Abstract
The criterion used to define MILD combustion in non-premixed condition is analysed using Direct Numerical Simulation (DNS) of MILD combustion of methane-diluted air established with internal exhaust gas recirculation. The simulations reveal multiple interacting reaction zones in MILD combustion which are extremely different from conventional combustion. Furthermore, DNS deduced S-curves highlight the role of chemically active species. Specifically, the temperature rise is accompanied with an increase in the scalar dissipation rate of mixture fraction, which is quite contrasting to the classical S-curve from the classical flame theories. This observation is explained on a physical basis.
Highlights
To mitigate the environmental impact of conventional combustion, alternative combustion concepts and technologies are explored constantly
NZ varies over several orders of magnitude and it is apparent that reactions occur in regions of high scalar dissipation rate (SDR)
From the analysis presented here using Direct Numerical Simulation (DNS) of Moderate or Intense Low-oxygen Dilution (MILD) combustion with internal exhaust gas recirculation (EGR), it is clear that the past definitions of MILD combustion deduced using one-step chemistry in simplified configurations present various shortcomings
Summary
To mitigate the environmental impact of conventional combustion, alternative combustion concepts and technologies are explored constantly. MILD combustion involves both preheating and dilution of reactant mixture with recirculated burnt products. As a result of this recirculation, which can be both external or internal to the combustion chamber, the oxygen content decreases drastically limiting the temperature rise and the NOx production. Due to these particular conditions involving exhaust gas recirculation (EGR) and highly preheated reactants, MILD combustion shows specific features such as the absence of a visible flame, heat release distributed over larger volumes and homogeneous temperature fields [4,5,6]. Given the high reactants temperature and the presence of chemically active species in the recirculated exhaust gases, autoignition plays an important role in MILD combustion in addition to propagating flames [7,8,9]
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