Study Of Dimethyl Ether Combustion Characteristics In The High-pressure Air Jet Controlled Compression Ignition Mode

Low nitrogen oxides(NO_x)and Soot emissions can be achieved simultaneously in the premixed compression combustion mode,but the combustion process is difficult to effectively control.High pressure air Jet Controlled Compression Ignition(JCCI)can effectively control the ignition phase in the premixed compression ignition process.Dimethyl Ether(DME)is similar to diesel,which belongs to high-cetane number fuel.Compared with diesel,DME has a higher oxygen content,and the emission of Soot is less in the combustion process.Moreover,DME is renewable and is an ideal alternative fuel for diesel.However,the combustion and emission characteristics of DME under high-pressure air JCCI mode need to be further studied.Therefore,CFD simulation software Converge was used in this paper to establish the CFD model of high-pressure air jet compression on the working medium and the pre-mixture of DME in the constant volume bomb.The maximum local temperature rise under the high pressure air jet and its impacts on the combustion process of DME are simulated.The research results were summarized as follows:(1)The effects of the inlet passage length,the initial pressure and temperature of the working medium on the maximum temperature rise under the high-pressure air jet were studied.With the increase of the inlet passage length,the compression of the initial working medium was more intensive,and the maximum local temperature rise in the constant volume bomb was increased gradually.With the decrease of the initial pressure,the pressure difference increased,and the compression effect of the air jet on the initial working medium was more obvious.However,when the pressure difference was too large,normal shock waves would be generated,which would affect the flow of air jet.A higher initial temperature with a lower density of the initial working medium leaded to a faster air jet inlet velocity,and a higher maximum local temperature rise.(2)The effects of air jet pressure and duration on the combustion process of DME under high pressure air JCCI mode were studied.With a longer air jet duration,the compression of the pre-mixture became more intensive during the air jet phase,and the start of combustion(SOC)was advanced gradually.However,longer air jet duration brought in more"low-temperature"air.After mixing,the local temperature and equivalence ratio of the mixture were reduced,leading to a decline of later combustion rate,lower peak of heat release rate,longer combustion duration,lower local combustion temperature,and reduced production of NO_x.Due to the earlier SOC in the longer air jet duration,the combustion process was less affected by the mixing of"low temperature"air,and the high temperature area in the combustion process was wider,which promoted the oxidation of carbon monoxide(CO)and total hydrocarbons(THC).A higher air jet pressure,with a larger pressure difference caused a more intensive compression of the initial working medium.However,under the effect of"low-temperature"air mixing,with the increase of air jet pressure,the peak heat release rate decreased,the combustion duration extended,the production of NO_x decreased,and the production of CO and total THC increased.(3)The effects of the initial equivalence ratio and temperature on the combustion process of DME under high pressure air JCCI mode were studied.The increase of the initial equivalence ratio accelerated the combustion process,SOC was advanced gradually,and the local temperature in the combustion process increased,which promoted the formation of NO_xand the oxidation of CO and THC.With the increase of the initial temperature,SOC was advanced gradually,which was conducive to the combustion process.However,the combustion temperature was also higher at a higher initial temperature,which promoted the formation of NO_x and the oxidation of CO and THC.