Construction And Application Of Multi-component Skeletal Mechanism Of F-T Diesel And Semi-empirical Soot Model Of PAHs

The multi-component surrogate fuel can more accurately reproduce the physical and chemical properties of the real fuel,but the current soot models be used widely are developed based on single component fuel,which can not meet the requirements of multi-component fuel.Polycyclic aromatic hydrocarbons(PAHs),as an important precursor of soot model,play an important role in the formation of soot.By analyzing the properties of F-T diesel and the path of PAHs generated by oxidation of different molecules in the fuel,this paper constructs the F-T diesel surrogate fuel skeletal model including PAHs sub-mechanism,so that the model can be applied to different fuels and improve the adaptability of surrogate fuel skeletal mechanism.The computational fluid dynamics(CFD)calculation of the working process of F-T diesel engine using this model not only improves the calculation efficiency,but also has an important significance to further explore the combustion and emission mechanism of F-T diesel engine and the clean utilization of F-T fuel.F-T(Fischer Tropsch)diesel is an alternative fuel prepared by Fischer Tropsch synthesis,which has the characteristics of high efficiency and cleanness.In this paper,the multi-component surrogate F-T fuel was constructed by analyzing the components of F-T diesel,n-dodecane(n-C₁₂H₂₆),iso-octane(i-C₈H₁₈)and methyl cyclohexane(MCH)are selected to represent the n-alkane,iso-alkane and cycloalkane in the fuel,respectively.The proportion of each component in the alternative fuel is determined by the characteristics of the actual fuel,including the cetane number(CN),C/H ratio and component composition.In order to establish the skeletal mechanism model,firstly,based on the two-step direct graph(DRG)and the important index method of singular perturbation(CSP).The detailed n-dodecane mechanism contains 1279 species and 5056 reactions were reduced to a simplified n-dodecane model with 159species and 399 reactions.Based on the decoupling method,the n-dodecane skeletal mechanism is constructed,including simplified n-dodecane mechanism,C₂-C₃ sub-mechanism and detailed H₂/CO/C₁ sub-mechanism.Finally,186species and 502 reactions F-T diesel multi-components surrogate fuel-PAHs skeletal mechanism were construced by coupling the oxidation mechanism of iso-octane macromolecule,methylcyclohexane mechanism and PAHs formation mechanism.The mechanism was validated by using a variety of basic reactor experimental data,including ignition delay in the shock tube,primary species concentrations in jet-stirred reactors,the premixed laminar flame and HCCI engine over wide operating conditions.The results were consistent with the experimental data.Based on the engine model,a three-dimensional CFD simulation model is established,which is coupled with F-T diesel surrogate fuel-PAHs model to valiate the combustion characteristic,PAHs and soot generation.The simulations show that the mechanism can accurately predict the combustion performance and emission characteristics of F-T fuel in the engine.It can can increase the maximum cylinder pressure of the engine,improve the power performance,and significantly reduce the emissions of PAHs and soot by appropriately reducing the initial intake temperature and increase the injection advance angle.