Ignition and Flame Propagation of Dual Fuel Mixture Under Turbulence State

Direct numerical simulation(DNS)method was adopted to investigate the ignition and combustion process of methane/n-heptane mixture under the thermodynamic state closed to low speed marine engine. And the ignition characteristic and flame development of the dual fuel mixture were analyzed under turbulence state. The results show that, after the first-stage ignition, the cool flame occurs in thick fuel mixtures affected by the turbulence. Several high-temperature expanding kernels form at the side of premixed methane/air mixture after second-stage ignition. The high-temperature kernels tend to appear in the zones with lower mixture fraction gradient. However, it is difficult for kernels to form at higher mixture fraction gradient where heat and active radicals dissipate rapidly, which against the stable development of combustion reaction. By the effect of turbulence, the flame fronts spread forward in folds both to the two sides, and the fronts propagate faster around the high-temperature expanding kernel. The propagation of flame front in lean premixed methane/air mixtures is accelerated by the increasing oxygen content, and the extent of folded flame front is reduced. The downstream “Cool Flame” appears in thick fuel mixtures, forming the “Double Flame” structure together with the flame front appeared in n-heptane-side mixture. As the reaction progresses, double flame fronts propagate to thicker fuel mixtures and the distance of the two fronts is decreased. © 2022, Editorial Office of the Transaction of CSICE. All right reserved.