## 10.6. The Discrete Transfer Model

This model is based on tracing the domain by multiple rays leaving from the bounding surfaces. The technique was developed by Shah (1979) and depends upon the discretization of the equation of transfer along rays. The path along a ray is discretized by using the sections formed from breaking the path at element boundaries. The physical quantities in each element are assumed to be uniform.

These rays have to be traced through the domain in the same way that the photons would be tracked in the Monte Carlo model. Therefore, the model description for both Monte Carlo and Discrete Transfer is identical.

For the results to be accurate the elements must be chosen so that the radiation field is reasonably homogeneous inside them. This means, for example, that they must be small enough that the scattering optical depth is less than unity across each element.

Non-gray models are dealt with by treating each band as a separate calculation (possible because scattering and reflection are assumed to be coherent). Tracking is done only once, and the results for the bands are combined to give the total radiative heat transfer.

For details, see General Radiation Considerations.

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