Investigations On Plasma Assisted Oxidation Driven By Nanosecond Pulsed Discharge

Plasma assisted combustion is an emerging technology for regulating ignition and combustion characteristics of engines,which can shorten ignition delay times,extend lean burn limits,and enhance ignition and flame stabilization in high-speed flow.One of the main mechanisms of plasma assisted combustion is the chemical kinetic effect.Thus,research on chemical kinetics of plasma assisted combustion system is of significance to understand and apply this technology.In this work,chemical kinetics of plasma assisted oxidation of several typical compounds among the C1-C4 fuels including methane,ethylene,propane,n-butane and dimethyl ether were investigated experimentally and numerically.In the experimental part,an experimental platform based on the molecular beam sampling and time-of-flight mass spectrometry was designed for the detection of hydrocarbon and oxygenated intermediates in plasma.A well-characterized dielectric barrier discharge flow reactor under the excitation of continuous high-frequency and high-voltage nanosecond pulses was used to generate spatially uniform plasma.In the modelling part,kinetic models were developed by adding plasma reactions to the core combustion mechanism.A program was developed to simulate kinetic process by decoupling plasma kinetics and normal gas phase kinetics.More detailed information of hydrocarbon and oxygenated intermediates,especially some reactive species and species directly related to fuel reactions,were obtained through the analysis of mass spectra and photoionization efficiency spectra compared with literature.In the ethylene reacting system,species like ethenol,ketene,ethanol,methylhydroperoxide,ethylhydroperoxide,diacetylene and vinylacetylene were detected.In the dimethyl ether reacting system,species like methyl formate,ethyl methyl ether and dimethoxymethane were identified.For the propane case,species including allene,propyne,formaldehyde,ketene,propylhyroperoxides and oxetane were detected.For the n-butane case,species like 1,3-butadiene,1-butyne,formaldehyde,ketene,butylhydroperoxides,and tetrahydrofuran were identified.Species mole fractions were also derived.The modeling simulation results were compared with the experimental results and modelling analysis was performed to figure out main pathways responsible for fuel consumption and intermediates production.Based on the aforementioned experimental and modelling efforts,regulation mechanisms of plasma on fuel oxidation were summarized.Firstly,plasma generated radicals like O,H and OH promote H abstractions of fuel.The experimental evidence is the observation of large amounts of water in reacting systems.Secondly,species with high energy were produced during discharge process which participate in the dissociation reactions,leading to the formation of alkanes,alkenes and alkynes with moderate mole fractions.Thirdly,fuel radicals are produced so that the reaction channels of oxygen addition and the following oxidation reactions are opened.The detection of alkylhydroperoxides provides direct experimental evidence for this effect.Finally,the reactants may be activated so that some reactions with high energy barrier can be initiated.For example,the internal H-atom transfer process of alkylperoxy radicals leading to the formation of hydroperoxyalkyl radicals.Several kinds of cyclic ethers were observed in the experiments,which indicate the existence of hydroperoxyalkyl radicals.Currently,the predictive abilities of kinetic models are limited by the uncertainties of low tempeaure oxidation process in combustion core mechanisms and absence of reactions and cross sections of plasma reactions.Further efforts on theoritical computation and experiments are needed to improve the performance of kinetic models.