Molecular Dynamics Simulation And Modeling Of Soot Formation In Hydrocarbon Fuels Combustion

Soot formation is one of the most complex phenomena in the combustion process,which involves the interactions among combustion chemistry,hydrodynamics,mass transfer,heat transfer and particle dynamics.Therefore,despite decades of active research,the gap between our understanding of soot and the actual process is still large.Soot is mainly produced by incomplete combustion of fuels.The adverse effects of soot on the environment and human health and the industrial demand for carbon black production encouraging us to study the mechanism of soot formation and to model the soot generation.Firstly,the harm of soot to the environment and human health was expounded at the beginning of this article,and the background and the significance of soot research were introduced.Then,the experimental and measurement methods of soot were illustrated.Then we briefly introduced and summarized the various processes of soot formation and research progress of soot formation,and proposed two major tasks of this study:to establish a suitable soot model for the calculation of soot formation,and to study the interactions between polycyclic aromatic hydrocarbons(PAHs)and interactions between PAHs and soot,and trying to reveal the mechanism of soot nucleation and growth.In the aspect of soot model,this paper improved the nucleation model based on PAH collision,considering more PAHs involved in nucleation and condensation.And a fuzzy membership function was adapted to ensure the smooth mathematical transition between soot sections.The oxidation collision coefficient of different sizes of soot was modified by parameter fitting.The soot generation of ethylene and jet-Al flames was calculated by using the improved model,and the soot generation under microgravity was also studied.Detailed mechanism and detailed model of soot calculation is very computer resource consuming.Based on soot model,combustion model and the characteristics of soot and flames,input parameters are reduced by Variable clustering method.Some species are sensitive to soot formation,some are directly involved in the process of soot nucleation,growth and oxidation,and some are not related to soot formation.The flame state space described by 26 input parameters is obtained by clustering dimension reduction results.The distribution of soot number density with soot diameter approximately conformed to logarithmic coordinate cubic function.A vector space of soot state is composed of volume fraction and four fitting parameters describing soot number density distribution.When the mapping from flame state space to soot description space is established,a method of reducing soot mass to soot precursor was proposed based on mass conservation.Here,a deep neural network with four hidden layers is constructed.Then we built a sample library consisting of 120 soot flame cases,and complete the construction of loss function.The numerical results of a premixed Stagnant Flame and an opposed flame are compared between DNN model and detailed soot formation model,respectively.The results predicted by DNN model are almost consistent with those predicted by detailed soot model.It show that the calculation speed of DNN model is about 8-20 times faster than that of 35 sectional aerosol model.In the aspect of mechanism research,the reaction activities of PAHs and its derivatives,five membered ring PAH,resonance stable PAH radical(RSR)and oxygen-containing functional group PAHs were analyzed by using Fukui frontier orbit theory.Among the substituents,the order of reactivity is alkynyl>alkenyl>alkyl.The reactivity of PAHs with acetylene group is closely related to the size of PAHs.The larger of the PAHs are,the lower of the reactivity of acetylene group in that PAHs.For six membered PAHs,the position of acetylene group has little effect on the reactivity.The presence of five membered rings enhances the reactivity of PAHs.Some PAHs with five membered rings have stronger reactivity than RSR with the same size.The reactivity of RSR aromatics decreased with the increase of PAHs particle size.The oxygen atom at the edge of PAHs shows strong reactivity,and the oxygen atom shows a very large Fukui index,even more active than the corresponding arylacetylene.The oxygen in the CHO substituent is much more stable than that at the edge of PAHs.By using ReaxFF force field,we simulated soot nucleation and growth under different flame temperatures.At a lower temperature(1000K),the smaller PAHs can not condense and nucleate at about 1000K.When some larger PAHs are added into the system,the effect of non-bond interactions is more complicated than that of homogeneous nucleation,which leads to the increase of nucleation temperature limit.At a moderate flame temperature(1450k),non-bond interaction is not strong enough to maintain the stable aggregation of small PAHs.However,the chemical reaction between these PAHs and unsaturated aliphatic hydrocarbons and the addition of unsaturated aliphatic hydrocarbons lead to the growth of PAHs.Polycyclic aromatic hydrocarbons(PAHs)are connected by branched chains to form clusters or clusters connected by chemical bonds.Under the condition of high temperature(1750k),due to the attack of a large number of free radicals,PAHs lose hydrogen atoms and form free radicals.At the same time,the addition of unsaturated hydrocarbons makes PAHs chain grow,even ring grow.The appearance of five membered ring or even seven membered ring makes PAHs appear curved surface structure.The chemical dimerization of PAH radicals and the combination of aryl and aryl-ethynylenes lead to the formation of clusters at this temperature.The larger clusters continue to grow and form soot with initial nucleation.The free energy curves of PAHs dimerization and condensation on soot surface were studied by steered molecular dynamics.We observed two condensation modes.That condensation at low temperature tends to be H-type,because there is less free energy.In most cases,with the increase of simulation temperature,V-type condensation will appear in the middle.The structure of multilayer fullerenes enhances the condensation of PAHs,but has no significant effect on the free energy.Hydrogenated fullerenes are unstable,and the C-H bond on the surface will prevent the condensation of PAHs on the fullerene surface at high temperature.From the point of view of equilibrium constant,the reaction limits of condensation and homogeneous nucleation are analyzed.PAHs condensation process is more important than nucleation in soot mass increase.But the nucleation process provides the starting point and number density of initial mass increase for condensation.The definitions of enhancement factor and condensation coefficient are also introduced from the perspective of molecular dynamics statistics.Then,a lot of ReaxF-MD cases were built to simulate the condensation of PAHs on the surface of fullerene and soot.By counting the condensation of PAH molecules on the surface of fullerene and soot,condensation coefficients at different temperatures can be fitted.With the increase of temperature,the condensation coefficient of PAH decreases dramatically.The larger the PAH is,the more the condensation coefficient temperature curves moves to the right side.At the same time,the simulation shows that the effect of nitrogen addition on the condensation coefficient is not significant.