Research On Combustion Stabilization Mechanism With External Electric Field In Micro Space Based On The Electrical Characteristics Of Methane/Air Flame

Since the 21st century,the demand for miniature portable machinery and electronic equipment in the military and civilian fields around the world has become more and more vigorous.The development of micro-energy power devices with micro size,lightweight,and high energy density is the core of improving the performance of micro-devices.In recent years,micro-energy power devices based on micro-space combustion have received extensive attention due to their much higher energy,power density,and longer endurance than traditional electrochemical cells.However,a series of combustion instability phenomena caused by scale effects make it a scientific and technological challenge to organize efficient and stable combustion in micro spaces.The exploration and development of efficient combustion and stable combustion technology in micro spaces is of great significance to promote the development of advanced micro-combustion power plants.Electric field assisted combustion technology is a new technology based on the weak plasma characteristics of flame.Because it directly acts on the flame surface itself,it has fast response speed and strong active regulation ability.It is a very potential combustion control technology.Therefore,this paper takes the methane/air flame as the research object,and on the basis of clarifying its basic electrical characteristics and combustion characteristics in a micro space,the influence law and internal mechanism of an external electric field on the characteristics of a methane/air flame in a micro space are carried out.The experimental and numerical simulation research of this paper provides a new idea and new method for active control of stable combustion in micro space.Based on the external small electric field method and the internal probe method,the overall macroscopic electrical properties of the methane/air flame and the spatial concentration magnitude and distribution of charged particles in the flame were measured.The distribution differences of the representative positive and negative charged particles H₃O⁺,O₂^-and e^-in the flame and the inner mechanism of their formation were revealed by one-and two-dimensional numerical simulation studies.Through experiments and theoretical derivation,the conductivity of methane/air flame and the simplified calculation formula of the conductivity of methane/air flame is obtained.The results show that the charged particles in the methane/air flame are mainly distributed in the flame reaction zone,with a concentration of 10¹⁶m^-3.The flame exhibits a"diode phenomenon"under positive and negative measurement electric fields.Positive ions H₃O⁺are mainly distributed in the vicinity of the flame surface reaction zone,close to the fuel side;O₂^-negative ions are mainly distributed in the low-temperature reaction zone with high oxidant concentration.The conductivity value of methane/air flame is in the order of 10^-8S/m,and the flame conductivity is mainly determined by the concentration of free electrons and the collision frequency between them and neutral particles.Based on the latest experimental measurement results and database information,combined with corresponding theoretical calculations,a comprehensive reaction mechanism of methane/air flame chemistry-ionization was constructed.The ionization mechanism of methane/air flames was deeply studied,the formation and transformation mechanisms of various charged particles in methane/air flames were analyzed,and the effect of N₂ on the ionization characteristics of methane flames was discussed when the air was used as the oxidant.The results show that compared with the existing mechanism,the constructed new mechanism can improve the prediction accuracy of the charged particle concentration of methane/air flame by nearly 20%.It is clear that H₃O⁺and O₂^-are the most critical intermediate ions in the mutual conversion process of positive and negative ions,respectively.The two reactions found to have the greatest impact on the charged particle concentration in a methane/air flame are the ionization reactions:CH+O<=>CHO⁺+e^-and the neutral particle reaction O₂+H<=>O+OH.It is clear that N₂ has little effect on the distribution of H₃O⁺and e^-in the flame,but slightly increases the maximum concentration of these two charged particles,and the effect on e^-is more obvious,especially in the flame burnt gas region.By designing a micro confined combustion device with continuously adjustable space size,the combustion characteristics of methane/air flames during the continuous reduction of the micro confined combustion space are studied.The experiment found that in the process of continuous reduction of the combustion space,five flame modes will appear successively:steady flame,instantaneous oscillation flame,flame extinction,repeated flame extinction,and flame quenching.At the same time,the effects of wall spacing,premixed air volume,plate material and inlet velocity on flame characteristics were investigated.The results show that the decrease of the wall spacing and the increase of the premixed air volume will increase the frequency of the repeated quenching flame,and the effect of the inlet flow rate on the FREI flame frequency is nonlinear.The quenching distance of the methane/air flame is about 2 mm.Increasing the premixed air volume can effectively reduce the quenching distance,while the fuel inlet flow rate has little effect on the quenching distance of the methane/air flame.The thermal conductivity and catalytic performance of the wall material will affect the flame quenching distance,with thermal conductivity having a greater effect.Based on a micro confined combustion device with continuously adjustable space size,a micro space combustion experimental system under the action of an electric field was further built to study the effect of high-voltage electric fields on the characteristics of methane/air flames in micro spaces.Mechanism.The research shows that,for a steady-state flame in a micro confined combustion space,the effect of the DC electric field on the flame has a strong dependence on the direction of the electric field.For non-premixed flames,the DC negative electric field（the direction of the electric field is opposite to the gas flow direction）can increase the flame stability,while the DC positive electric field can cause flame oscillations up to 28Hz and even flame splitting.Reducing the combustion space or increasing the premixed air suppresses the flame oscillation caused by the DC positive electric field.The DC negative electric field can effectively increase the FREI flame frequency and reduce the quenching distance by nearly 20%.The effect of high-frequency AC electric field on the steady flame characteristics in micro spaces is obviously weaker than that of DC electric field.The best method for stabilizing combustion in an electric field is to use DC negative electric field.The main mechanism for stabilizing combustion in the electric field is the ion wind effect,and the thermal effect and chemical reaction effect can basically be ignored.A combustion-electric field coupling model was established,and the numerical simulation study of methane/air combustion in a micro space under the action of an electric field was carried out to explore the effect of the applied electric field on the flow field characteristics of the methane/air flame in the micro space,and the concentration and variation of key free radicals and charged particles.Distribution characteristics,and the influence of flame ionization key chemical reaction rates.The research results show that under the action of a DC positive electric field,the flow velocity of the methane/air flame reaction zone and the flue gas zone in the micro space increases significantly,nearly doubled,and the flame stretch rate near the wall increases significantly;the OH concentration at the root of the flame increases significantly.decrease.The effect of negative electric field is just the opposite,but the influence strength is obviously smaller than that of positive electric field.The AC electric field has the weakest effect on the flame,but still has a certain stretching effect on the flow field near the flame surface,while the distribution of OH and charged particles shows a shrinking trend under the action of the AC electric field.