View Issue Details
| ID | Project | Category | View Status | Date Submitted | Last Update |
|---|---|---|---|---|---|
| 0004119 | OpenFOAM | Bug | public | 2024-07-29 07:48 | 2024-07-29 13:22 |
| Reporter | psturm | Assigned To | henry | ||
| Priority | normal | Severity | minor | Reproducibility | N/A |
| Status | closed | Resolution | no change required | ||
| Summary | 0004119: Calculation of heat capacity in the janaf thermodynamic model | ||||
| Description | There seems to be an inconsistency between the calculation of the specific heat capacity Cp and the provided coefficients in the janaf thermodynamic model. From looking at the source code I believe Cp is always meant to be the mass heat capacity in units of J/kg/K and Cp is calculated in the janaf model as Cp = (((a4*T + a3)*T + a2)*T + a1)*T + a0 However, the JANAF database contains Cp in J/mol/K and also the coefficients given in the tutorials (e.g. "\tutorials\multicomponentFluid\simplifiedSiwek\constant\speciesThermo") and in the documentation point to the fact that Cp is calculated as a molar heat capacity in units of J/mol/k. Using the methane (fuel) example of the user guide (https://doc.cfd.direct/openfoam/user-guide-v12/thermophysical): Cp = R*((((a4*T + a3)*T + a2)*T + a1)*T + a0) (note the additional R here!) lowCpCoeffs (5.14988 -0.013671 4.91801e-05 -4.84744e-08 1.66694e-11 -10246.6 -4.64132); T = 300 K gives a value of 35.8, which is the correct molar heat capacity in J/mol/K. The mass heat capacity, however, is Cp = R/molWeight*1000*((((a4*T + a3)*T + a2)*T + a1)*T + a0) = 2229 J/kg/K. So, are these molar values converted to the correct units somewhere else in the code? If not, using the janaf thermodynamic model will result in wrong Cp, enthalpy and entropy values. | ||||
| Tags | No tags attached. | ||||
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Can you provide an example case which demonstrates that the heat capacity values used in OpenFOAM are incorrect? |
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Hello, There is a call in the constructor called convertCoeffs that converts the polynomial parameters to mass basis: https://github.com/OpenFOAM/OpenFOAM-dev/blob/e45bad9beb38b1aaceb36e94a12656ecd62cd062/src/thermophysicalModels/specie/thermo/janaf/janafThermoI.H#L68 If this flag is on, it would multiply the polynomial values with the specific gas constant of methane, giving you the correct answer of 2229 J/kg/K. However the flag is by default is set to false: https://github.com/OpenFOAM/OpenFOAM-dev/blob/e45bad9beb38b1aaceb36e94a12656ecd62cd062/src/thermophysicalModels/specie/thermo/janaf/janafThermo.H#L187 I wonder if the construction of the janafThermo from the compressible modules takes care of this conversion by setting it to true? |
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Here it is: https://github.com/OpenFOAM/OpenFOAM-dev/blob/e45bad9beb38b1aaceb36e94a12656ecd62cd062/src/thermophysicalModels/specie/thermo/janaf/janafThermo.C#L74 The janafThermo.C converts the molar polynomials to mass-basis polynomials without a conditional within janafThermo, so returned value should be correct. |
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Yes, you are completely right. The coefficients are converted to mass-basis and then Cp is calculated using the converted coefficients. Stupid enough, I have missed this conversion step on the coefficients. So all seems correct. Apologies for bothering. |
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User support request |
| Date Modified | Username | Field | Change |
|---|---|---|---|
| 2024-07-29 07:48 | psturm | New Issue | |
| 2024-07-29 08:25 | henry | Note Added: 0013327 | |
| 2024-07-29 12:03 | blttkgl | Note Added: 0013328 | |
| 2024-07-29 12:18 | blttkgl | Note Added: 0013329 | |
| 2024-07-29 12:26 | psturm | Note Added: 0013330 | |
| 2024-07-29 13:22 | henry | Assigned To | => henry |
| 2024-07-29 13:22 | henry | Status | new => closed |
| 2024-07-29 13:22 | henry | Resolution | open => no change required |
| 2024-07-29 13:22 | henry | Note Added: 0013331 |
