Studies On The Stability Of Jet Diffusion Flame Under Dilution Condition

Jet diffusion flame is widely used in industrial production and daily life because it is easy to control.However,some uncontrolled jet diffusion fires will cause serious casualties,economic losses,and environmental damage.A lot of research has been carried on the stability of jet diffusion flame.However,due to the complex physical and chemical reaction process,the stability mechanism of lifted flame is still a hot topic in the research.Thus,it is important to research the stability of jet diffusion flame with dilution to enhance the understanding of the combustion mechanism.In addition,the pollution emissions during the combustion of fossil fuels not only damage human health but also threaten human survival.Adding diluent in gas fuel can effectively reduce nitrogen oxides and soot emissions,but adding diluent in gas fuel can significantly reduce the stability of flame.Therefore,it is of great significance to study the stability of jet diffusion flame with dilution for energy security utilization and sustainable development.This paper concerns the effects of different dilution gases and ambient pressures on the stability of jet diffusion flame by means of experimental research,theoretical analysis and numerical calculation.Based on the study of liftoff height,blowout limit,and self-excitation oscillation,combined with the classical theory of combustion dynamics,the evolution mechanism of jet diffusion flame behaviors with different dilution gases and environmental pressures under dilution condition is revealed,and dimensionless characterization models are established to characterize the behaviors.Specific research include:The liftoff height and blowout limit of diffusion jet flame under different dilution gases are investigated.Propane diluted with helium,nitrogen,and carbon dioxide is supplied.The stability of jet diffusion flame is researched by changing the type of dilution gas,mole fraction of fuel,and jet velocity.The results show that there are deviations from the fully turbulent liftoff height prediction model proposed by Kalghatgi et al at high dilution ratio.By analyzing the underlying mechanism and key factors of dilution effects,the fuel mole fraction responds to the combustion intensity at the bottom of the flame,so the premixed flame prediction liftoff height model proposed by Kalghatgi is revised by considering the fuel mole fraction.At the same time,the applicability of the premixed flame model proposed by Kalghatgi in predicting blowout limit under different dilution gases and high dilution conditions is verified.In addition,the liftoff height and self-excitation oscillation of diluted jet diffusion flame under different ambient pressures are investigated.Propane diluted with helium is supplied with air coflow.The stability of laminar diffusion jet flame was studied for various environmental pressures,fuel mole fractions,and jet velocities.The results show that the liftoff height of the laminar jet diffusion flame increases nonlinearly with the jet velocity,and decreases with the decrease of ambient pressure and fuel mole fraction.By analyzing the effects of laminar flame propagation speed,fuel velocity,and fuel mass fraction,the dimensionless analysis model of laminar diffusion flame liftoff height under different environmental pressure and fuel mole fraction is proposed.At the same time,the oscillation frequency of liftoff height is extracted by Fast Fourier Transform.The results show that the buoyancy force on the flame gradually decreases with the decrease of ambient pressure.The buoyancy-induced self-excitation phenomenon disappears under very low ambient pressure,while the heat loss selfexcitation frequency decreases with the decrease of pressure.By analyzing the effects of nozzle diameter,average liftoff height,dimensionless jet velocity,and heat loss factor,a dimensionless model of heat-loss-induced self-excitation frequency under different ambient pressure and fuel mole fraction is proposed.