Numerical Study Of Ignition Of Dual Fuel And Combustion And Emmision Of Diesel Methanol Dual Fuel

In order to improve thermal efficiency of fuel,diesel natural gas dual fuel(DNDF)and diesel methanol dual fuel(DMDF)in compression engine have been paid high attention.Experiment results show that both natural gas and methanol delay the ignition of diesel.But the reason for that remains unclear.In addition,the numerical model of DNDF has been studied widely,but the numerical study on DMDF is very insufficient.So far,there has been many experimental studies about DMDF combustion,but they need to be analyzed deeply in theory and further described as the mathematical model.At present there are still some insufficiencies in the theoretical studies on DMDF combustion.The ignition mechanism of DMDF is not clear.The ignition delay correlation of DMDF has not been built.The numerical studies on DMDF combustion only focus on three dimensional models which are not validated well.So the main work in the dissertation included three aspects.Firstly,the ignition characteristics of DNDF and DMDF were studied on constant volume combustion chamber,and the ignition delay correlations of them were built.The ignition mechanisms of various kinds of dual fuels were studied,such as diesel-methanol,diesel-natural gas,diesel-LPG,diesel-ethanol and diesel-butanol.Secondly,the three dimensional combustion and emission model for DMDF engine was built based on KIVA code and validated with experiments.Parameter studies which included the ignition process,injection timings and EGR rates were done with this model.Thirdly,the Quasi-dimensional combustion model of DMDF engine was built based on FORTRAN language and validated with test bench experiments.Results are shown below.Firstly,the experimental study on constant volume combustion chamber shows that methanol and methane both delay the ignition of diesel and methanol delays more strongly than methane.The ignition delay correlations of DMDF and DNDF are well fitted.The reason for methanol and methane delaying ignition of diesel is that they both consume a lot of OH radicals by fuel dehydrogenation,but don't produce comparable OH radicals in low temperature oxidation,which results in the net reduction of OH radicals in the radical pool.The reason for methanol delaying the ignition of diesel more strongly than methane is that methanol dehydrogenation consumes more OH radicals than methane dehydrogenation,and reaction CH3O+O2=CH2O+HO2 in DNDF oxidation can contribute a lot of OH radicals to dual fuel radical pool,while there is not such reaction in DMDF oxidation.The numerical studies of various kinds of dual fuels show that the differences between the effects of high octane fuels on the ignition of diesel depend on fuel dehydrogenation and lowtemperature chain branching reactions of high octane fuels.In the low temperature oxidation,the order of the ability of high octane fuels delaying the ignition of diesel is ethane> propane> n-butane> n-pentane> n-hexane.Alcohol fuels delay the ignition of diesel more strongly than alkane fuels which have the same carbon atom number.Secondly,based on chemical reaction mechanism and semi-empirical emission model,the three dimensional model can predict well the combustion and emission of DMDF engine.The chemical reaction mechanism includes 65 reactions and 43 species.This mechanism also includes reactions related to NOX and C2H2 which was used as soot precursor.This model was validated with premixed laminar flame species concentration and test bench experiments.Parameter studies results show that,in DMDF engine,the chemical interaction factor is the most important factor for the retarding of diesel ignition timing compared to others,such as the high octane value of methanol,the lower oxygen concentration and the higher specific heat capacity in the intake charge.In DNDF engine,the effects of chemical interaction and high octane value of methane are comparable.With EGR rates increasing,NOX emission in DMDF mode decreases as same as in diesel mode,while the soot emission in DMDF mode increases less than in diesel mode.So using EGR in DMDF engine is better for emission than in diesel engine.Thirdly,the built quasi-dimensional combustion model of DMDF engine can predict well the DMDF combustion.The combustion rate of DMDF was modeled by a double Wiebe function model for diesel combustion and a flame propagation model for methanol combustion.In addition,methanol autoignition in heavy load in DMDF engine can be modeled well by three Wiebe functions.