Experimental And Kinetic Studies On Cool Flame Characteristics Of Hydrocarbon Fuels

With the development of society and economy,the relation between energy and environment are becoming more and more serious.Low-temperature combustion is regarded as a potential development direction of combustion technology because of its unique advantages of improving energy conversion efficiency and reducing pollutant emissions effectively.However,the high complexity of the fuel low-temperature oxidation process has led to an unclear understanding of its mechanism,which has seriously hindered the development of low-temperature combustion technology.The spontaneous combustion and cool flame characteristics of hydrocarbons controlled by low temperature kinetics are related to the safety,reliability,and economy of low temperature combustion process,and therefore become the focus in the study of low temperature combustion kinetics.In view of the lack of cool flame experimental data of hydrocarbons and the lack of understanding of their formation and development mechanisms,the formation and development of the cool flame of typical fuels were studied using a constant volume combustion vessel,and the key factors affecting their cool flame characteristics were analyzed.Finally,the promotion and inhibition mechanisms of the cool ignition process of hydrocarbons by adding typical components were investigated.The main work and results are as follows:(1)A constant-volume combustion vessel was designed and built to study the low-temperature oxidation characteristics of methane and dimethyl ether at different initial pressures,temperatures,and equivalence ratios to obtain the corresponding ignition limits under different conditions,and to map the cool flame region of the fuels.The results show that the cool flame can be obtained under both very rich and very lean conditions;the cool flame can exist in a wider pressure range in the rich condition than in the lean condition;meanwhile,the lower pressure limit of the cool flame region decreases with the increase of the initial temperature.(2)Numerical simulations of the methane cool ignition process were carried out using different reaction mechanisms,and a kinetic mechanism that can describe the methane cool ignition process more accurately was selected by comparing the numerical results with the experimental data;and the numerical study of the methane cool ignition characteristics was carried out using this mechanism,focusing on the influence of heat loss on the methane cool ignition mode,and the key reaction paths affecting its ignition characteristics were obtained through sensitivity analysis and path analysis.The key reaction paths affecting the ignition characteristics were obtained through sensitivity analysis and path analysis.The results show that there are three ignition modes of methane:hot ignition,two cool ignition and one cool ignition;the key paths affecting the cool ignition characteristics of methane are CH₃?CH₃O₂?CH₃O₂H?CH₂O and CH₃?CH₃O?CH₂O.(3)Different kinetic mechanisms were used to simulate the low-temperature premixing and diffusion ignition processes of DME.Through the comparisons of numerical results and experimental data,the mechanism of DME is optimized,and the optimized mechanism can describe the DME cool ignition process more accurately.The heat release analysis and path analysis of the cool ignition process are carried out through this mechanism.The results show that the key reaction pathways affecting the cool ignition characteristics are:CH₃OCH₃?CH₃OCH₂?CH₃OCH₂O₂?CH₂OCH₂O₂H?O₂CH₃OCH₂O₂H?HO₂CH₂OC HO?OCH₂OCHO,CH₂OCH₂O₂H?HO₂CH₂OCHO,CH₃OCH₂O₂?CH₃OCHO/CH₂O and CH₃OCH₂?CH₂O.(4)The cool ignition process of n-heptane was simulated to study the effects of different fuel/nitrogen ratios,initial velocities of fuel and oxidizer on the ignition characteristics of n-heptane,and the heat release analysis and reaction path analysis of the n-heptane cool ignition process;the effects of adding ozone and methanol on the cool ignition process of n-heptane were analyzed.The results show that there are three different types of flames,cool flame with double heat release peak,hot flame with double heat release peak and cool flame with single heat release peak in the low-temperature oxidation of n-heptane;the key reaction pathways of n-heptane cool ignition process are the generation of fuel radicals?oxidation of fuel radicals to peroxyalkyl radical?isomerization of peroxides?secondary oxygenation of hydroperoxides?dissociation of the products after secondary oxygenation?dissociation of aldehydes.Ozone accelerates the cool ignition process by decomposing O radicals at low temperatures and leads to the third-stage ignition of n-heptane;methanol reduces the low-temperature oxidation rate of n-heptane by inhibiting the generation of OH radicals.The experimental results obtained in this work provide data support for the development of the low-temperature mechanism of fuel oxidation,and the revelation of the key reaction pathways of fuel oxidation in the low-temperature region can effectively promote a deep understanding of the cool flame properties of hydrocarbon fuels,and the understanding of the mechanism of additives affecting the low-temperature oxidation of hydrocarbon fuels provides theoretical support for the exploration of the cool flame regulation technology of hydrocarbon fuels.