LES/TPDF investigation of the effects of ambient methanol concentration on pilot fuel ignition characteristics and reaction front structures

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Large-eddy simulations with a transported probability density function model is applied to study the ignition process of an n-heptane spray in a constant volume chamber with a premixed methanol-air atmosphere. Three reacting spray cases with initial methanol air equivalence ratio ranging from 0 to 0.3 are investigated at an initial temperature of 900 K. The case setup is based on the Engine Combustion Network Spray-H configuration. The effects of the ambient methanol-air equivalence ratio on the ignition characteristics and the reaction front structures in n-heptane/methanol dual-fuel combustion are studied in detail. It is found that the ambient methanol affects the low temperature chemistry of n-heptane, which results in a change of spatial distribution of
key species such as heptyl-peroxide. With the presence of methanol in the ambient mixture cool flame is found in the entire fuel-rich region of the n-heptane jet, while when methanol is absent in the ambient mixture, the cool flame is established only around the stoichiometric mixture close to the n-heptane injector nozzle. In general, both low- and high-temperature ignition stages of n-heptane ignition are retarded by the methanol chemistry. An increase in methanol-air equivalence ratio leads to a decrease of the peak heat release rate of the n-heptane first-stage ignition. The chemistry of methanol inhibits the n-heptane ignition by decreasing the overall hydroxyl radicals (OH) formation rate and reducing the OH concentration during the transition
period from the first-stage ignition to the second-stage ignition. As a result, the
transition time between the two ignition stages is prolonged.


Original languageEnglish
Publication statusAccepted/In press - 13 Oct 2020