On the Unity Lewis Number Assumption in reactingFoam

rsc · September 30, 2015, 08:16

The diffusive term in the species conservation equation of the reactive term is formulated as:

Code:

fvm::laplacian(turbulence->muEff(), Yi)

This was attributed to the unity lewis assumtion in previous posts.
http://www.cfd-online.com/Forums/ope...tive-flow.html
http://www.openfoam.org/mantisbt/view.php?id=277

I'm relatively new to CFD so forgive me if this is a stupid question. Doesn't unity lewis number mean that this diffusion coefficient should be equal to the thermal diffusivity (alpha)? How come viscosity is used here - does anyone have a reference for this?

rsc · October 5, 2015, 10:18

*bump
Heh, I guess I'll take that as a no.

My (uneducated) guess is that the diffusion coefficient Di=mu may be valid in the turbulent case, but not in the laminar one. I did try setting Di=alpha (mixture average thermal diffusivity) and did not notice any substantial difference in the output, but I didn't do a quantitative comparison.

My case involves a light gas (hydrogen) though, so it is likely that both approximations are equally wrong. I guess I have no choice but to implement a proper transport model with binary diffusion coefficients.....

September 30, 2015, 08:16	On the Unity Lewis Number Assumption in reactingFoam	#1
rsc New Member Join Date: Sep 2015 Posts: 3 Rep Power: 10	The diffusive term in the species conservation equation of the reactive term is formulated as: Code: fvm::laplacian(turbulence->muEff(), Yi) This was attributed to the unity lewis assumtion in previous posts. http://www.cfd-online.com/Forums/ope...tive-flow.html http://www.openfoam.org/mantisbt/view.php?id=277 I'm relatively new to CFD so forgive me if this is a stupid question. Doesn't unity lewis number mean that this diffusion coefficient should be equal to the thermal diffusivity (alpha)? How come viscosity is used here - does anyone have a reference for this? atulkjoy and tbraun84 like this.

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October 5, 2015, 10:18		#2
rsc New Member Join Date: Sep 2015 Posts: 3 Rep Power: 10	*bump Heh, I guess I'll take that as a no. My (uneducated) guess is that the diffusion coefficient Di=mu may be valid in the turbulent case, but not in the laminar one. I did try setting Di=alpha (mixture average thermal diffusivity) and did not notice any substantial difference in the output, but I didn't do a quantitative comparison. My case involves a light gas (hydrogen) though, so it is likely that both approximations are equally wrong. I guess I have no choice but to implement a proper transport model with binary diffusion coefficients.....