Effect Of Mixing Ozone On Combustion Characteristics Of Premixed Methane/Oxygen And Methane/Air

Micro-burner is the core device of micro power system.The combustion stability of hydrocarbon fuel in micro-burner is directly related to the performance of the whole system.However,with the reduce of the scale,the surface volume ratio increases greatly.The residence time of the internal gas is much shorter than that of its combustion in the conventional scale,which leads to the decrease of the combustion efficiency.Additionally,the influence of the wall on the flame increases,resulting in the quenching of a large number of free radicals near the wall and the decline of flame stability.These unfavorable conditions put forward higher requirements for the organization and enhancement of combustion process.There are many activated components in the plasma that can accelerate the oxidative cracking of fuel,which can be effectively used to strengthen ignition,stabilize flame and accelerate flame propagation.Ozone is one of the active components.Due to its reactivity and long life,it can be transported to the flame zone without substantial loss.In view of many problems existing in micro-scale combustion and the strong advantages of plasma combustion,the strong oxidation of ozone was discussed in this paper.On the basis of analyzing the relevant research developments at home and abroad,the stable combustion range of mixture before and after mixing ozone was compared and analyzed.The influence of ozone on the combustion characteristics of the mixture was obtained,and the effect of initial parameters such as ozone concentration and equivalence ratio on the combustion process was systematically studied.Considering the influence of mixing ozone on the combustion limit,the experimental platform and the combustion calculation model coupled with ozone reaction mechanism were established respectively,and the experimental observation and numerical simulation were carried out in the microchannel.The flammability limits of methane/oxygen mixture and methane/oxygen/ozone mixture under different channel lengths were obtained,and the influence of ozone on near-limit combustion was explored from two aspects of thermal effect and chemical kinetics effect.Simultaneously,the basic combustion properties of the mixture such as the outer wall temperature before and after mixing ozone were compared and analyzed.The results showed that:mixing ozone can effectively broaden the combustible limit of methane/oxygen mixture;When the ozone concentration was increased to 5%,the reaction temperature could be raised by up to 110 K.The reaction temperature and flow rate also increased with the increase of ozone concentration,and the high temperature region was moved slightly towards the inlet.Near the flammability limit,the kinetic effect of ozone showed greater potential.Compared with the thermal effect,the increase of O and other free radicals was almost dozens of times of the temperature.Moreover,the amount of OH radicals,the temperature on the centerline of the outer wall,the temperature of the outlet gas and the heat loss of the wall all showed an increase after ozone mixing.At lower equivalence ratio and lower inlet velocity,the improvement was more obvious.Based on the established three-dimensional model,and the effects of ozone concentration,equivalence ratio and initial temperature on the combustion characteristics of the mixture were investigated by numerical simulation.It was displayed that the maximum heat release rate increased linearly at first,and then the growth rate slowed down gradually.That is,when the concentration of ozone increased to a certain level,there would be saturation phenomenon,and the effect began to decline.It was found that 9%is an optimal concentration of ozone,which could not only ensure the promotion effect,but also save the economic cost to the greatest extent.When the equivalence ratio was about 1.0,the heat released and reaction temperature of the mixture were the highest.However,the excess methane could not be fully burned in the lean oxygen,the higher the equivalence ratio was,the lower the complete combustion efficiency was.The increase of inlet temperature reduced the chemical energy input into the combustion chamber,resulting in the decrease of the heat released by reaction and the temperature inside the combustor.But preheating was conducive to radial heat transfer,the temperature difference between the wall and the channel was decreased,the generation of CO was decreased,the generation of CO₂ was increased,and the efficiency was improved.In order to further understand the flame propagation characteristics after ozone mixing,a one-dimensional premixed flame model was established by CHEMKIN.The effects of ozone mixing on the laminar combustion characteristics of methane under different oxidants were analyzed,including laminar burning velocity,adiabatic flame temperature and laminar flame thickness.The influence mechanism of ozone on the laminar burning velocity was investigated.The results indicated that,whether the oxidant was air or oxygen,the addition of ozone could improve laminar burning velocity and adiabatic flame temperature of the mixture,and reduce the flame thickness.However,the promotion effect of ozone on the combustion process of methane-oxygen mixture was only about one tenth of that of methane-air mixture,and the difference was even more striking under conditions of fuel-lean and fuel-rich.The existence of ozone molecules in the preheating zone made O and other free radicals and some important intermediate products increase significantly,which initiated and accelerated the chain-branching reaction,thus speeding up the whole combustion process.At last,the sensitivity of the elementary reactions was also discussed.It was shown that H+O₂<=>O+OH was the biggest contributor to the enhancement of burning velocity and the most important reaction of the whole system.In this paper,the combustion characteristics of the mixture in micro-scale after mixing with ozone were analyzed.The basic combustion data such as flammability limit were obtained,and the influence law of mixing ozone on combustion process was summarized.Also,the mechanism of ozone enhanced combustion was preliminarily revealed from the perspective of reaction kinetics.The relevant conclusions will provide theoretical basis for engineering application and improving flame stability.