Investigation On The Ignition Characteristics Of Methane-T425 Mixtures

A study on the ignition characteristics of natural gas-diesel dual fuel mixtures is of great significance for understanding the ignition process of dual fuel mixtures and optimizing the in-cylinder combustion of dual fuel engines.In this paper,methane and T425（mixtures of 57.5%n-heptane and 42.5%toluene in mole fraction,i.e.T425）are selected as the surrogate fuels for natural gas and diesel,respectively.The experimental study was conducted to investigate the ignition characteristics of methane-T425mixtures via shock tube setup at initial temperature of 1150¹550 K,initial pressure of1.0³.0 MPa,equivalent ratio of 0.5².0 and methane content of 0¹00%.And related chemical kinetic analysis and reaction path analysis of the ignition process for methane-T425 at the elevated temperature were also carried out based on the CHEMKIN PRO software and MEHL mechanism.Besides,the simulations were carried out under moderate and low temperatures in order to investigate the phenomena of negative temperature coefficient（NTC）and multi-stage ignition.The main conclusions of the present study are as follows.（1）The ignition delay time of the methane-T425 mixture increases nonlinearly with the increase of the methane content.At low methane content,the variations of the ignition delay time profiles of methane-T425 mixtures are negligible as the methane content increases.But when the methane content exceeds a certain value（about 82.5%）,the ignition delay time of mixtures increase significantly.（2）According to the reaction path analysis of the ignition process of methane-T425mixtures during high temperature,it is found that with the increasing methane content,the competition for OH radicals between methane and n-heptane and toluene has led to great change of the reaction channels for n-heptane and toluene.Meanwhile,the amount of the stable compound C₂H₆ which is formed by CH₃ radicals,the main dehydrogenation product of methane,is increased significantly,so that the reactivity of the system is lowered,thereby delaying the ignition of the mixture.（3）The ignition delay time of methane-T425 mixtures can be divided into three regions:low-temperature region,intermediate-temperature region and high-temperature region,according to the studies of the ignition process of methane-T425 mixture at different initial temperatures.During the low-temperature and high-temperature regions,the ignition delay time of mixtures are shortened exponentially.In the intermediate-temperature region,the ignition delay time of mixtures show NTC behaviors at low methane content（below about 82.5%）,with the increasing methane content,the NTC phenomenon will finally disappear.（4）At high-temperature regions,the ignition delay time of the methane-T425 mixtures are shortened with the increasing initial pressure.But as the pressure increases,the impact on ignition delay time of the mixtures from the initial pressure is weakened gradually.According to variation between ignition delay time of the mixtures and equivalence ratio at different initial temperature,the ignition delay time decreases first and then increases with the increase of equivalence ratio.And when elevated the initial temperature,the equivalent ratio corresponding to the shortest ignition delay time for the mixtures decreases gradually.（5）The reaction path analysis of ignition process of methane-T425 mixtures at medium temperature show that when the methane content is higher than 82.5%,the nonexistence of the NTC behavior of the ignition delay time for mixture result from the interaction between the negative temperature coefficient phenomenon of n-heptane and the positive temperature coefficient phenomena of the methane and toluene.（6）The rise of HO₂ and OH radicals which are formed by the moderate temperature chain reactions and low temperature chain reactions of n-heptane triggers a small ignition of the mixture,i.e.the first stage of the ignition process.While the final ignition of the mixtures at different initial conditions are dominated by the decomposition of H₂O₂（H₂O₂（+M）=2OH（+M））.