A Numerical Investigation On The Ignition/Extinction Of Dual Fuel Based On The Low-temperature Reaction Pathway Of N-heptane

With the depletion of petroleum resources and worsening of environmental pollution,energy conservation and environmental protection have become the mainstream concept of society.Natural gas-diesel dual fuel engine is gradually becoming the preference for future ship power due to its economy and ultra-low emissions properties.However,the dual-fuel engine suffers from poor ignition characteristics and extinction occurrence at low engine loads which can lead to the deterioration of engine power and emissions.The present work performs the ignition/extinction mechanism investigation of methane/n-heptane based on natural gas-diesel dual fuel engine and analyzes the extinction mechanism of dual fuel engine with pre-chamber.The mechanism that methane influences n-heptane ignition is investigated under engine-like condition first.A numerical study using CHEMKIN codes is performed to study the effects of methane addition on the IDs of n-heptane/air mixture.Results show that the prolonged IDs of n-heptane/air mixtures are observed after methane added,and the increment of the IDs in the NTC（Negative Temperature Coefficient）regime is significantly higher than that in the high temperature region.This phenomenon can be attributes to the fact that the production and consumption rates of the key radicals decrease significantly in the NTC regime after methane added,while it is negligible in the high temperature region.Secondly,a numerical simulation is conducted to seek the ignition/extinction mechanism in n-heptane ignite methane process.The Adaptive Mesh Refinement Object-oriented C++（AMROC）codes simulated the ignition process under different initial temperatures and equivalence ratios.It is found that the extinction phenomenon occurs at the case with low methane/air initial equivalence ratio of 0.35 and initial temperature of 1000K.And the peak of flame front speed decreases with the increase of initial equivalence ratio.However,the distance propagated into the methane/air mixture is promoted.With the increase of initial methane/air temperature,the ignition point moves toward the side with higher n-heptane concentration.As the initial temperature increases,the mass fraction of key species(C₇H₁₅O₂、KET)increases and the peak value of mass fraction moves to the side with higher n-heptane concentration.Finally,the ignition/extinction mechanism is investigated in dual fuel engine with pre-chamber.The effects of intake temperature,pilot fuel amount,pilot fuel injection timing and swirl ratio on the ignition/extinction and combustion process mainly in the pre-chamber were studied.Results show that:（1）With the increase of intake temperature,the n-heptane low-temperature reaction channel are weakened,but the high temperature reactions of n-heptane are enhanced,leading to the intense combustion of n-heptane.Thus,the impact of pilot fuel is improved.In addition,the increasing intake temperature enhances the initial reaction of methane that promote the production of important intermediates such as CH₂O and H₂O₂,which can promote the reaction of n-heptane.（2）With the increase of pilot fuel amount,the low-temperature reaction of n-heptane increases and heat release in the initial stage is improved,which can trigger the high-temperature reaction of n-heptane earlier,leading to the enhancement of jet-flame.More hot spots appear in the cylinder,which can contribute to the ignition of methane.（3）Under the misfire condition,increasing the pilot fuel amount（from 0.9%to 1.5%）together with the advanced injection timing（SOI=-12°CA）promotes the ignition and combustion process obviously.（4）As the swirl ratio increases,the mean pressure,temperature and heat release rate（HRR）in the cylinder increase first and then decrease,indicating that the effect of swirl ratio on the in-cylinder ignition and combustion is non-monotonic.Meanwhile,the increase of swirl ratio promotes the consumption of key intermediates in the n-heptane low-temperature reaction pathway,such as C₇H₁₅O₂ and KET.Based on the n-heptane low-temperature reaction pathway,the present work reveals the ignition/extinction and flame development process of methane/n-heptane mixture,which provides a theoretical basis of ignition stability of dual-fuel marine engine with pre-chamber.