A new evaluation method of average path length of the Patch's effective mean absorption coefficient from each gas element to its surrounding walls is improved. In this method, the average path length is obtained as the length that makes the radiation intensity carried along the path length equal to the average radiation intensity on surrounding walls. It is used to estimate equivalent for 3D gray radiative heat transfer analysis. The results of gray analysis using this method are closer to those of nongray analysis than those of gray analysis using uniform gray absorption coefficient evaluated from the Hottel chart.
