Study On The Ignition Characteristics Of N-heptane In O₂/CO₂ Environment

With the increasing global environmental protection issues,O₂/CO₂ combustion has become one of the emerging directions to achieve energy-saving and emission-reduction of diesel engines.Based on O₂/CO₂ combustion,a closed-cycle diesel engine system with liquid oxygen-carbon sequestration has received much attention after its proposal.The closed-cycle diesel engine is suitable for closed environments such as underwater or tunnels since it is able to solve the problem of diesel engine emissions in closed spaces.However,the closed-cycle diesel engine is prone to occur ignition problems in O₂/CO₂ environments,which seriously hinders the practical application and development of the diesel engine.In addition,due to the complex components of diesel fuel,it is convenient to use n-heptane to characterize diesel fuel.Therefore,it is urgent to study the ignition characteristics of n-heptane in O₂/CO₂ environments at this stage.This paper studies ignition characteristics of n-heptane based on O₂/CO₂ environments.First,based on the self-ignition theory,a novel model of the ignition delay time in O₂/CO₂environments is established,which takes into account the third-body effect of CO₂ in O₂/CO₂ environments.In addition,a computational fluid dynamics simulation model is established for a constant volume combustion chamber.Secondly,the constant volume combustion chamber test system is built,and n-heptane ignition under four different test conditions is recorded by a high-speed camera.Through the experimental results,the n-heptane ignition processes under different conditions are analyzed and the proposed ignition delay time model is verified.Moreover,comparing the computational fluid dynamics simulation results,the temperature distribution and equivalence ratio distribution of the n-heptane flame are also analyzed.Finally,the chemical kinetics simulations of n-heptane ignition in O₂/CO₂ environments are carried out.The chemical and the third-body effects of CO₂ on n-heptane ignition are compared and analyzed through the artificially defined CO₂ molecules.The results show that the TBE model proposed for n-heptane ignition under O₂/CO₂environments matches well with the experimental results and the maximum error is 8.94%in 57%O₂/43%CO₂ environment.Secondly,the maximum temperatures of n-heptane flame under different O₂/CO₂ environments show little difference.However,the distribution of the high temperature area downstream of the flame is affected and the flame shapes are changed under different O₂/CO₂ environments.In addition,when the CO₂ vol.%in the environment is less than 50%,there is no significant change in the maximum equivalence ratio.When the CO₂ vol.%is higher than 50%,the maximum equivalence ratio will increase with the decrease of O₂ in the environment.Finally,the chemical effect of CO₂ has a limit impact on the reaction intermediate products,while the impact of the third-body effect of CO₂ is greater than the chemical effect.Moreover,the analysis of production rate shows that H+O₂→O+OH（R11）is the most affected reaction by the third-body effect of CO₂ for OH radicals.The reaction path analysis shows that the third-body effect of CO₂ will inhibit OH radicals generating path except the decomposition reaction of H₂O₂.