Thermodynamic Property Calculation For Hydrocarbon And Oxygenated Fuels: Methods And Applications

Thermodynamic properties,including standard enthalpy of formation,heat capacity and entropy,are essential for the construction of combustion chemical kinetic models.The method on the basis of electronic structure and statistical mechanics calculations is an effective method for obtaining thermodynamic properties.In this thesis,the methods for calculating thermodynamic properties of hydrocarbon and oxygenated fuels are explored and applied.First,the C4-C10 alkanes for which accurate experimental values are available are selected to evaluate the effects of different hindered rotation treatment methods on predicting thermodynamic properties.These methods include single structure harmonic oscillator approximation method,three one-dimensional methods and the multistructural method with torsional anharmonicity.The influence of different torsional potential energy surfaces and vibrational frequency analysis methods on the computed results is discussed.The results show that different treatments of hindered rotation have little influence on the standard enthalpy of formation but impact much the heat capacity and entropy at different temperatures.The one-dimensional methods proposed in this thesis can predict the heat capacity and entropy accurately and effectively.Then,several representative C2-C4 oxygenated compounds are selected to compare the performance of isodesmic reaction and atomization reaction combined with various quantum chemical methods on predicting the standard enthalpy of formation.The results show that isodesmic reactions predict the standard enthalpy of formation more accurately because of the better error cancellation effect compared with atomization reactions.The standard enthalpy of formation of oxygenated species can be calculated accurately and effectively by using the density functional theory M06-2X combined with isodesmic reactions.In order to explore the reliability and practicability of the calculation method,the thermodynamic properties of the important species in the dimethyl carbonate model are calculated,and the effects of thermodynamic properties from different sources on the simulation of ignition delay time of dimethyl carbonate are compared.It is found that the standard enthalpy of formation predicted by MN12-L is more accurate than that by M06-2X when the change of the O-CH₃ bond in RCOO-CH₃ is involved in the constructed isodesmic reaction.The simulation results are more consistent with the experimental data when these newly computed thermodynamic properties are adopted.In addition,the heat capacities and entropies of alkanes and alcohols are calculated and added into the database of an intelligent prediction model of thermodynamic properties.The results show that the new model can get accurate prediction results.Finally,M06-2X and the PI-N method are used in calculating the barrier heights and partition functions of the hydrogen addition reactions of unsaturated fatty acid methyl esters,and the obtained rate constants are incorporated in the combustion models.The mole fractions of several combustion products simulated by the new model agree better with the experimental data.In this thesis,through the research and application of thermodynamic property calculation methods for hydrocarbon and oxygenated fuels,the accurate and efficient calculation methods of thermodynamic properties are determined,which are very helpful for the construction and improvement of the combustion chemical kinetic models.