Study On Mechanisms Of Flame Tip Opening Of Lean H₂/Air In A Micro-Combustor With Cavities

Since the micro combustor have an increase in specific surface area by 2-3 orders of magnitude,the increasing of heat lose and shortened residence time make the micro combustion unstable.In all the methods of improving the combustion efficiency in a micro combustor,the micro combustor with cavities is considered to be a very effective structure.The low velocity zone and recirculation zone generated by the cavities could effectively anchor the flame.However,in the micro combustion,the lean H₂/air flame will show the flame tip opening phenomenon as the inlet velocity increases.In this paper,based on the micro combustor with cavities,the effects of stretch rate,heat regenerative,and Lewis number effect on the flame tip opening of lean H₂/air flame were studied.In this paper,the influence of the cavity structure,e.g.the depth of cavity and the angle between the inclined wall and the horizontal plane,on the combustion efficiency and the flame tip opening phenomenon is numerical studied.The studies on the cavity depth show that,the combustion efficiency increases with the depth of the cavity at low inlet velocity and vice versa at high inlet velocity.This is due to the fact that the effect of heat loss and heat regenerative on the combustion efficiency is greater when the inlet velocity is smaller,while the residence time and stretch effects dominate at larger inlet velocity.To the angle?90°and 135°?between the inclined wall and the horizontal plane,it was found that,compared to the combustor with the angle is 135°,due to the larger recirculation zone in the cavity and the larger high-temperature areas,the combustor has a higher combustion efficiency at low inlet velocity when the angle is 90°.However,when the inlet velocity increases,the flame is easily blown at the corner of the cavity when the angle is 90°.Thus,the combustion efficiency of combustor with the inclined wall cavity is higher.Second,by changing the state of the gas mixture,e.g.inlet temperature and inlet pressure,the physical properties of the mixture gas,the effective Lewis number(Le_eff),and the adiabatic combustion temperature can be changed.An increase in Le_eff will improve the flame tip opening phenomenon and reduce the leakage of unburned hydrogen from the flame tip.The increase of the adiabatic combustion temperature will increase the average temperature in the combustor.This will lead to an increase of the combustion reaction speed,and the hydrogen consumption rate.In addition,as the inlet pressure reduced from 1.0 atm to0.5 atm,at higher inlet velocities,as the ambient pressure decreases,the hydrogen combustion efficiency increases first and then decreases.This is because when the pressure is relatively high,the reaction rate of hydrogen will increase with the decrease of the pressure,and when the pressure is reduced to 0.5 atm,the gas density will decrease,the heat generated by the combustion will be very small.Therefore,a sudden increase in heat loss results in a significant decrease in the rate of combustion.Changes in the physical properties of the mixture may affect the initiation and reaction rates of the elementary reactions and affect the stability of the flame tip.Increasing the inlet temperature or the oxygen concentration in the oxidant can significantly improve the flame tip opening phenomenon in the micro combustor with cavities.When the diluent in the oxidant is changed from the N₂ to He,the effective Lewis number and the adiabatic combustion temperature increase and the stretch rate decreases.As a result,the stretch rate decreases,and the flame tip opening is suppressed.Finally,we also studied the thermal management of the combustor.In the study of the external surface emissivity,the results show that lowering the emissivity will increase the combustion efficiency.This is mainly due to the reduction of the emissivity which will increase the heat regenerative effect of the combustor,and reducing the heat loss of the combustor.The double wall combustor,which consists of a high thermal conductivity material as the inner layer and a low thermal conductivity material as the out layer,has a better use of the thermal insulation effect and the heat regenerative effect.Therefore,optimizing the heat loss and heat regeneration of the double-walled combustor with appropriate materials can effectively improve the combustion efficiency,and can effectively reduce the tip opening phenomenon at high inlet velocity.