One-dimensional simulation on the propagation of cool and hot flames for methanol/n-heptane dual-fuel premixtures

Numerical simulations of stratified methanol/n-heptane dual-fuel mixtures with various stratification configurations are conducted using a skeletal chemistry mechanism. The transient initiation and propagation of cool and hot flames are studied under various thermal and compositional stratifications under engine-like conditions. The ignition delay times (IDTs) for stratified dual-fuel premixtures are significantly reduced when the initial temperature is within the negative temperature coefficient (NTC) region. Different initiation regimes are identified under various initial conditions. When the initial temperature of the n-heptane mixture is below or within the NTC region, the propagation of a cool flame front dominates the early stage of ignition. When the initial temperature is within the NTC region, hot flames are formed behind the cool flame front where the reactivity is enhanced due to the thermal stratifications. The lower temperature at the mixing layer tends to shorten the IDT. A double-flame structure for the coexistence of cool and hot flames is observed. At higher temperatures, the high reaction rate for the n-heptane/air premixture plays the dominant role during the decomposition process. Thus, high-temperature kernels appear in the n-heptane/air mixture. By increasing the equivalence ratio of the n-heptane/air mixture, the double-flame structure transitions to a single-flame structure in which the hot flame propagation plays the dominant role.