Study On Characteristics Of CH₄ Laminar Flame Under O₂/H₂O Condition

Due to the increasingly serious energy and environmental problems,automobile exhaust,as the main source of air pollutants,is also subject to increasingly strict emission regulations.Pure oxygen combustion technology is a clean combustion technology with great application prospects.At the same time,researchers propose that the introduction of water vapor in the process of pure oxygen combustion and its application in the engine can effectively reduce pollutant emission and flame temperature,and improve the thermal efficiency of circulation.At the same time,natural gas engine has been paid more and more attention because of its good emission performance.The technology has been used successfully in turbines to generate electricity,but rarely in internal combustion engines.In order to widely apply this technology to natural gas engines,a lot of basic research data on the flame characteristics of CH₄in O₂/H₂O atmosphere are needed.In this paper,the laminar flame characteristics of CH₄in O₂/H₂O atmosphere are studied.The basic data of laminar flame velocity and flame structure under different dilution gas,dilution ratio,preheating temperature,equivalence ratio and water vapor concentration are obtained by constant volume combustion bomb experiment and numerical calculation with CHEMKIN software.The influence of these data on the flame characteristics and combustion process of CH₄is analyzed.The physical and chemical effects of diluent gas on the reaction mechanism and reaction path were compared.It provides theoretical guidance for the establishment of detailed chemical reaction mechanism suitable for O₂/H₂O combustion.The laminar flame velocity of CH₄under different conditions was calculated by using constant volume combustion bomb test-bed and CHEMKIN software.Under different dilutions,the laminar flame velocity decreases with the increase of dilution ratio.In the case of N₂dilution,the laminar flame velocity decreases linearly with the increase of dilution ratio;in the case of H₂O dilution,the laminar flame velocity decreases linearly in the initial stage,and gradually becomes gentle in the later stage;in the case of CO₂dilution,the laminar flame velocity decreases exponentially with the increase of dilution ratio.At the same dilution rate,CO₂dilution has the most obvious effect on reducing laminar flame velocity,followed by H₂O dilution.Compared with N₂dilution,the maximum value of OH radical changed little in H₂O dilution,while the maximum values of O and H decreased significantly.Similarly,the maximum values of OH,O and H decrease significantly when CO₂is diluted.When CO₂is diluted,the consumption path of CH₂（s）changes,and the primary reaction R153 is the main reaction of CH₂（s）consumption in O₂/CO₂atmosphere;when H₂O is diluted,the conversion of CH₃to CH₄is promoted through the primary reaction R52,which slows down the overall reaction speed and reduces the laminar flame speed.With the increase of CO₂content,the thermal diffusion effect increases gradually,which becomes the main reason for the decrease of laminar flame velocity.When the water content increases,the physical effect decreases and the chemical effect increases.The laminar flame velocity first increases and then decreases with the increase of equivalence ratio,and reaches the peak at about 1.0.With the increase of equivalence ratio,the relative mole fraction of H radicals increased,while the relative mole fraction of O and OH radicals decreased.The decrease rate of OH was higher than that of O,while the relative mole fraction of HO₂radical remained unchanged.With the increase of preheating temperature,the laminar flame velocity increases exponentially,and the peak concentration of main free radicals H and OH in the flame increases obviously,while the free radical O increases slightly,and the free radical HO₂remains basically unchanged.With the increase of water vapor concentration,the physical effect of water vapor decreases and the chemical effect increases,but the physical effect is the main reason for the decrease of laminar flame velocity.With the increase of water vapor content,the peak concentration of main free radical H,O and OH gradually decreased,but at high water vapor concentration,water vapor promoted R86:OH+OH=O+H₂O to reverse,and the relative molar concentration of free radical OH gradually increased.