| In recent years,with the rapid development of the automobile industry,the car ownership has gradually increased,and the demand for diesel is also increasing.Simultaneously,people are facing increasingly prominent energy and pollution problems.When traditional cars powered by diesel burn,they emit large amounts of harmful gases such as CO,SO2 and NO,as well as micro-nano particles.These exhaust emissions are direct factor of haze formation.The development of new clean energy is an important measure to reduce the burden of fossil energy and improve the air pollution caused by exhaust emissions from traditional automobiles.Biodiesel is an environmentally friendly clean fuel,which can be used as an alternative fuel and fuel additive for fossil diesel.When biodiesel used as a fuel additive mixed with diesel,the emission of pollutants can be effectively reduced without changing the engine structure.The density,viscosity,surface tension data and interface properties of biodiesel are the basic parameters of fuel pipeline and internal combustion engine design,and are also important factors affecting fuel injection and combustion efficiency.It is almost impossible to carry out detailed research on the properties of fuel,because biodiesel and fossil diesel are very complex mixed systems,consisting of hundreds or even thousands of hydrocarbons,and the composition and properties of the fuel vary according to the production process,the source of the feedstock,and the standard of use.Therefore,in this paper,one or two fatty acid esters and alkanes which are close to the actual fuel properties are used as physical model fuels to replace the complex fuel to study the thermophysical properties of biodiesel and biodiesel as additives in the mixing system with fossil diesel.This paper adopts the method of molecular dynamics simulation,the research contents include:Taking ethyl hexanoate system as an example,the randomness of molecular simulation was studied,and the effects of the number of molecules,simulation time,cutoff radius and neighbor list radius on the simulation results were explored;The properties of density,viscosity and surface tension of ethyl hexanoate,ethyl heptanoate,ethyl octanoate,ethyl laurate,methyl butyrate,methyl octanoate,n-decane,n-dodecane and n-hexadecane were simulated.The change rules of three properties and the simulation deviation distribution were analyzed;The AMBER force field,GROMOS force field,and OPLS-AA force field were used to simulate the thermophysical data of n-dodecane,n-hexadecane,and ethyl heptanoate.The simulated data were compared with the experimental data,and the simulation deviations of the three force fields were analyzed.Finally,the optimal parameters of force field were selected to simulate the mixed system of alkane and ester;The thermal physical properties of six mixed systems(ethyl hexanoate and ethyl dodecanoate,ethyl octanoate and ethyl dodecanoate,n-decane and methyl octanoate,n-decane and methyl decanoate,n-hexadecane and ethyl hexanoate,n-hexadecane and ethyl heptanoate)with molar fraction ratios of 0.25-0.75,0.50-0.50,0.75-0.25were obtained.The interface layer thickness of the mixed system was calculated using the"10-90"principle.The variation law of the interface thickness and the interface characteristics of the mixed system were studied.The main conclusions of this article are the following:(1)It is found that the simulation results of the system with a large number of molecules are more accurate.Considering the accuracy and efficiency of the simulation,the appropriate particle filling number can be selected;If the simulation time is too short,it is difficult for the system to reach the equilibrium state,which will cause a large simulation deviation.Especially for the simulation of viscosity and surface tension,sufficient simulation time is necessary.According to the research on the cutoff radius and the radius length of neighbor list,the optimal value of cutoff radius is 1.4 nm,while the length of neighbor list radius has almost no effect on the simulation results;There is a certain randomness in the simulation results of molecular dynamics.For a system with a small number of particles in a box of a certain size,the randomness is greater than that in a system with a large number of particles,and the randomness of simulation at low temperature is greater than that at high temperature.(2)The simulated date of density,viscosity,and surface tension are consistent with the experimental date,and the deviations are within the allowable range.The density of fatty ethyl ester decreased with the increase of the number of carbon atoms,and the surface tension and viscosity gradually increase with the increase of carbon atoms.The density,viscosity and surface tension of fatty acid methyl ester and alkane all increased with the increase of the number of carbon atoms.(3)The deviations of density of fatty acid esters simulated by OPLS-AA force field is about±1%.The relative deviation of simulated surface tension value is within 15%of the absolute value of deviation of data points,except that the methyl butyrate is larger at 298.15 K.The simulation has reached the allowable range of deviation,OPLS-AA force field cannot accurately simulate fatty acid ester viscosity and thermal properties of alkanes,especially long esters and alkanes.GROMOS is the best force field for the reappearance of the three thermal properties of alkanes among OPLS-AA,AMBER and GROMOS force field,and the simulation of the thermal properties of fatty acid esters is accurate.Therefore,GROMOS force field is the best choice for the simulation of the binary mixture of alkanes and esters.(4)It is found that the density,viscosity and surface tension of the mixed system are between the two pure materials,and the thermal property data are closer to the pure materials with large mole fraction.The interfacial properties of the mixed system were analyzed,and the changes of molecular density and interfacial thickness were obtained. |
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