Numerical Simulation And Characteristics Of Plasma Assisted Combustion Excited By Nanosecond Repetitively Pulsed Discharge

Plasma assisted ignition and combustion is a promising solution that utilizes plasma kinetic,thermal and transport effects to control ignition delay and improve combustion efficiency.Literature research found that few studies consider controlling the non-equilibrium excitation of plasma or changing the original gas composition to improve the efficiency of plasma assisted ignition and combustion.Therefore,it is essential to deeply explore the non-equilibrium excitation mechanism of discharge plasma and the effect of inert gas on the discharge process,optimizing the enhancement effect of nanosecond repetitively pulsed（NRP）discharge plasma on ignition and combustion.Firstly,combined with related literatures,this article introduces the concept,classification and generation method of plasma in detail,focusing on NRP discharge plasma,and introduces the mechanism and research status of NRP discharge plasma assisted ignition and combustion.Secondly,with reference to the existing simulation model construction methods,the zero-dimensional plasma assisted combustion loosely-coupled simulation model is established to systematically calculate and analyze methane-air plasma assisted ignition and combustion excited by synergistic NRP and DC discharge and inert gas diluted methane-air plasma assisted ignition and combustion excited by NRP discharge.This paper draws the following important conclusions.（1）The selective excitation of different particles in plasma by synergistic discharge is better than that by NRP discharge,and N₂（v）,O₂（v）,CH₄（v）,O₂（a¹Δ_g）and O₂（b¹Σ_g⁺）can be produced more effectively.Particle density evolutions of the neutral radicals,the vibrationally excited species and the electronically excited species have the similar three development stages.The reaction of N₂electronically excited species helps to produce abundant atoms O and H,which promotes the formation of methane oxidation intermediates.The kinetic effect of the discharge plasma on ignition enhancement is highly efficient because it can break though the threshold of the thermal effect.Both the kinetic effect and the thermal effect of the NRP discharge on ignition enhancement are weaker than those in the synergistic discharge.e+O₂=e+O+O（¹D）in the NRP discharge and e+O₂=e+O₂（a¹Δ_g）in the synergistic discharge play the crucial roles in the methane-air plasma enhance ignition.The influence of the vibrationally excited species in the non-equilibrium plasma on the ignition enhancement should be mainly attributed to the thermal effect.（2）The dilution of inert gas is conducive to increase the formation rate of the electron,the radical and the electronically excited species in methane-air NRP discharge plasma.The densities of the electron and the O₂electronically excited species are the highest in the He diluted methane-air NRP discharge plasma,and the densities of the radical and the short-life electronically excited species in Ar diluted methane-air NRP discharge plasma are the highest.Radical CH₃plays an essential role in the formation of hydrocarbons C_xH_y（x≤3）in the methane-air NRP discharge plasma.The addition of inert gas has the vital influence on the main formation pathway of the radical in methane-air NRP discharge plasma.Inert gas diluted methane-air NRP discharge plasma is better than methane-air NRP discharge plasma in reducing ignition delay,and the effect of Ar diluted methane-air NRP discharge plasma on reducing ignition delay is more obvious than that of He diluted methane-air NRP discharge plasma.In the process of plasma assisted ignition,CH₃+O₂=CH₃O+O and H+O₂=O+OH are the main reactions to reduce the ignition delay,and the dilution of inert gas has a great influence on the normalized sensitivity,especially for the above reactions.The reactions of H+CH₄=CH₃+H₂and OH+CH₄=CH₃+H₂O have inhibiting effect on methane-air ignition both with and without non-equilibrium NRP discharge plasma.There are three points of innovation in the whole research.Firstly,the more complete zero-dimensional global model of NRP discharge is established.This model integrates the particles and reactions considered in many literatures,which is a relatively more complete model.Secondly,the more accurate rate coefficient of ionization reaction is applied.The main ionization reaction rate coefficient is obtained by downloading the latest collision cross section data and fitting with the help of Bolsig+,instead of obtaining the reaction rate coefficient from the references in the previous research.Thirdly,the more reliable loosely-coupled numerical model of combustion is constructed.More plasma particles are added to the combustion model,rather than just a small number of known particles in previous studies.After the plasma reaction is obtained in the reference,and coupled with GRI3.0,a more perfect kinetic plasma assisted combustion model is acquired.