Study On Instability And Transition Mechanism Of Methane/Oxygen Premixed Flame In Microscale Combustor

With the rapid development of the MEMS industry,the demand for efficient and convenient micro power devices is becoming increasingly urgent,which makes micro power systems become a research hotspot.Combustion-based micro-power devices have attracted extensive attention due to their high energy density and convenient energy supplementation.However,the micro combustor,which is the core component of the micro-power device,has reduced size,high surface-to-volume ratio.And the flame-wall interaction is also strong in the micro combustor.These facts reduce the combustion stability in the micro-combustion chamber and cause flame instability,even the flame extinguishing,which in turn will reduce the efficiency of the micro power device.Therefore,it is necessary to clarify the causes of unstable flame and the transition mechanism between stable flame and unstable flame,as well as to clarify the influence of scale effects on combustion,so as to provide a theoretical reference for improving the stability of microscale combustion.Therefore,based on the investigation of the research works of unstable flame,this paper carried out related research on the flame with re-extinction and re-ignition（FREI）and the transformation of flame forms in micro combustors with experimental and numerical simulation methods.The main research contents are as follows:（1）A micro-scale combustion visualization experiment platform was built,and the flame propagation characteristics of the methane/oxygen premixed flame in the micro-combustor were studied.A high-speed digital camera and an infrared thermal imager were used to obtain data such as flame shape,flame propagation speed,and temperature distribution on the combustor wall.Based on the experimental research,the numerical model is established,and the reliability of the numerical model is verified by comparing with the experimental data.（2）Experimental methods have been used to observe the various flame at microscale,and the critical conditions for the transformation of flame forms have been clarified.Through the visualized micro-scale combustion experiment platform,various flame forms such as diffusion flame,stable flame,partial FREI flame,overall FREI flame and flame flashback extinguishment were observed,and the equivalent ratio range and the inlet Reynolds number range of different flame forms were measured.The influence of the shape and the characteristic size of the combustor on the flame shape were obtained in the experiment.It was found that the entrance Reynolds number range of the FREI flame in the rectangular combustor and the tube combustor are different.The increase of the combustor length will inhibit the formation of overall FREI flame,and the increase of the channel height of the rectangular combustor and the diameter of the tube combustor will reduce the Reynolds number range of flame flashback and extinguishment.（3）The effects of the shape and size of the combustor on the characteristics of stable flame and FREI flame were studied experimentally.The results show that the length of the combustor has relatively little effect on the flame position of the stable flame,and the increase of the length of the combustor will reduce the frequency of FREI flame,and the effect of the length increasing on the reduction of the flame frequency is greater when the amplitude of the FREI flame is smaller.The increasing of the channel height of the combustor and the diameter of the tube combustor has similar effects on the flame characteristics.The increase of the channel height and the diameter will increase the distance between flame position of the stable flame and the exit of the combustor and reduce the wall temperature at the corresponding of the flame front.For the FREI flame,the increase of channel height and diameter will significantly reduce the flame frequency.（4）The effect of hydrogen addition on the flame characteristics of FREI flame was studied by experimental method.The increase of hydrogen addition ratio will inhibit the appearance of stable flame at low inlet Reynolds number conditions,but the effect of has hydrogen addition is limited at high inlet Reynolds number conditions.For in the flame form of FREI,the increasing of the hydrogen addition ratio can improve the uniformity of the wall temperature distribution and increase the wall temperature at the upstream of the combustor.When the amplitude of FREI flame is relatively small,the frequency of FREI flame is larger with the increase of hydrogen addition ratio.However,when the amplitude of FREI flame is relatively large,the propagation distance of the flame is increased with hydrogen addition due to the influence of heat recirculation through wall,which also increase the flame propagation time.This leads to the reducing of FREI flame frequency with the increasing of hydrogen addition ratio.（5）By comparing the experimental and simulation result,a reliable simulation model was obtained,and the combustion characteristics of the methane/oxygen premixed gas in the tube combustor were studied by using the numerical model.According to the change of flame propagation velocity,the flame propagation process of FREI flame can be divided into four stages,namely acceleration stage,deceleration stage,reverse propagation stage and weak reaction stage.The results show that in the acceleration stage and the deceleration stage,the elementary reaction R11:CH₄+H=CH₃+H₂ provides the largest proportion of positive heat release rate,while in the reverse propagation stage and the weak reaction stage,R5:H+O₂+M=HO₂+M and R10:CH₄=CH₃+H provides the largest proportion of positive heat release rate.In addition,In the process of repeated flame propagation,the reduction of methane consumption at the flame front directly leads to the reduction of flame temperature,which in turn causes the change of flame propagation speed.Meanwhile,there is a strong correlation between the flame propagation speed of repeated extinction and the heat release rate of the reaction and the heat flux through wall.（6）The transition mechanism between FREI flame,flame flashback and stable flame was analyzed through numerical simulation.The results show that the decrease of equivalence ratio turn FREI flame into flame flashback.The increase of the equivalence ratio will make the FREI flame transform into a downstream stable flame,while increasing the inlet flow rate will make FREI flame turn into an upstream stable flame.The change of the equivalence ratio affects the flame form transformation by change the proportion of the elementary reaction R11 in the methane reaction pathway as well as the generated amount of free radicals H which is the main reactant of R11.Meanwhile,the increase of the equivalence ratio increases the positive temperature sensitivity of the elementary reaction and reduces the negative sensitivity,so that the reduction of the reaction intensity at a higher equivalence ratio will cause a greater decrease in the temperature of the flame front.So,the increase of the equivalence ratio in turn has a significant impact on the flame extinguishing.The change of the inlet flow rate will change the temperature and velocity of the incoming flow in the combustor,and the increase of the incoming flow temperature will increase the flame propagating velocity.Thereby the changing of the propagation speed of the flame front causes the flame form transformation from FREI to stable.