# How the momentum source term due to droplet evaporation is calculated?

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July 3, 2020, 09:57
How the momentum source term due to droplet evaporation is calculated?
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Alessandro
Join Date: Jul 2016
Posts: 11
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Hi Foamers.

The momentum source terms due to the dispersed phase can be identified as those due to the mass transfer and the forces acting on the particle. In literature, this term is expressed as in the figure in the attachment (Experiments and Numerical Simulations of Diluted Spray Combustion, B. Merci, D. Roekaerts, A. Saidiki., p.17)

In OpenFOAM the momentum source term is written in the UEqn.H as parcels.SU(U).
SU(volVectorField& u) const is defined in line 563 of ~/OpenFOAM2.4.0/src/lagrangianba/intermediate/clouds/Templates/KinematicCloud/KinematicCloudI.H:

Code:
tmp<fvVectorMatrix> tfvm(new fvVectorMatrix(U, dimForce));
fvVectorMatrix& fvm = tfvm();

fvm.source() = -UTrans()/(this->db().time().deltaT());

return tfvm;
UTrans() is updated twice when solving the lagrangian libraries. First in the KinematicCloud, for the momentum spray source term due to the force acting on the particle and later in parcels/Templates/ReactingParcel/ReactingParcel.C (line 689):

Code:
td.cloud().UTrans()[cellI] += dm*U0;
U0 is (line 579):
Code:
const vector& U0 = this->U_;
and at line 731 the this-> U_ pointer is calculated as the particle velocity.

The expression does not completely look like that found in literature, where the difference between the carrier and the particle velocity is considered. Could, please someone double-check if my reading of the following lines of code is correct?

Attached Images
 momsource.JPG (13.2 KB, 24 views)

Last edited by pappo1890; July 3, 2020 at 18:01.

 Tags evaporation, openfoam, spray source term, su(u), utrans