A Study On Burning And Extinction Behaviors Of Stagnation-point Diffusion Flames With Varied Stretch Rates

The effects of flame curvature,unsteadiness in the flame and flow,as well as hydrodynamic normal or shear strain effects on the flame can cause flame stretch rate.Generally,flames in normal gravity are often in the diffusion form with high stretch rate,while for microgravity fires in spacecraft,low-stretch diffusion flames are frequently seen.Furthermore,a dynamic transition from high to low（or from low to high）stretch flame exists in both normal-and micro-gravity fires.The stagnation-point diffusion flames are established to be uniform and one-dimensional,which facilitates the detailed observations of the flame stretch and other important parameters.Thus,stagnation-point diffusion flames with varied stretch rates is an important and longstanding topic in the fundamental research of combustion and fire.The available studies on stagnation-point stretched diffusion flames are scarce,most of which focus on high-stretch blowoff.The studies of low-stretch diffusion flames are much fewer,in particular,the different characteristics and transition mechanism of low and high stretched diffusion flames are still not clear.Therefore,this work established three experimental platforms for typical condensed-phase solid fuels and alkane gas fuel,where the stagnation-point diffusion flames from low to high stretch rates in a buoyancy flowfield were obtained,by changing the radius of solid fuel and gaseous "Tsuji" burner.In addition,the heat,gas-phase components,oxygen and pressure conditions can be changed in those platforms.The steady burning,instabilities,and extinction behaviors of these flames were investigated systematically.Firstly,for steady burning of solid fuels,according to the mixture fraction theory,the relationships between flame standoff distance,thermal layer thickness and stretch rate were revealed.Furthermore,the theoretical steady burning rate was derived to be raised to the power of 0.5 with stretch rate.These theoretical relationships were confirmed by the experimental measurements.Moreover,for thermally thin/thick solid fuels,the steady flame temperature as well as the solid and gas phase heat transfer flux in the energy balance along with varied stretch rate were analyzed.Secondly,the effects of oxygen and pressure upon the characteristics and mechanisms of flame extinction of solid fuels were studied.（1）For thermally thin material,with increased stretch rate and pressure,the critical flame temperature increases while the critical oxygen concentration decreases,which results in a constant critical Damk?hler number（Dacrit）along with varied stretch rate at a given pressure,and Dacrit decreases with reduced pressure.The mechanisms of flame extinction can be evaluated by a non-dimensional solid surface heat loss ratio parameter,Floss:when Floss>50%,flame quenching is dominated by heat loss,corresponding to lower stretch rate and pressure.In contrast,flame blowoff is controlled by chemical reaction.（2）For thermally thick material,the flammability map is divided by local and total extinction limits,which critical oxygen concentrations both have "U" shape relationships with stretch rate,and the turning point stretch rate is about 10 s-1.The mechanisms of flame extinction can be determined by this turning stretch rate:when stretch rate is less than 10 s-1,the large volume heat loss causes declined flame temperature and flame is quenched.When stretch rate is more than 10 s-1,the increase of stretch rate results in decreased chemical resident time and thus flame is blown off.In the comparative study of thermally thin and thick solid fuels,it is obvious that the solid in-depth pyrolysis and conductive heat loss are absent in thermally thin fuels,which further influences the distribution of pyrolysis gases and gas-solid phase energy balance.Thus,there are differences in the flame structure,heat transfer flux,and extinction limits between thermally thin and thick materials with varied stretch rates.Thirdly,for the stagnation-point diffusion flame of gaseous fuel,the steady burning,flame instabilities and extinction behaviors at different N₂/CO₂ dilution level in the fuel stream with varied low stretch rates were studied.（1）At low dilutions,the quasi-steady flame temperature,radiation,standoff distance and heat transfer flux are all related to the combined effects of stretch rate,fuel injection rate,uF,and dilution gas type.（2）When the dilution is increased,two instability patterns appear:one is the stripes and holes pattern at higher uF,and smaller stretch rate and C02 dilution are prone to show this pattern,which is dominated by Rayleigh-Taylor instability and influenced by thermal-diffusion instability.The other is the periodic hole pattern at lower uF,and higher stretch rate and N₂ dilution are easier to exhibit this pattern,which is dominated by heat loss instability and affected by thermal-diffusion instability.（3）The flame becomes total extinction when the dilution is further enhanced.The dilution limits gradually increase with increased uF and plateaus at high levels,where N₂>CO₂.Extinction in the high dilution limits part with high uF is due to largely decreased heat release and substantial flame heat losses.Whereas,extinction in the low dilution limits part with low uF is due to both flame and burner surface heat losses to ambience,furthermore,the extinction limits decrease with the increase of stretch rate in this part.Finally,the steady burning and extinction behaviors of stagnation-point low stretch diffusion flame were studied when the oxygen levels in the oxidizer stream was reduced from high to low by N₂ dilution.When the oxygen concentration gradually decreased from high to low,sooting limit first appeared,then the instability limit,and finally the extinction limit,which divided the flammability map into four regions.By comparing the results of gas and solid fuels,it was found that,the gaseous burner has the potential to simulate a real solid fuel’s stagnation-point stretched diffusion flame,which can be achieved by adjusting the type of diluent gas,fuel dilution level,and fuel injection rate to emulate the surrogate solid’s burning rate and further its heat of gasification.The results of the present work have revealed the characteristics and mechanisms of steady burning,instabilities,and extinction behaviors of stagnation-point diffusion flames with varied stretch rates.Also,the differences and transition mechanisms between high and low stretch rate,and the quantitative and qualitative differences of stretched flames between thick and thin solid fuels were unveiled.Furthermore,it provided a method of using gaseous burner to emulate a real solid fuel by a stagnationpoint stretched flame.This work enriched the basic knowledge in the fundamental understanding of stretched flames in the combustion and fire research field,provided a scientific guidance for the development of prediction and prevention capability in fire engineering,and gave an experimental and theoretical support to simulate a microgravity low-stretch flame in a terrestrial environment.