DNS study of freely-propagating turbulent lean-premixed flames in the broken reaction zone regime

Abstract

The novel engines nowadays featured with higher efficiency are operated under the superpressure, supercritical, supersonic, and near-limit combustion condition that is situated in the broken reaction zone regime. In the broken reaction zone regime, the turbulence Kolmogorov length is shorter than the reaction zone thickness and the fluctuating RMS velocity is higher than the propagating speed, as such small-scale vortex could tear up the continuous front surfaces and the combustion is dispersedly distributed, which is highly deviated off the flamelet theory assumption. Hence, the relevant study would provide some guiding implications for the refinement of turbulent premixed combustion models under the extreme conditions. In this article, the propagation and heat/radical diffusion physics of a high-pressure dimethyl ether (DME)/air turbulent lean-premixed flame with Ka = 200 are investigated numerically by DNS with detailed fuel chemistry and transport model. A wide range of statistical and diagnostic methods, including the Lagrangian fluids tracking, Joint Probability Density Distribution (JPDF), and chemical explosive mode analysis (CEMA) will be applied to reveal the deflagration front structure, the local combustion modes, dynamics evolution, as well as the roles of heat/mass transports and cool/hot flame interaction in the turbulent combustion regimes, which would be beneficial to the design of novel engines with high performances. It is found that in the broken reaction zone regime, the reacting front structure as well as its inner diffusion processes has changed significantly. The reaction zone thickness increases remarkably, and HRR and fuel consumption rate in the cool-flame zone are increased by 16% and 19% respectively. The diffusion effect not only enhances flame propagation, but also suppresses local HRR or fuel consumption. The strong turbulence interplaying with diffusive transports is the underlying physics for the enhancements in cool- and hot-flame fronts. In the turbulence field with strong flame folding and eddy mixing, it created intermixed reacting fronts with various progress variables; the heat/radical diffusions is the underlying mechanism for flame intensification/thickening phenomena. For the cool-flame front, diffusive transports of heat, CH₂O, CH₃OCH₂O₂, and CH₃OCH₃ are of the governing significances for the flame thickening and combustion enhancement. For the hot flame front, heat conductivity is most dominant and the diffusions of CH₂O, H₂O₂, and CH₃OCH₃ are of less importance with an inhabitation impact.