Numerical Investigations On The Combustion Mechanisms Of Diesel And Typical Oxygenated Fuels

Diesel combustion coupled with oxygenated fuels can effectively reduce the soot emission,which also helps to make the engine combustion more efficient and clean.In this work,the combustion mechanisms of diesel and typical oxygenated fuels were investigated,through performing engine combustion experiments,zero-dimensional(0D)chemical kinetic analyses,and three-dimensional(3D)numerical simulations.Based on the engine combustion experiment of diesel surrogate fuels,a reduced toluene reference fuel(TRF)-n-dodecane-cyclohexane-polycyclic aromatic hydrocarbons(PAHs)mechanism was proposed.The 3D modeling investigation demonstrated that the lower volatility and longer molecular structure for the diesel surrogate fuel resulted in the higher soot emission.Moreover,a method for analyzing the chemical kinetics process in the fuel combustion was proposed.More turbulent and wrinkled structures were predicted by the LES approach,in contrast to the more smooth and concentrated structures predicted by the RANS approach.However,despite their different predicted spatial distributions,these two approaches yielded similar combustion characteristics,and three primary combustion regions were categorized during the initial high-temperature ignition period,including the upstream exothermic region(Region 1),upstream endothermic region(Region 2),and downstream exothermic region(Region 3).Through using the SVUV-PIMS technique,new species like DMF252 J,DMF25CH2,and fulvene were identified in the low-pressure laminar premixed flames of 2,5-dimethylfuran(DMF)-oxygen-argon,based on which a revised detailed DMF combustion mechanism was formulated.In addition,a reduced TRF-DMF-PAHs mechanism was developed.The 0D modeling results showed that DMF has the higher tendency to produce soot under homogeneous combustion condition,owing to its cyclic structure,which tends to generate phenol and cyclopentadienyl radical despite its oxygen content,promoting the formation of PAHs and soot eventually.However,under engine combustion condition,compared with pure diesel combustion,diesel blended with DMF yielded the longer ignition delay and mixing period,due to the low reactivity of DMF,which restrained the soot formation process.A reduced TRF-methanol-ethanol-propanol isomers-butanol isomers-n-pentanolPAHs mechanism and a reduced TRF-ethanol-n-butanol-DMF-methyl decanoate(MD)-PAHs mechanism were developed.The 0D modeling results demonstrated that the alcohol fuel with a shorter carbon-chain structure exhibited the lower sooting tendency since it had higher oxygen content and produced the fewer hydrocarbon intermediates.However,when n-heptane was blended with different alcohol fuels with the same oxygen content,the similar sooting tendency of the blended fuels was observed,due to their same carbon-hydrogen-oxygen content.However,compared with ethanol,n-butanol,and MD,DMF exhibited the highest sooting tendency,owing to its special molecular structure.The 3D combustion modeling study of diesel blended with different alcohols with the same oxygen content showed that the blended fuels exhibited similar combustion characteristics at the low exhaust gas recirculation(EGR)rates,due to the same carbonhydrogen-oxygen content.However,at the high EGR rates,their soot emissions exhibited a positive correlation with their corresponding autoignition reactivity,which can be sequenced by methanol > ethanol > n-butanol > n-pentanol > n-propanol.The following 3D combustion modeling study of biodiesel blended with ethanol,n-butanol,and DMF also confirmed that the lower fuel reactivity led to the longer ignition delay and mixing process,which inhibited the soot formation.However,although ethanol has a higher reactivity compared with DMF,more heat was absorbed during its evaporation process,which resulted in the longer ignition delay and thus the lower soot emission.