| Aviation fuel not only should have a higher energy density, but also a good heat absorption capacity. However, fuels as combustible coolant, its limited physical heat has couldn’t meet the cooling requirement of hypersonic flight. Therefore, developing the chemical heat sink of fuels is one of the important content of fuel research. Endothermic hydrocarbon fuel chain alkanes (C7H16～C18H38) were taken as an object of study this paper. On the basis of study on the reaction mechanism, product distribution and heat sink of n-decane by employing the density functional theory (DFT), the effects of carbon chain length and branched chain on endothermic ability of chain alkanes (C7H16～C18H38) have been also researched preliminarily. The research main conclusions of this thesis are as follows:1. The geometry optimizations and vibrational frequencies of reactions, products and transition states involved in the pyrolysis of n-decane were performed using the density functional theory B3LYP with6-311G(d,p) basis set. On this basis, the single point energy was calculated by using the B3LYP/aug-cc-pVTZ method, and the potential energy profile was also constructed. The rate constants of all reactions with Eckart correction were calculated by the TheRate program package. The heat capacity and entropy (Cθp,m and Sθ) at different temperatures were obtained by statistic thermodynamics. In order to calculate the standard formation enthalpy (△fHθ298K) for all species, isodesmic reactions were designed. The Chemkin Ⅱ program was used to model the product distribution, combined with the Heatsink program to compute the heat sink. The effects of the temperature and pressure on the heat sink and product distribution were discussed. The results show that the C-C bond breaking process is the initial step of thermal pyrolysis reactions and H-abstraction reaction is easier to operate than the β-scission reaction. Tunneling effect has a significant influence on the rate constant of H-abstraction reaction at low temperature. The thermal cracking initial temperature is500℃and the reactions mainly occur in the range of600-700℃. The major products are hydrogen, methane, ethylene, ethane, propylene and1,3-butadiene and the product distributions vary with temperatures. Raising the production of olefins especially ethylene is helpful for the improvement of heat sink. Heat sink within600-700℃changes significantly, while the pressure has little impact on it. The total heat sink of n-decane is2.33MJ-kg-1at600℃and2.5MPa obtained by simulation calculation, with the conversions of25.9%, which could meet the cooling requirement of aircrafts at5-6Mach number.2. Based on the study on the thermal decomposition of n-decane, the effects of carbon chain length and branched chain on endothermic ability of normal alkanes and isoalkanes (C7H16～C18H38) were studied at the same methods of theory. The results show that the physical heat sink has a linear increase with temperature and there is almost no difference between the same carbon number of normal alkanes and isoalkanes. The chemical heat sink varies with the structure of the compounds. The more branched chains or branched carbon numbers the compound has, the greater the chemical heat sink is. |
PDF Full Text Request