Influence of wall thermal conditions on the ignition, flame structure, and temperature behaviors in air-fuel, oxygen-enhanced, and oxy-fuel combustion under the MILD and high-temperature regimes

Abstract

In this work, variations of wall thermal conditions and oxidant compositions are considered to investigate the temperature distribution, ignition process, and flame structure under MILD and high-temperature combustion (HTC) regimes. Studies are performed by the computational fluid dynamics (CFD) approach with OpenFOAM open-source code. Also, kinetic calculations by well-stirred-reactor (WSR) is used to quantitatively describe results. Results show that by the transition from HTC to the MILD regime, Tmax and Tave have intense (more than 15%) and uniform (less than 3%) drops, respectively. This indicates that moving from HTC to the MILD regime can be considered as an effective way to maintain the thermal conditions while reducing the emission of pollutants. The MILD regime is formed at Tw = 1500 K and Tw = 1700 K under air-fuel and oxy-fuel combustion, respectively, indicating that oxy-fuel MILD combustion is associated with fewer heat losses. Enhancement of heat loss creates an intense variation of reaction pathways under the MILD regime, while it is constant for HTC regime. Higher heat loss leads to diminishing the role of reactions O + H2O ⇌ 2OH and O2+CH2OH ⇌ HO2+CH2O on the ignition delay time (IDT) under insulation reactor and increases the contribution of reactions H + OH + M ⇌ H2O + M and O + CH3 ⇌ H + CH2O.