Study On The Chemical Kinetic Mechanisms Of Methane/n-heptane Mixtures

Natural gas has been emerging as a promising alternative fuel recently,due to its economic and environmental advantages.In the natural gas-diesel dual-fuel engines,it is important to study the mutual oxidation processes and the kinetic coupling between natural gas and diesel for the better design and optimization of the dual-fuel engines.As a result,the methane/n-heptane mixture was selected as the surrogate model of natural gas-diesel dual-fuel in the present study,and then a detailed analysis of the chemical kinetic mechanism of methane/n-heptane oxidation was performed.Besides,a reduced chemical kinetic mechanism of the methane/n-heptane mixture was developed under the relevant condition.The mutual oxidation processes of the methane/n-heptane mixtures were experimentally investigated in an atmospheric jet-stirred reactor?JSR?,and the NUI mechanism was employed to perform the numerical studies under the same condition by using the perfectly-stirred reactor model.The results show that the onset temperature of the oxidation of methane decreases with the increase of the n-heptane content in the methane/n-heptane mixture significantly,and a nonlinear relationship between the onset temperature and the n-heptane content is observed.Different from methane,all the H-abstractions of n-heptane are observed to start at lower temperature.According to the chemical kinetic analysis of the methane/n-heptane mixtures,it confirms that a small amount of n-heptane addition could accelerate the methane oxidation dramatically through the radical pool enrichment.The promoting effect becomes insensitive to n-heptane addition once the radical pool is established.The reaction path analysis shows that with the increase of n-heptane content,the H abstraction of methane by OH and the oxidation of CH₃ by HO₂ are enhanced at high temperature,while the H abstractions of n-heptane by OH are inhibited to some extent at low temperature.The sensitivity analysis of the mole fractions of methane and n-heptane indicates that those n-heptane related reactions,in which OH and other radicals are produced will have a positive effect on the oxidation of methane,while those methane related reactions,in which OH and other radicals are consumed will have a negative effect on the oxidation of n-heptane.Based on the directed relation graph with error propagation?DRGEP?method,a reduced chemical kinetic mechanism of the methane/n-heptane mixture consisting of212 species and 995 elementary reactions was developed from the detailed NUI mechanism.In addition,the reaction path analysis and the sensitivity analysis were also adopted to carry on the further reduction,resulting in a specific simplified mechanism consisting of 185 species and 768 elementary reactions.The numerical results from the reduced mechanism and detailed mechanism calculated in CHEMKIN software were compared with the experimental data for the validation.It is seen that the reduced mechanism behaves well in the prediction of the mole fractions of the major species during the oxidation processes and the ignition delay times,as well as the laminar flame speeds of the methane/n-heptane mixtures at different n-heptane contents.