Nonpremixed MILD Combustion in a Laboratory-Scale Cylindrical Furnace: Occurrence and Identification

Abstract This paper reports a comprehensive investigation into the occurrence and identification of nonpremixed MILD combustion of C3H8/air in a cylindrical furnace. The combustion mode is altered by varying burner arrangement, air-fuel nozzle separation, equivalence ratio and thermal input. Based on the experimental observations, flow analysis and chemical calculations, a new criterion is proposed to identify the occurrence of nonpremixed MILD combustion using local heat release rate (HRR) particularly for CFD simulations. Importantly, unlike the case of CH4, the nonpremixed MILD combustion of C3H8 exhibits a negative HRR (HRR−) region that departs distantly from the positive HRR (HRR+) region, due to complex pyrolysis. This is distinct from the conventional flame where the HRR− region is adjecent to the HRR+ one. The new criterion notably improves the mode identification of nonpremixed combustion operating in various furnaces at different conditions. Moreover, the current study highlights the importance of local HRR in linking the experimental observation, CFD simulation and chemical calculation of MILD combustion.