A new 3D method of modeling convective-diffusive (CDT) heat transfer and zonal radiation transfer (ZRT) employing different calculation schemes and multi-scale curvilinear grids is presented. The coarse multiblock unstructured grid calculation domain allows use of a conservative and an accurate zonal radiation transfer method with only modest computational effort that requires only a small fraction of total processor CPU time. The blockages (e.g., bars in a furnace) and screens have their own very coarse grids. This reduces the time for defining their intersections with rays. Structured fine grid is used for convective-diffusive (CDT) calculations. The main difficulty (i.e., in coupling between CDT and ZRT numerical computations) is successfully overcome using a simple algorithm. The zonal radiation transfer method is based on a fast algorithm for calculating view factors and total exchange areas. The present approach is fast, efficient and accurate for gas fired furnaces and complex internal configurations of the work pieces with many blockages and screens. The utility of the method is demonstrated by calculating the heating of a hundred round metal bars arranged in a continuous natural gas fired furnace. Good agreement between calculations and industrial experiments is demonstrated.

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