[Benben]

Hello,

I found a weird comportement of reactingFoam in openfoam 6 (maybe is it similar in older versions).

I setup a case with H2O and N2 as species, reaction off, and a closed box for the mesh.
On every walls of the box, boundary conditions are "no slip" for U, "zero gradient" for P and T.
The only thing is that i initialized the interior of the box to full N2 (Y[H2O] = 0.), and set the walls to a fixed fraction of H2O (Y[H2O] = 0.2).
This way the H2O should diffuse inside of the box, and there should be no convective flux.

The H2O is diffusing correctly but the temperature goes crazy for no reason.

My guess is that there should be a molecular diffusivity term in the energy equation, and there is none. So the species that diffuse dont bring with them there enthalpy.
I attached the (very light) test case for people to be able to test it. You just have to run "reactingFoam" inside, the mesh is already generated. It will create only 5 time steps.
Box.zip

[Benben]

I found this thread :
having trouble using reactingFoam with reactions turned off
That talks about a similar problem.

"reactingFoam algorithm is an efficient, approximate method for turbulent reacting flow. However, in the laminar limit the approximation breaks down, leading to temperature inconsistencies.

In the laminar case, if you have a composition gradient, the only way to ensure that the diffusion does not give rise to temperature errors is by enforcing a Lewis no = 1 constraint, e.g. update the sensible enthalpy eqn to:

fvScalarMatrix hsEqn
(
fvm::ddt(rho, hs)
+ mvConvection->fvmDiv(phi, hs)
// - fvm::laplacian(turbulence->alphaEff(), hs)
- fvm::laplacian(turbulence->muEff(), hs) // unit lewis no.
==
DpDt
+ chemistrySh
);
"

[Benben]

This modification in the code of the solver doesnt resolve the problem

[zeng-ujs]

Originally Posted by Benben

This modification in the code of the solver doesnt resolve the problem

Hi,Benben.

Recently i met the same problem when i ignition with pilot flame . the temperature dropped crazy and total reached less than 200K. Firstly my thought was there maybe something wrong with boundary, so i set a 2D case to exam it , and amazing everything is fine . i have no idea what should i do, do you have solved this problem?