Study On Oscillation-Induced Extinction Mechanism Of Spherical Diffusion Flame In Micro-Gravity

Flame extinction has existed extensively in the various combustion devices including boilers,gas turbines,internal engines,and aero-engine;and it is highly relevant with stability,safety,and efficiency of combustion devices.It is of great importance to study flame extinction which is a hot-spot issues in the field of combustion theory and science.Flame extinction is one of limit combustion phenomenon which often happened within a small spatial domain or a very short time interval.Flame extinction is not only correlated with chemical dynamics,but also with flow or heat/mass transport.Besides,for most hydrocarbon fuels,the low-temperature chemistry played an important role in extinction process.Hence,there are many factors affecting extinction,and the extinction mechanism is extremely complex.Flame extinction can be triggered by various mechanisms,including the radiative extinction,stretch-induced extinction,jet blow-out extinction,transport induced extinction,local extinction and re-ignition,premixed propagating front extinction,and oscillation-induced extinction,etc.Existing research indicated that the first six extinction modes have been investigated thoroughly,and their governed mechanisms have been deeply understood.However,as another type of extinction,i.e.the oscillation-induced extinction which existed in a wide range of combustion equipment,it has not attracted much attention,and our understanding on its inducing mechanism is still quite uncertain.Hence,it is necessary to study the mechanism of oscillation-induced extinction deeply and systematically.To meet research needs,this study employed the laminar,reduced-pressure,and micro-gravity spherical diffusion flame to study the kinetic and transport mechanism that controlled the oscillation-induced extinction.Moreover,DME was employed as target fuel.The main contents and conclusions are as follows:（1）The S-curve of DME spherical diffusion flame was achieved through steady-state simulation,then the transient simulation was conducted on near-extinction flames on the S-curve,observing the phenomenon of oscillation.The results indicated that the hot-flame extinction was controlled by a single-frequency oscillatory mode with a constant frequency（1 Hz）,nevertheless the cool-flame extinction was governed by dual-frequency oscillatory mode that had fairly distinct frequencies.The hot-flame extinction was governed by the competition between the high-temperature exothermic/endothermic reactions and the chain branching/termination reactions involving small molecules;the cool-flame extinction was governed by the low-temperature chain branching/termination reactions in the negative temperature coefficient（NTC）region.（2）Based on the steady-state solution on the S-curve,we used chemical explosion mode analysis（CEMA）method to transform the Jacobian matrix of the chemical reaction source term of control equation,then diagnosing the combustion mechanism of chemical explosion mode（CEM）,which could further reveal the oscillation instability of DME spherical diffusion flame under near-extinction condition.The results indicated that for hot-flame branch,when the X^*_O2 was enough low,CEM with positive eigenvalues firstly emerged at the maximum temperature location because of incomplete combustion.However,for cool-flame branch,CEM always existed within its reaction zone.Within the near-extinction region on the hot-flame branch or cool-flame branch,the eigenvalues of CEM were complex number,indicating that the near-extinction flames will probably exhibit an intense oscillation nature,and the oscillation region of cool-flame branch was broader than that hot-flame branch.Besides,the local extinction area can be separated from the flame zone using the profile of eigenvalues of CEM efficiently;Positive eigenvalues are almost simultaneous with flame oscillations,indicating that the flame oscillation under the near-extinction condition was universal.（3）Base on development of hydrogen doping technology,it is well-known that the limit combustion characteristics（extinction and ignition）of traditional fuels could be dramatically changed by mixing a small amount of hydrogen with fuel.Hence,the effects of hydrogen addition on the extinction of DME spherical diffusion flame were studied.The resules showed that both hot-or cool-flame flammability limits could be extended due to H₂ addition.Furthermore,H₂ addition led to a narrower oscillation zone of hot-flame branch,but led to a wider oscillation zone of cool-flame branch.