Molecular Simulation And Experimental Measurement Of Transport Properties For Alternative Fuels

The over exploitation and use of fossil fuels has not only caused serious environmental problems,but also led to the shortage of energy.In the face of the intensification of the energy crisis,finding fuels that can replace traditional energy has become a hot issue for scholars all over the world.Biodiesel has excellent physical and chemical properties and is considered to be a high-quality alternative fuel.Besides,alcohols can not only be used as fuel alone,but also mixed with other fuels to improve their physical and chemical properties.Therefore,it is very significant to explore its thermophysical properties.As the basic thermophysical property of material,thermal conductivity is the basic parameter of internal combustion engine design.Viscosity is also one of the basic physical properties,which affects the atomization characteristics of fuel and mechanical wear.At present,there are few studies on the thermophysical properties of alcohols and biodiesel through the combination of experiment and molecular dynamics simulation.In this paper,the thermal conductivity of fatty alcohol is accurately measured by transient hot wire method;Then,the thermal conductivity and viscosity of seven fatty alcohols and three fatty acid methyl esters（methyl decanoate,methyl laurate and methyl myristate）are calculated by molecular dynamics simulation method,and the effects of molecular chain length and temperature on thermal conductivity are analyzed on microscopic level.The results results show that:1.According to non-equilibrium molecular dynamics（NEMD）method,the feasibility of calculating the thermal conductivity of organic matter is proved by a large number of simulation operations in the OPLS-AA force field.The simulation results show the same trend with the experimental data,and the relative deviation between them is less than 7%.2.In the simulation of heat conduction,it is found that at the same temperature and pressure,the proportion of molecular interaction gradually increases and plays a major role in energy transmission with the increasing number of carbon atoms.In the molecular interaction,the intermolecular interaction term gradually decreases,while the intramolecular interaction term gradually increases,and the intersection of the ratio of them to molecular interaction terms coincides with the lowest point of the thermal conductivity of the seven alcohols;With the increase of temperature,the proportion of molecular interaction contribution decreases,while the convection contribution increases gradually.The increase of temperature leads to the intensification of molecular thermal motion and the increase of energy dissipation caused by kinetic energy.Besides,the increase of temperature leads to the decrease of liquid density.In other words,the number of molecules per unit volume decreases,the energy transmission efficiency decreases,and the thermal conductivity decreases gradually.3.Through the calculation and analysis of the radial distribution function of alcohol molecules,the changes of four main atom pairs（O-O,O-H,O-C and C-C）in alcohol molecules with the number of carbon atoms and temperature are investigated.The results show that O-O,O-H and C-C atom pairs are not affected by the change of carbon atom number,but the peak value of O-O and O-H atom pairs increases with the increase of carbon protons,while ethanol is slightly larger than 1-propanol,and the peak value of C-C atom pairs is relatively concentrated;The peaks of O-C atom pairs of other six alcohols decrease with the increase of the number of carbon atoms except ethanol,and the peak position is gradually away from the central atom;However,with the increase of temperature,the peak value of the four atom pairs decreases and the peak position does not change.4.The thermal conductivity of methyl decanoate,methyl laurate and methyl myristate are simulated in the pressure range of 0.1 to 15MPa and the temperature range of 332K to 372K according to the NEMD method,.The maximum relative deviation between the simulation results and the experimental data is-8.87%.Based on the above results,it is found that the thermal conductivity of the three fatty acid methyl esters decreases and shows a linear relationship with the increase of temperature.5.The viscosity of 1-propanol to 1-octanol was calculated by RNEMD method at 298K.Besides,the variation of viscosity with temperature and pressure was calculated by taking1-propanol as an example.Compared with the experimental data,the simulation results are in good agreement with the relative deviation within 15%which prove that this model can be used to calculate the viscosity of alcohols.6.The thermal conductivity of seven fatty alcohols（ethanol,1-propanol,1-butanol,1-pentanol,1-hexanol,1-heptanol and 1-octanol）were accurately measured at standard atmospheric pressure and temperature range from 293K to 363K.Based on the experimental data,it is found that the thermal conductivity of seven liquid fatty alcohols decreases with the increase of temperature.At the same temperature and pressure,the thermal conductivity of the seven liquid fatty alcohols decreased firstly and then increased.In other words,the thermal conductivity of C₂（ethanol）to C₅（1-pentanol）decreased gradually,and the thermal conductivity increased slowly with the extension of molecular chain.