湍流状态下双燃料混合物自燃和火焰传播过程

Ignition and Flame Propagation of Dual Fuel Mixture Under Turbulence State

采用直接数值模拟(DNS)方法,研究了低速机缸内热力学状态下甲烷/正庚烷混合物的着火及燃烧过程,分析了湍流状态下双燃料混合层中的着火特性及火焰发展过程.结果表明:第一阶段着火后,湍流作用下混合气偏浓区域生成冷焰;第二阶段着火后,甲烷/空气预混气侧生成多个高温膨胀核心.混合分数梯度平缓区域更易生成高温核心,而混合分数梯度较大时会增大标量耗散率、加强热量和活性基团的耗散,不利于燃烧反应的稳定发展.湍流作用下火焰前沿形成褶皱向两侧传播,热膨胀核心位置的火焰前沿传播较快.甲烷/空气预混气侧含氧量增加导致火焰前沿传播加快,前沿褶皱程度逐渐降低;正庚烷侧火焰前沿在传播下游存在冷焰反应区域,形成“双火焰”结构,随着反应进行,火焰前沿传播进入更浓的混合物中,双火焰面之间的距离逐渐缩短.

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.