A fast spectral approximation of narrow-band model for thermal radiation calculation

A fast spectral approximation of a narrow-band computer model is proposed for prediction of thermal radiation in a generally nonisothermal and nonhomogeneous combustion environment. The approximation is made by assuming each discretized spatial element along a line of sight to be locally "gray" within each small narrow-band spectrum interval. For each spatial element, the local equivalent spectral absorption coefficient of the combustion products, including carbon dioxide, water vapor, and soot, is calculated based on formulas from the narrow-band model. Compared with the previously developed fast narrow-band model, FASTNB, which is an order of magnitude faster and gives almost exactly the same result as Grosshandler's original narrow-band model RADCAL, this approximation method provides calculation result with small deviation at substantially faster speed. Furthermore, the speed-up ratio increases linearly with the number of discretized spatial elements along a line of sight. This makes this approximation method particularly useful in computational combustion, where the grid number used in computation is steadily becoming larger and larger as the computation goes toward large eddy simulation (LES) and direct numerical simulation (DNS).