Pyrolysis And Coking Kinetics And The Effects Mechanism During The Pyrolysis Process Of N-Heptane

Endothermic liquid hydrocarbon fuels have a high heat sink.After the endothermic cooling of the scramjet engine,the autogenous cracking produces more flammable gases,which improving the combustion efficiency and engine operating efficiency.The selectivity of the gas which generated in the endothermic pyrolysis process of hydrocarbon fuel is very sensitive to the reaction conditions.The change of product composition has a huge influence on the combustion characteristics of the combustion chamber and characteristics of the active cooling thermal management.At the same time,the high temperature Cleavage coking of fuel in the active cooling channel is also one of the serious problems in hypersonic engine.Therefore,that how to grasp accurately the fuel cracking characteristics and predict the influence of composition changes on endothermic performance,and suppress coke formation are very important for engine operation.For this reason,this paper mainly focuses on the study of the active cooling characteristics of hydrocarbon fuels in hypersonic scramjet engines,and the requirement to improve combustion performance and to reduce coking carbon deposits.The main research directions are flow pyrolysis and coking of hydrocarbon fuels in actively cooled microchannel.Experiments and models of thermal cracking and coking of n-heptane in microchannel have been carried out.The main research direction was to carry out experimental and model studies on thermal cracking and coking of n-heptane in microchannel.The influence of the change of reaction conditions in the pyrolysis of n-heptane pyrolysis was analyzed.The main content includes the following aspects:Firstly,the concepts and differences of cracking and combustion are introduced.The application of hydrocarbon fuel cracking in scramjet engines,the construction of a model for reaction mechanism of hydrocarbon fuels,and the concept of coking and soot are introduced.The process of forming coke and soot was analyzed.Secondly,the experimental device and test method of n-heptane in a micro tube pyrolysis were introduced and the product components were analyzed.The results were in good agreement with the related literature.Then,the main product and the content of n-heptane change with the flow rate,temperature and pressure were analyzed.The ratio of olefins and alkane formation with flow rate and temperature changes was analyzed.An n-heptane pure cracking mechanism of a 166 step reactions with 44 components was constructed,and the main product and n-heptane conversion obtained under different reaction temperatures,pressures,and flow rates were verified.The calculated results are better than that of most of the n-heptane oxidation mechanism in literature.A constructed mechanism model was used to analyze and explain the dynamics of the n-heptane pyrolysis products at different temperatures and pressures.The effects of temperature and pressure on the residence time,rate,average molecular weight and density changes in the n-heptane pyrolysis process were analyzed and explained.The reaction conditions for increasing the olefin yield and conversion under a range of temperatures and pressures were optimized.The gas-phase and liquid-phase product composition of the n-heptane pyrolysis products at a high conversion rates and their variation with temperature and flow rate were measured.A detailed pyrolysis mechanism model for the formation of 226 components and 1113 steps of n-heptane cleavage to polycyclic aromatics(Coronene)was constructed.The n-heptane pyrolysis process was numerically simulated using the construction model,and the A comparison of the calculation results of the detailed mechanism model for oxidation and polycyclic aromatic hydrocarbons was shown.The former was better than the latter.The calculations of the main benzene and polycyclic aromatic hydrocarbons with temperature and pressure changes on two models were compared.The reaction pathways of benzene at different equivalence ratios,generation rate and sensitivity of polycyclic aromatic hydrocarbons were analyzed.The main pathway for the formation of benzene to helium in the cracking process of n-heptane was obtain.Finally,the coking conditions of n-heptane pyrolysis were measured and analyzed.The particle shape and distribution of the coke in different parts of the cracking pipeline were compared.The effect of the stainless steel tubes of different materials,before and after passivation on the coking morphology of cracking was compared.From the point of view of flow and wall chemical reactions,two types of fuel cracking coking models have been developed:solid-phase deposition and wall-surface reaction.And the coking characteristics in the channel were predicted,the agreement with experimental values was relatively high.The influence of temperature,flow,diameter of tube and pressure on the coking amount was analyzed using the construction model.The results showed that the conditions of low temperature and high flow have a very significant effect on the suppression of coking,and the effects of diameter of tube and pressure appear non-monotonic.The degree of fuel cracking has a great influence on the range of the two coking models of solid-phase deposition and wall-reaction,and there is a coupling effect and complementarity in the actual channel.