Nonpremixed MILD combustion in a laboratory-scale cylindrical furnace: Occurrence and identification

• Published in: Energy (Oxford) Vol. 216; p. 119295
• Main Authors: Cheong, Kin-Pang, Wang, Guochang, Si, Jicang, Mi, Jianchun
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.02.2021; Elsevier BV
• Subjects: Combustion; Combustion mode; Criteria; energy; Equivalence ratio; Flameless combustion; Furnaces; heat; Heat release rate; Heat transfer; Mathematical analysis; MILD combustion criterion; Nonpremixed; Nozzles; Pyrolysis; Heat release rate; Combustion mode; MILD combustion criterion; Flameless combustion; Nonpremixed
• ISSN: 0360-5442; 1873-6785
• DOI: 10.1016/j.energy.2020.119295

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. •Occurrence of nonpremixed C3H8 MILD combustion and its identification are systematically investigated.•Unlike burning CH4, MILD combustion of C3H8 includes a negative heat release rate (HRR) region.•A new criterion based on local HRR is proposed for identifying the nonpremixed MILD combustion.•Local HRR reasonably links the experimental and numerical results of MILD combustion.