Study On Combustion Mechanism And Characteristics Of N-heptane Mixed Hydrogen Under Low Temperature Conditions

Diesel engines have high efficiency and wide power coverage,and are widely used in heavy machinery and military fields.However,their NOx emissions are high,and NOx emissions can be reduced by blending.Combining hydrogen and diesel in traditional internal combustion engines can not only relieve fuel pressure,but also reduce pollutant emissions.The cetane number of n-heptane is close to that of diesel,and it is often used to predict the ignition process of diesel.Research on hydrogen-assisted n-heptane combustion will help expand the scope of use of n-heptane and diesel fuel,and lay a theoretical foundation for further research on blended fuels.Based on the Chemkin-Pro simulation platform,the reduced mechanism of n-heptane suitable for low temperature is obtained by the sensitivity analysis method and the absolute rate of production;The six common hydrogen-air reaction mechanisms were compared and analyzed,and the Conaire mechanism,which was simplified and had better prediction accuracy,was selected and verified by coupling it with the n-heptane oxidation mechanism and the nitrogen oxide reaction mechanism.The n-heptane/hydrogen mechanism（BJTH）containing 146 species reactions is obtained.Based on the IC reactor model,coupled with the reduced mechanism of BJTH,the in-cylinder combustion process of diesel fuel mixed with hydrogen under low temperature conditions is simulated and analyzed.The simulation results show that when the hydrogen blending ratio is 0-0.4,the addition of hydrogen will increase the ignition temperature and pressure of the blended fuel.Under the same conditions of initial temperature,pressure and equivalence ratio,the ignition delay time increases with the increase of hydrogen content.Blending proper amount of hydrogen is beneficial to improve the thermal efficiency of the engine.When the hydrogen blending rate is 0.2,the thermal efficiency reaches its peak,and then the amount of hydrogen blending increases and the thermal efficiency decreases.Under various hydrogen doping conditions,the main production pathway of CH₂O is the decomposition of C₃H₅O/C₂H₅O（R54/R64）.With the increase of hydrogen content,the decomposition reaction rate of R54/R64 increases significantly,corresponding to the backward movement of the crankshaft angle,and the delay of the cold flame.OH mainly comes from the decomposition reaction of H₂O₂（R85）.As the amount of hydrogen added increases,the importance of R85 increases,the reaction time is delayed,and the hot flame time is moved back.The study on the emission characteristics of n-heptane under low temperature conditions found that CO mainly comes from the decomposition of C₂H₃CO.The increase in hydrogen mixing rate and the increase in CO emissions are mainly due to the competition between elementary reactions.The main source of NO is the decomposition of NO₂.The increase in hydrogen mixing rate and the decrease in NO emissions are mainly due to the increase in hydrogen mixing,prolonged combustion duration and shortened high temperature in the internal combustion engine cylinder.The main source of C₃H₆ is the decomposition of C₅H₉,the hydrogen doping rate increases,and the peak value of C₃H₆ decreases.The reason is that the initial reaction NC₇H₁₆ decreases under the condition that the total calorific value of each working condition is equal.At present,the research on n-heptane hydrogen-mixed fuel mainly focuses on the experimental means to analyze the influence of hydrogen addition on the combustion delay period of n-heptane,and there is still a lack of research on the chemical reaction mechanism of n-heptane hydrogen-mixed fuel.The research work in this paper is conducive to in-depth understanding of n-heptane combustion process and combustion path,reveals the influence mechanism of hydrogen addition on n-heptane combustion and emission characteristics,and lays a certain theoretical foundation for the research of mixed fuel.