Investigation On The Ignition Characteristics Of Methane/n-heptane Mixtures

As a clean alternative fuel,natural gas has little sulfur,dust and other harmful substances with a large reserve,and thus it has received widespread attention around the world.However,as natural gas has high octane number,it's difficult to be ignited by compressing.The dual fuel engine used on the ship is usually ignited by diesel,and fueled with natural gas/diesel mixtures.As the model fuel of natural gas/diesel mixtures,the ignition characteristics of methane/n-heptane mixtures under intermediate-high temperature conditions were investigated using a heated shock tube in this study.And a detailed chemical kinetic model was chosen to simulate the ignition delay time and conduct chemical kinetics analysis.The accuracy of this kinetic model was further verified at low temperature.Based on this kinetic model,the two stage ignition characteristics of methane/n-heptane mixtures and chemical analysis were investigated.The main conclusions are as follows:(1)In the intermediate-high temperature conditions,the ignition delay time of methane/n-heptane mixtures decreases with temperature increasing,increases with equivalent ratio increasing,decreases with pressure increasing,and nonlinearly decreases with n-heptane blending ratio increasing.The smaller the proportion of n-heptane,the greater effect of its change on the ignition delay time of the mixed fuel.N-heptane is usually consumed at the beginning of the reaction under intermediate-high temperature conditions,while methane is consumed rapidly before ignition.When the blending ratio of n-heptane is relatively high,some methane can be generated in the consumption process of n-heptane.(2)In the low-intermediate temperature conditions,the first-stage ignition delay with the negative temperature coefficient behavior of methane/n-heptane mixtures can be observed in temperature lower than 750 K.In the second-stage ignition delay,the negative temperature coefficient behavior can be clearly observed when temperature range from 600 K to 800 K.Similar with the intermediate-high temperature,the influence of the n-heptane blending ratio on the two stage ignition delay time is nonlinear,which increases with the n-heptane blending ratio decreasing.The oxygen-addition reaction of methane and n-heptane under low temperature conditions is the most important reaction during the first-stage ignition,and a large amount of hydroxyl and hydrogen superoxide is generated during the low temperature oxidation of n-heptane.Little methane is involved in the first-stage ignition,while most of n-heptane is incompletely oxidized in this process and further fully reacted in the second ignition process.