Optical Diagnosis And Combustion Mechanism Analysis Of Methane Blended With Dimethyl Ether For Lean Premixed Combustion

The shortage of fossil fuels and the serious pollution caused by fuel combustion are serious energy and environmental problems at the moment.Lean Premixed Combustion（LPC）technology is a low-pollution combustion technology with higher combustion efficiency and lower nitrogen oxide emissions.However,in practical applications,methane LPC technology faces problems such as unstable combustion and flame blow-out.Compared with methane,dimethyl ether has a faster maximum laminar flame propagation speed,a lower auto-ignition temperature and a wider combustion limit range.Based on this,this study uses dimethyl ether to replace part of the methane fuel to enhance the combustion stability of methane LPC technology and expand the lean burn limit.In this paper,OH*chemiluminescence analysis method and Tomographic Particle Image Velocimetry（Tomo-PIV）were used for combustion optical diagnosis,and Chemkin software was used to analyze combustion mechanism of the methane/dimethyl ether blended fuel.The chemical reaction mechanism of the combustion process was analyzed to explore the effect of the blending of dimethyl ether on flame morphology,thermal-state three-dimensional flow field,combustion mechanism,and emission generation mechanism.The results can be listed as follows:（1）Direct high-speed photography of flame morphology and OH*chemiluminescence intensity measurement were carried out for combustion with different air flow rates,equivalence ratios and blending ratios,and the effect of blending dimethyl ether on flame morphology was studied.Under low equivalence ratio conditions,40%blending ratio of dimethyl ether can effectively change the flame morphology,making it from a"V"flame to a relatively more stable"M"flame.When the equivalence ratio is 0.6,the blending of dimethyl ether has the most obvious change in flame morphology.Blending with dimethyl ether can effectively increase the OH*luminous intensity,increase the area occupied by high OH*luminous intensity,reduce flame fluctuations,and improve combustion stability.（2）Under low equivalence ratio conditions,the effect of different dimethyl ether blending ratios on the thermal-state three-dimensional flow field was studied.From upstream to downstream of the combustion chamber,the radial distribution of the average axial velocity of the flow field transforms from a symmetrical double-peak structure to a single-valley structure.And the closer it is to the downstream,the closer the wave peak position is closer to the outside region,and the peak value is lower with a further downstream position.The radial distribution of the average tangential velocity in the flow field gradually transforms from a single-peak and single-valley structure to a horizontal straight line with a value of zero.The spatial flow field structure under the working conditions of various dimethyl ether blending ratios is similar,and the blending of dimethyl ether has little effect on the thermal-state flow field for the same equivalence ratio.（3）The chemical reaction kinetics analysis of the methane/dimethyl ether combustion was also carried out.With the increase of the dimethyl ether blending ratio,the generation rate and total peak value of hydrogen radical,hydroxyl radical and methyl radical are increased significantly,and the total peak value appears earlier.When the dimethyl ether blending ratio is 40%,it also has a good effect of promoting fuel pyrolysis rate and accelerating the process of fuel oxidation.With the increase of the blending ratio,the reaction rate of CH₃OCH₂ radical from the pyrolysis reaction R444（CH₃OCH₂=CH₃+CH₂O）after dimethyl ether dehydrogenation increases rapidly,becoming the reaction that generates the most methyl groups.Besides,the formation mechanism of CO and NO_x was analyzed.Although the blending of dimethyl ether increases the production of CO and NO_x,the total production level of CO and NO_x is generally low under low equivalence ratio conditions,which is the lean burn conditions.With the increase of the blending ratio,the reaction rates of R30（HCO（+M）=H+CO（+M））and R31（HCO+O₂=CO+HO₂）to form CO are accelerated.At the same time,the main reactions R754（N+OH=NO+H）and R756（N₂+O=N+NO）,which generate thermal NO_x,also have a slightly faster reaction rate,so the total amount of CO and NO_x productions slightly increases.Based on the above research,it is shown that the 40%blending ratio of dimethyl ether at the equivalence ratio of 0.6,leads to stable combustion and a relatively lower amount of CO and NO_x production.