Experimental And Numerical Study On The Laminar Premixed Dimethyl Ether/Methanol Flames

With the increasingly severe in the energy situation and the worsening in environmental pollution,new clean alternative fuels gain the more attentions.Dimethyl ether has become the most popular vehicle alternative fuel in recent years because of its simple molecular structure,high oxygen content,high cetane number and low emissions.However,because of the low boiling point of dimethyl ether,there are still some problems when burning it on the engine,such as gas resistance,corrosion of rubber,easy overflow and poor lubrication.Compared with the dimethyl ether,methanol has many advantages,such as good dynamic performance,good anti-explosion,high security,etc.The mixture of dimethyl ether and methanol can get better combustion and emission performance.At present,studies on combustion and emission characteristics of dimethyl ether/methanol blended fuels in engine performance have been carried out.However,the fundamental study on the fuel blends was lack and there are few reports on the details of the combustion of dimethyl ether/methanol blends.In this paper,dimethyl ether/methanol laminar premixed flame is studied experimentally and numerically.The laminar flame speed of the blended fuel under different initial conditions was measured in a combustion bomb.The chemical reaction kinetics of dimethyl ether/methanol mixture was analyzed by gas phase chemical reaction analysis software Chemkin II and the latest published chemical reaction mechanism Analysis was made both in the macroscopic and the microcosmic,the experimental phenomenon and the specific reaction mechanism and the reaction path analysis.The reliable experimental data can provid the basic research of the combustion of the dimethyl ether/methanol mixed fuel,and can provide theoretical basis and suggestion for engine operation.The main results are as follows:?1?The premixed laminar flame speed of dimethyl ether/methanol/air mixture was measured and calculated and the macroscopic combustion characteristics of the blended fuels were analyzed.The results show that the simulation results are in good agreement with the experiment.The flame speed of the blended fuels increases with the increase of methanol addition as the mixing degree within 60%,while decreases when that is greater than 60%.With the increase of equivalent ratio,the laminar flame speed of the mixtures increases first and then decreases.With the increase of methanol addition,the peak value of the laminar flame speed shifts to the richer mixture.With the increase of methanol addition,the adiabatic flame temperature of mixture decreases,and which increases first and then decreases with the increase of equivalence ratio.?2?The flame structure of dimethyl ether/methanol blended fuel under different initial conditions was studied based on the verified chemical reaction mechanism and Chemkin II.The effects of methanol addition and equivalence ratio on the flame structure and fuel reaction path were mainly analyzed.The results show that the burning rate of laminar flow is mainly influenced by free radicals such as H,OH and O in the mixture.With the increase of methanol addition,mole fraction of the free radicals H and OH decreases,that leads to the decrease in laminar flame speed.With the increase of methanol addition,the mass combustion flow is gradually sensitive to the sensitivity of the reaction R1:H+O₂=O+OH.The sensitivity analysis of the temperature can found that the products area is at the axial distance 1mm;fuel addition would affect on the reaction path of the two fuels,especially for methanol.By mixing with dimethyl ether,a part of CH₃ will combine with OH free radical to produce CH₃OH,and then influence the reaction path.