Combustion And Flame Propagation Characteristics Of Premixed CH₄/O₂ In Micro-combustors

Micro Electro Mechanical Systems(MEMS)which based on the combustion of hydrocarbon fuel is developing rapidly.As the core component of MEMS,the stability of micro-combustor is of vital important.After the miniaturization of the combustor,the homogeneous combustion inside the chamber faces many challenges.Such as the increase of the heat loss-to-production ratio due to the increment of surface-to-volume ratio,the decrease of the fuel residence time owing to the reduction of the internal space and the possibility of flame quenching when the characteristic size of the combustor is close to the quenching distance.Therefore,exploring the relevant laws of homogeneous combustion and flame propagation at the micro-scale is of great significance to provide theoretical basis for designing highly efficient micro-combustors.Based on the related research of scholars at home and abroad,this paper discussed the combustion and flame propagation characteristics of premixed methane/oxygen in micro-combustors.Moreover,this paper investigated the effects of mixing hydrogen with methane and oxygen on the flame propagation characteristics.Firstly,three types of stainless steel micro-combustors that with the same hydraulic diameter were designed.Experimental and numerical researches were conducted to investigate the premixed combustion characteristics of methane/oxygen in these combustors.Results show that for the combustion process inside the micro-combustors of circular,square and rectangle sections,the flammability limits of the premixed gases decrease successively at the same equivalence ratio.The homogeneous combustion intensity in micro-combustors of circular,square and rectangle sections reduce in turn when the flow fluxes are the same.At the same distance from the entrances,the flow rates along the centerlines of the circular,square and rectangle micro-combustors decrease sequentially.On the section that is perpendicular to the gas flow rate,the velocity in combustor of circular section is larger and the velocity gradient in combustor of rectangle section is smaller.Homogeneous combustion intensity inside the combustors and the mean temperature of the external wall increase firstly and then decrease when the equivalence ratio increases.In addition,homogeneous combustion intensity in micro-combustors of square,rectangle and circular sections decrease sequentially when the equivalence ratios are the same.When inlet velocity increases,homogeneous combustion intensity inside the combustors and the mean temperature of the external wall increase accordingly.When at the same inlet velocity,homogeneous combustion intensity in micro-combustors of square,rectangle and circular sections decrease successively.Secondly,experimental investigations of premixed methane/oxygen flame propagation characteristics were carried out in quartz glass micro-combustors.The impacts of diameter,section shapes,equivalence ratio and inlet velocity on flame propagation states and stationary positions were obtained.The results show that jet flame,pulsated flame,stationary flame and oscillatory flame are observed in experiment.Stationary flame is obtained at higher equivalence ratio and its position is closer to the combustor entrance with the increase of diameter.Comparing with the combustor of rectangle section,stationary flame in micro-combustor of circular section occurs at higher equivalence ratio and its position is closer to the entrance.Stationary flame can only be obtained at low inlet velocity when equivalence ratio is high and the flame can be stabilized at higher inlet velocity when equivalence ratio decreases.At low inlet velocity,stationary flame is closer to the combustor entrance when equivalence ratio varies from 1.85 to 1.925 and its position is closer to the exit with increment of inlet velocity.Lastly,the impacts of mixing hydrogen with methane/oxygen on flame propagation characteristics in quartz glass micro-combustors were analyzed by means of experimental research.The results indicate that under the condition of hydrogen addition,stationary flame appears at higher equivalence ratio and its position is closer to the combustor exit at higher mixing ratio.With the increment of diameter,stationary flame occurs at higher equivalence ratio but the increment magnitude of equivalence ratio decrease gradually.Furthermore,the influence of diameter on flame stationary position is reduced with the increase of diameter.Comparing with the combustor of rectangle section,stationary flame in micro-combustor of circular section is obtained at higher equivalence ratio and its position is closer to the entrance at the same equivalence ratio.The section shapes of the combustor affect flame stationary positions mostly when the mixing ratio is 25% and the inlet velocity is less than 0.75 m/s.Moreover,the inlet velocity range is narrower and the flame position is closer to the entrance at higher equivalence ratio.In this paper,combustion characteristics of premixed methane/oxygen in micro-combustors were analyzed systematically.The flammability limits of premixed gases in micro-combustors of different section shapes were maintained and the effects of equivalence ratio and inlet velocity on the combustion characteristics were obtained.Moreover,flame propagation characteristics of the premixed methane/oxygen and methane/hydrogen/oxygen were discussed in detail.The relevant laws of the occurrence of stationary flame and its positions were obtained.The related conclusions are supposed to provide theoretical evidence for the design of micro-combustors.