Analysis Of The Combustion Mechanism Of Diesel Fuel In Oxygen-Carbon Dioxide Atmosphere

CO₂ produced by diesel engine of power machinery under closed space such as tunnel or pit is a kind of gas harmful to human health,however,liquid oxygen solid carbon closed cycle diesel engine can effectively control CO₂ emission and thus reduce human health hazards,but there are problems of misfire,unstable combustion and unclear mechanism of diesel fuel combustion under CO₂ atmosphere.Therefore,it is essential to explore new diesel combustion methods and combustion mechanisms.In order to study the effect of CO₂ on diesel fuel combustion in CO₂/O₂ atmosphere,a quantum chemical model of diesel fuel combustion in this atmosphere is proposed,which considers the effect of CO₂ pyrolysis（CO₂→CO+O·）,the new reaction mechanism of OH radical generation from pyrolysis products（CO+O₂+H·→CO₂+OH·）,the reaction mechanism of OH radicals with diesel surrogate fuel molecules and the dynamic change of CO₂ concentration.Firstly,the potential energy surface for the thermal disintegration of CO₂ molecules are calculated by the coupled cluster theory of quantum chemistry,and the intermediate,transition state structures and the energy barriers to be traversed for the reaction of CO to OH radicals are calculated by the density function theory of quantum chemistry.Chemically reactive sites of n-heptane pyrolysis molecules and toluene are predicted by average local ionization energy and molecular surface electrostatic potential.Arrhenius parameters of the new reaction path are calculated by molecular reaction dynamics.The mechanism is coupled and the coupling mechanism is simplified by sensitivity analysis and species concentration analysis to obtain the combustion mechanism of diesel fuel under CO₂/O₂ atmosphere.Simulation of diesel fuel combustion under CO₂/O₂ atmosphere is performed by computational fluid dynamics,and combustion characteristics parameters such as temperature map,concentration field and flame float length are obtained.Secondly,the experimental platform of visual optical constant volume combustion chamber is built.The combustion background atmosphere（50%vol.CO₂+50%vol.O₂、47%vol.CO₂+53%vol.O₂、43%vol.CO₂+57%vol.O₂、39%vol.CO₂+61%vol.O₂和35%vol.CO₂+65%vol.O₂）,initial temperature 850 K,initial pressure 3 MPa,injection pressure 120MPa,injection volume 18.4 mg and injection pulse width 0.5 ms are established.The experiment is repeated ten times under the same working condition,and the ignition and flame propagation process of diesel fuel under different CO₂ atmosphere are obtained by using high-speed camera.The experimental data are postprocessed by writing Python code and other image processing programs.Finally,the combustion mechanism and combustion characteristics of diesel fuel under CO₂/O₂ atmosphere are analyzed.The results show that CO₂ pyrolysis occurs at high temperature,and the Boltzmann distribution of the products and reactants can reach 98.3%.CO₂ can directly participate in the chemical reaction,and its chemical effect shows that CO₂can promote the combustion of diesel fuel under CO₂/O₂ atmosphere.The minimum average local ionization energy and surface electrostatic potential of oxygen atom of CO are 13.56 e V and-0.37 e V,while that of carbon atom are 12.62 e V and-0.51 e V,respectively,indicating that the chemical reactivity of carbon atom of CO is greater than that of oxygen atom.The maximum,minimum and average errors between the calculated and experimental values of the flame lift-off length of diesel combustion under 35%CO₂+65%O₂ atmosphere are 8.9%,0.5%and 1.4%,respectively,which are all within the acceptable range.The natural luminosity of the flame fluctuates under CO₂/O₂ atmosphere.CO₂ will cause unstable combustion and turbulence,and also lead to the surface wrinkles of the flame and the increase of flame perimeter.