Experimental And Numerical Investigations On Small Ethanol/Ether Fuel And Unsaturated Hydrocarbon

Methanol and dimethyl ether,which are the simplest alcohol ether molecules,can replace some fossil fuels.Adopting methanol/dimethyl Ether dual-fuel mode is an effective means to control combustion and greatly reduce soot.However,it is still unclear to completely understand the auto self-ignition and interaction mechanism of the mixture,because of shortages experimental data of ignition delay time(IDT)under different mixing ratios at high temperature and high pressure.Allene and Propyne are simple isomers of unsaturated hydrocarbon.Previous studies have been shown that they are important precursors of soot products.However,there is a lack of experimental IDT data under high pressure and high temperature,so the effect of key kinetic parameters on soot formation is not quite clear.In this paper,experiments and simulations have been conducted to study the combustion characteristics of these fuels operation,and also the main influencing factors and interaction mechanism ware analyzed.Firstly,New high pressure shock tube(HPST)ignition delay data for methanol(MeOH)and dimethyl ether(DME)are acquired at engine-relevant conditions(T=700K?1300 K,P=15 bar and 30 bar,and equivalence ratios of?=0.5,1.0,and 2.0 in synthetic dry air).The detailed chemical kinetic model(Aramco Mech.?2.0?Zzh 1.0)is capable of accurately predicting this wide range of data for the reactions of MeOH.In order to differentiate the first stage ignition times from pre-ignition times,the endwall-pressure traces that used to determine first stage ignition times are divided into three types based on the characteristics of HPST pressure trace during the DME measurements.Hydroxyl radical plays a more important role in the initial reaction stage of fuel oxidation,such as the major consuming reactions of MeOH and DME are abstraction of H atoms from fuel molecules by hydroxyl radicals.The reaction path of methanol is broadly unchanged at the different temperature rang while the reaction of DME with oxygen proceeds through different reaction channels.Unimolecular decomposition of methoxymethyl radicals(CH3O(?)H2<=>CH2O+(?)H3)serves as initiation reaction at high temperature(T>1000 K),addition of molecular oxygen to methoxymethyl radicals(CH3O(?)H2+O2<=>CH3OCH2(?)2)form alkyl-peroxyl radicals acts as initiation reaction at low temperature(T<1000 K).Secondly,new wide range of new ignition delay time data for 25/75,50/50 and75/25 mixtures of both methanol and DME fuels in‘air'are presented.Pressures from 7to 41 atm and equivalence ratios of?=0.5,1.0 1.5 were studied in the temperature range 700–1300 K.The experimental results showed that,when the fuel equivalent ratio of the mixture is constant,the reactivity increases(ignition delay decreases)as the DME fraction in the fuel blend is increased at low temperature range and a totally opposite effect is produced after the DME was added into methanol at high temperature.When mixing dimethyl ether by methanol,DME promotes MeOH oxidation by introducing low temperature chain-branching steps to produce hydroxyl radical.In particular,HO₂CH₂OCHO unimolecular decomposition(HO₂CH₂OCHO<=>(?)CH₂OCHO+(?)H)plays an essential role at low temperature because hydroxyl radical is major contributors for abstraction of H atoms from methanol.The reaction of addition of hydroperoxy radical to methyl radical which come from unimolecular decomposition of DME molecular((?)H3+H(?)2<=>CH4+O2)acts to inhibit reactivity of the MeOH/DME mixtures at high temperature.With the increase of the proportion of methanol in the mixture,the negative temperature coefficient(NTC)and the first stage ignition delays of DME are gradually reduced and weakened.When the ratio of methanol is up to 50%,the NTC and the first-stage ignition delays are no longer observed.The non-linear change of overall ignition delay with varying DME blending ratios is observed in both experiments and simulations.When the proportion of dimethyl ether is 0?30%,the effect on IDT of the mixed system is the greatest.Thirdly,New high pressure shock tube(HPST)ignition delay data for allene(C₃H₄-A)and propyne(C₃H₄-P)are studied at engine-relevant conditions(T=850 K?1400 K,P=10 bar and 30 bar,and equivalence ratios of?=0.5,1.0,and 2.0 in synthetic dry air).There exist similarities and peculiarities of combustion characteristics of allene and propyne,these properties may be strongly influenced by their isomeric structures.Numerical simulation results show that hydrogen atom abstraction from C₃H₄-A/P are important consumption pathways in all temperatures,abstraction by hydroxyl,hydroperoxyl radical results in their IDT decreases.However,H-atom abstractions by hydrogen and oxygen gradually show a higher level of sensitivity with increasing temperature.The propargyl radical is a key species in the soot formation,the channel producing it also changes according to the temperature.The H-atom abstractions by hydrogen form propargyl radical is the dominant channel at high temperatures.The formation of propargyl radical mainly comes from the H-atom abstractions by hydroxyl radical and hydroperoxyl radical(C3H4-A/P+(?)H<=>(?)3H3+H₂O and C₃H₄-A/P+H(?)₂<=>(?)₃H₃+H₂O)at low temperature.The reactants,intermediates and product profiles of the oxidation of allene and propyne in a JSR were compared as a function of temperature in the range of 800?1200 K and at pressures of1 and 10 atm.The results show that there are similarities and differences in the rate of formation and the mole fraction of these intermediate species,which further verify the features in the reaction channels of C₃H₄-A and C₃H₄-P.A numerical model consisting of a detailed kinetic reaction mechanism was used to describe allene and propyne ignition in shock waves and oxidation in a jet-stirred reactor.There is relatively good agreement between the Aramco Mech2.0 mechanism and the experimental data.However,the current mechanism still under-predicts reactivity at in lean-fuel oxidation.This studies provides a large quantity of high-pressure IDTs data for a wide range of MeOH/DME dual fuels,allene and propyne.It also provides chemical kinetic mechanisms for them,and can act as the basis for the further development of chemical kinetic mechanisms for dual fuels and unsaturated hydrocarbon.