Re-Recognition of the MILD Combustion Regime by Initial Conditions of Tin and XO2 for Methane in a Nonadiabatic Well-Stirred Reactor

A more straightforward combustion map for identifying moderate or intense low-oxygen dilution (MILD) combustion regime in a well-stirred reactor (WSR) using initial inlet temperature (T-in) and oxygen mole fraction (x(O2)) has been proposed based on previous mathematical criteria provided by Cavaliere and de Joannon (Prog. Energy Combust. Sci. 2004, 30, 329-366). Furthermore, the detailed evolution of different combustion regimes under the nonadiabatic condition has been comprehensively examined. Results show that there exists a critical X-O2 (X-O2*), below which MILD combustion can be established unconditionally as long as T-in exceeds the self-ignition point (T-in) and beyond which T-in needs to be remarkably promoted to fulfill the mathematical criteria of MILD combustion. Thus, the two regions are termed unconditional MILD combustion (UMC) and conditional MILD combustion (CMC), respectively. For the adiabatic condition, X-O2* is calculated to be 9.7%, indicating that MILD combustion will be more easily achieved with an oxygen-diluted oxidizer than the oxygen-enriched counterpart. Interestingly, X-O2* is found to climb as the heat loss ratio (HLR) increases, suggesting that enhancing the HLR of the WSR would help expand the UMC region, namely, more readily establishing MILD combustion. In addition, high-temperature combustion (HTC) can shift to CMC or even UMC by just enlarging HLR, providing a potential solution to realize MILD combustion in practical applications. However, the combustion regime would further shift to unsteady combustion (USC) or even no reaction (NR) regions once the heat is overextracted. Hence, it would be a challenge for MILD combustion application in intense heat extraction scenarios, such as boilers. Interestingly, higher T-in and lower X-O2 are found able to widen the UMC region under larger HLR conditions. Moreover, CO2 or H2O dilution would result in a wider UMC region compared to N-2 dilution, while it is more pronounced for CO2 due to its highest X-O2*. Besides, the shifting of the combustion regime from HTC to MILD combustion by heat extraction would be more effective with CO2 dilution than either N-2 or H2O dilution.