Extension of the reduced integration scheme to calculate the direct exchange areas in 3D rectangular enclosures with nonscattering media (Q1666384)

Summary: The evaluation of direct exchange areas (DEA) in zonal method is the most important task due to the heavy computer cost of multi-integrals together with the existing of singularities. A technique of variable transformation to reduce the fold of integrals, which was developed originally by \textit{H. Erkku} [Radiant heat exchange in gas-filled slabs and cylinders. Cambridge, MA: Massachusetts Institute of Technology (PhD Thesis) (1959)] to calculate the DEAs of uniformly zonal dividing cylindrical system, was extended by \textit{W. Tian} and \textit{W. K. S. Chiu} [``Calculation of direct exchange areas for nonuniform zones using a reduced integration scheme'', J. Heat Transfer 125, No. 5, 839--844 (2003; \url{doi:10.1115/1.1599368})] for nonuniformly zonal dividing cylindrical system with large thermal gradients. In this paper, we further extend the reduced integration scheme (RIS) to calculate the DEAs in three-dimensional rectangular system. The detail deductions of six-, five-, and fourfold integrals to threefold ones are presented; the DEAs in a rectangular system with assumption of gray medium are computed by the Gaussian quadrature integration (GQI) and the RIS comparatively. The comparisons reveal that the RIS can provide remarkable higher accuracy and efficiency than GQI. More interestingly and practicably, the singularities of DEAs can be decomposed and weakened obviously by RIS.