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Mark March 30, 2004 09:31

Absorption Coefficient

I'm having trouble getting a converged solution for radiation in a furnace. Combustion is modelled using eddy dissipation with ke turbulence model. I have obtained a second order solution to the combustion flow field but can't get converged radiation. I'm using DO model with default values.

I know the absorption coefficient to be approximately 0.35 in the furnace drum, where the flame exists, but the absorption coefficient drops to zero in some parts of the model domain. The point at which the absorption Coeff. is equal to zero is at the material discharge away from the flame.

My question is, can I apply a constant absoprtion coefficient value of 0.35 without effecting the accuracy of the solution too much??? Or can I appy constant absorption coefficient to certain sections and WSGGM to others?

Thanks for your help,


Otilia March 30, 2004 18:35

Re: Absorption Coefficient
Check if there is a combo box next to the absorption coefficient in the materials panel. If that is the case you can use a UDF to define a value of the absorption coefficient as a function of grid coordinates.

However, I do not think that this will help with your convergence problems. Check underrelaxation factors and reaction schemes. Are you using pdf? If yes, check pdf file.

Allan Walsh March 31, 2004 12:55

Re: Absorption Coefficient
What do you mean by not converge? Energy balance is off? Residuals are high? Wall radiant heat fluxes change?

Radiation may be the last to converge. Perhaps there is no steady-solution to your problem. What sort of accuracy do you need?

I have used the WSSGG model in models of more than 50 different furnaces, albeit with the P1 radiation model, and have not seen the determination of the absorbtion coefficent to be a source of instability.

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