Numerical Simulation Study Of Chemical Reaction Kinetics Of N-Heptane/Methanol Dual Fuel

Methanol is a promising alternative fuel for internal combustion engines,however,there are limited studies involving the complex chemical coupling between diesel/methanol dual fuels,which hinders further combustion analysis.In this paper,the ignition and laminar flame speed characteristics of n-heptane/methanol dual fuel were investigated by constructing a dual fuel combustion reaction model based on CHEMKIN software using n-heptane as the reference fuel for diesel fuel.With the help of kinetic analysis tools,the interaction mechanisms between chemical reactions in the oxidation of different fuels were explored from the chemical reaction kinetics level.Closed Homogeneous Batch Reactor was used to simulate the ignition delay times of mixtures with different methanol substitution percent（0～80%）varying between 0.3～2equivalence ratios and initial temperatures of 650K～1300K.The results show that the inhibitory effect of methanol on first-stage ignition at the initial low temperature is mainly due to the competition between methanol and n-heptane for OH radicals;the higher concentration of HO₂ radicals produced by methanol mixing promotes the formation of H₂O₂ and its subsequent decomposition at high temperature to produce a large amount of OH radicals,which promotes second-stage ignition.When a small amount of methanol is mixed at the initial high temperature,the reaction rate of radicals increases,which has the effect of promoting ignition;however,mixing too much methanol will increase the consumption of active radicals,which is not conducive to ignition.The first stage of ignition of the mixture after methanol blending becomes sensitive to changes in the equivalence ratio.This is due to the fact that the low-temperature reaction chain of n-heptane can provide more OH at higher equivalence ratios,resulting in a relative weakening of the methanol inhibition.The sensitivity analysis showed that most of the radical reactions were insensitive to the change of equivalence ratio at the initial low temperature,and only the sensitivity coefficient of the peroxone generation reaction increased with the increase of equivalence ratio.At the initial high temperature,the ignition promoting reaction R391 and the ignition inhibiting reactions R397 and R385had a greater effect on the ignition and were particularly sensitive to the change of the equivalence ratio,and their promoting effect on the ignition became stronger with the increase of the equivalence ratio.Premixed Laminar Flame-Speed Calculation was used to study the laminar flame speed of blended fuels with different methanol substitution percent.The results show that the laminar flame speed of blended fuels is between that of pure fuels and increases with the increase of methanol substitution percent.Blending methanol increases the concentrations of OH and HO₂ in the mixtures,with a particularly significant increase in the concentration of HO₂ radicals.reaction rate analysis of HO₂ shows that increasing the methanol content increases the concentration of CH₂OH,promoting the reaction CH₂OH+O₂=CH₂O+HO₂ to produce HO₂.Sensitivity analysis of laminar flame speed shows that small molecule reactions,such as R377,R400 and R408 are important reactions that enhance the flame reaction activity,while the reactions of R409,R397 and R384 decrease the predicted value of laminar flame speed of the mixtures.