if I can dig-up the respective expressions in the next couple of days. |
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There is already support for the cp-cv function: cpMcv but currently no support for the pressure effect departure function for this, cp or enthalpy. We could add any or all of these as required. |
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cp, enthalpy and entropy now support departure functions: commit e538f281b8d7e489c57b0805131810693fdf4657 Author: Henry Weller <http://cfd.direct> Date: Wed Feb 17 10:48:30 2016 +0000 thermophysicalModels: Added support for thermodynamic departure functions and these have been implemented for the Peng-Robinson EoS: commit ab3ed6d75ea30e696b5fea3c77045d7f9bb732e6 Author: Henry Weller <http://cfd.direct> Date: Wed Feb 17 15:17:52 2016 +0000 PengRobinsonGas: Added thermodynamic departure functions |
| Date Modified | Username | Field | Change |
|---|---|---|---|
| 2014-10-07 19:24 | dkxls | New Issue | |
| 2014-10-30 12:48 | dkxls | Tag Attached: thermo | |
| 2014-10-30 12:48 | dkxls | Tag Attached: entropy | |
| 2015-03-07 15:44 | henry | Note Added: 0004001 | |
| 2015-03-07 21:52 | henry | Note Added: 0004005 | |
| 2015-03-09 18:47 | dkxls | Note Added: 0004062 | |
| 2015-03-09 19:04 | henry | Note Added: 0004063 | |
| 2015-03-24 00:17 | liuhuafei | Issue cloned: 0001626 | |
| 2015-10-22 10:06 | henry | Status | new => closed |
| 2015-10-22 10:06 | henry | Assigned To | => henry |
| 2015-10-22 10:06 | henry | Resolution | open => fixed |
| 2016-02-25 10:27 | henry | Note Added: 0005979 |
