# Radiation: DO Model, Absorption coefficient.

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 August 28, 2018, 10:59 Radiation: DO Model, Absorption coefficient. #1 Senior Member   nm Join Date: Mar 2013 Posts: 100 Rep Power: 13 In the DO Model, the absorption coefficient is the requested input (unit is 1/m) rather than absorptivity (which is unitless, 1-emissivity-reflectivity). Now I am confused as to how to obtain the property. Integrating this over the complex geometry and obtain it sounds meaningless as that is the purpose of the DO model, to discretize and solve the radiation equation. Fluent manual says it can be easily obtained from heat transfer books which unfortunately is not true. Can anyone who have experience working with absorbing media please provide some insight. Much appreciated. Thanks.

August 28, 2018, 13:03
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Lucky
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Quote:
 Originally Posted by nvarma Now I am confused as to how to obtain the property. Integrating this over the complex geometry and obtain it sounds meaningless as that is the purpose of the DO model, to discretize and solve the radiation equation.
The DO model solves the radiative transfer equation over solid angles (which means integrated). Hence the inputs to the DO model is the absorption coefficient and scattering coefficient (spatially integrated quantities) rather than the absorptivity.

If you want to use absorptivity, use full ray tracing.

If you consider the absorptivity a basic property of matter than the absorption coefficient is related to it by a length scale. And this length scale can be estimated using formulas in heat transfer texts. I agree this is a dumb way to proceed, resorting to empirical fudge factors almost and using them in a a CFD code, but that is the price you pay when you don't want to solve the ray tracing problem. Or you can do the integration yourself to determine what this length scale ought to be.

 Tags fluent 14.5.7, radiation do model