Study On The Mechanism Reduction And Numerical Simulation Of CH4/DME Blended Fuel In Micro-combustion

In recent years,hydrocarbon fuel has attracted the attention of domestic and foreign scholars by its high energy density,light weight and long power supply.Extensive research has been done on the micro power system based on hydrocarbon fuel.The performance of micro burner as a core component of micro power system is closely related to the energy output of the system.However,different from conventional scale,micro-scale combustion is faced with challenges such as flame quenching and instability.In the face of these challenges,many measures to enhance combustion and stabilize flame have been put forward by researchers.Our research group has proposed a new blending way,premixed methane/DME/air in micro combustor.In the previous experimental work,it has been found that the addition of DME can greatly expand the combustible limit and effectively promote the combustion.Due to limited testing techniques on the experimental platform,the flame dynamics of premixed methanemixed DME combustion is not fully understood.Compared to the experimental method,the numerical simulation can obtain detailed information of the combustion process more convenient.Science the mixed mechanism of methane/DME which is suitable for micro scale field has not been developed yet,one of the works in this paper is to develop the mechanism of methane/DME mixture suitable for micro-scale combustion.Subsequently,a numerical simulation study was carried out on the flame dynamics of methane/DME/air premixed combustion.Moreover,the entropy generation rate of the micro combustor is calculated and the efficiency of the system is analyzed.The main research contents and innovations are as follows:(1)The detailed chemical reaction mechanism of methane/DME was simplified by DRGEPSA software.Based on the sensitivity analysis of laminar flame speed,the mechanism of methane/DME mixture(consisting of 25 species and 96 reactions)was developed for micro-combustion.The mechanism can accurately predict the ignition delay time and laminar flame speed at atmospheric pressure with various DME addition ratios and equivalence ratio from 0.7 to 1.5.A three dimensional numerical model of premixed methane/DME/air in a micro-planar combustor is constructed for model verification.The results show that the model agrees well with the experimental results,no matter the flame shape or the blow out limit.(2)In the stainless steel micro-planar combustor,the basic comparison of flame shape between methane with or without DME addition is accomplished by changing the DME addition ratio and equivalence.U-shaped flame and double-peak U-shaped flame are found in the combustion of methane with DME addition.When the equivalence ratio is 0.9,the unsteady inclined flame vanishes.The principle of lean and rich combustion condition on methane with DME addition is discussed,which explains the main reason of DME condition can promote methane/Air combustion.(3)The formation and transformation mechanism of U-shaped and inverted Ushaped flame are summarized by giving structural schematic when the equivalence is 0.9.The influence of DME blending ratio on the transformation process of inverted Ushaped flame is discussed from two aspects: flame position and degree of asymmetry.It is found that blending of DME can effectively delay the movement of the inverted Ushaped flame to the outlet and strengthen its deformation resistance.(4)By changing the DME blending ratio and inlet velocity,the entropy generation rate of methane blended with DME in the micro combustor is calculated.The mechanism of the DME condition to the entropy generation induced by chemical reaction,mass diffusion and thermal conduction is discussed in detail.The influence of DME blending ratio and flow rate on the exergy efficiency of the micro combustor is further discussed.It is found that when the inlet velocity is less than 0.3 m/s,exergy efficiency decreases with the increase of DME blending ratio.When the inlet velocity is greater than 0.3 m/s,exergy efficiency shows an opposite trend.The input energy of methane blending with DME increases with the increase of inlet velocity.Therefore,high DME blending ratio and inlet velocity are ideal working condition to improve the exergy efficiency.