Radiation specular reflection question
 

Hy all
I would like to use specular reflection properties in a solar concentrator simulation.
At first I try to test Starccm+ on tests cases. My test is very classical given in [Robert Siegel] Thermal Radiation Heat Transfer book.
It is two concentric spherical grey surfaces at constant fixed temperature radiating to each other with no participating media. Test result is on the radiation flux to compare.

I tried both methods S2S and DOD. With coeficient of reflection fixed to zero I get almost same result with both methods and it is very close to Analytic Seigel solution.
But when the coeficient of specular reflection is fixed to 1. (perfect mirror) the values given by Starccm is really diferent from analytical solution of Seigel. In the analytical solution the flux diminish greatly compare to diffuse case as with starccm+ it diminish barely. I improved mesh and tried many tune but results keeps the same.

Maybe I make a mistake somewhere. I found very few about specular radiation computation with starccm+, only the doc in fact. I am starting to have doubt about the model.
Does anybody can confirm it works well ? Or maybe someone have allready use it with success and can take of this doubt.

Thanks for answer.