Evaluation of the apparent emissivity of the internal surface of a parallelipipedic isothermal cavity

The apparent emissivity of a parallelepipedic hollow radiation source is computed to all orders of multiple reflection. An iterative method is used to solve accurately the apparent emissivity integral equation, and a special discretization scheme is constructed to handle exactly all geometrical factors. Explicit expressions are given which allow the difficulties associated with the singularities appearing near the cavity edges to be avoided. The case of a flat five-plate cavity is considered in detail. It is shown that this particular shape is adequate to produce high and uniform apparent emissivity on the face opposite to the aperture.