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IDProjectCategoryView StatusDate SubmittedLast Update
0000958OpenFOAMBugpublic2013-08-13 16:362013-08-20 15:05
Reporterdkxls Assigned Touser2 
PriorityhighSeveritymajorReproducibilityalways
Status resolvedResolutionfixed 
PlatformLinux x86_64OSopenSUSEOS Version12.2
Summary0000958: [ThermoParcel]: Wrong heat conductivity at droplet surface due to const. Prandtl number assumption
DescriptionThe ThermoParcel uses a constant Prandtl number assumption for the heat conductivity (kappas) at the droplet surface, which gives wrong values in high temperature spray simulation (e.g. engines).

In Foam::ThermoParcel<ParcelType>::calcSurfaceValues, kappas should obtained from the thermo class, like Cp and mus. The Prandtl number can then be calculated as:
    Pr = Cpc_*mus/kappas;
This does not only give correct values for kappas and Pr, but the Prandtl number can also be removed from the constant properties entirely, as it is in fact not a constant property anymore.
TagsHeat transfer, Lagrangian, spray, sprayEngineFoam, sprayFoam

Activities

dkxls

2013-08-13 18:42

reporter   ~0002397

After a more thorough review of the source code, I have two remarks:
1)
The Prandtl number is computed at two independent place, 1) in the
ThermoParcel (calcSurfaceValues) and 2) in the ReactingParcel template
(correctSurfaceValues), where the second computation is only done if
BirdCorrection is 'true'.
The two computations differ largely and I cannot find a reference for the 'Bird
correction' way. Especially the Cp averaging based on mole fractions, where the
quantity itself has units [J/kg/K] seems odd! However, as said I don't have a
reference for that.

2)
If I am correct, then there is no access to the cell pressure from within the
ThermoParcel template. Hence the kappa computation in ThermoParcel would require
a volScalarField for kappa in the same way as Cp, which seems unfeasible.
Thus I would suggest a redefinition of 'calcSurfaceValues' in ReactingParcel and
compute the heat conductivity like this:
  kappas = 0;
  forAll(Ys, i)
  {
      kappas += Ys[i]*thermo.carrier().kappa(i, pc_, T);
  }
Note: Mass averaging for the mixture heat conductivity is just a rough
approximation, but probably still a good tradeoff between accuracy,
computational time and code complexity.

user2

2013-08-20 15:05

  ~0002429

Thanks for the report - kappa calculation updated by commit 4324b6

Issue History

Date Modified Username Field Change
2013-08-13 16:36 dkxls New Issue
2013-08-13 16:37 dkxls Tag Attached: Lagrangian
2013-08-13 16:37 dkxls Tag Attached: spray
2013-08-13 16:37 dkxls Tag Attached: sprayEngineFoam
2013-08-13 16:37 dkxls Tag Attached: sprayFoam
2013-08-13 16:37 dkxls Tag Attached: Heat transfer
2013-08-13 18:42 dkxls Note Added: 0002397
2013-08-20 15:05 user2 Note Added: 0002429
2013-08-20 15:05 user2 Status new => resolved
2013-08-20 15:05 user2 Fixed in Version => 2.2.x
2013-08-20 15:05 user2 Resolution open => fixed
2013-08-20 15:05 user2 Assigned To => user2