Experimental Study Of The Propagation And Instabilities Of Laminar Expanding Flames

Understanding the properties of laminar premixed flames is of essential importance in the pollutant reduction and stability control of the flames in engines.In this dissertation,the outwardly expanding spherical flame is adopted in an experimental study of the propagation and instability of premixed flames.Specifically,this dissertation aims at improving the accuracy of laminar flame speed measurements,revealing the response of the laminar flame speed to flame chemistry,and providing physical insights into the flame-front instability as well as global pulsation in flame acceleration.In this dissertation,the extrapolation uncertainty in laminar flame speed measurements was first quantified and reduced,using hydrogen/air and propane/air flames.Results show that the extrapolation uncertainty consists of model error and random error.A small value of the parameter,|L_b/R_f|,allows for the neglect of the model error by increasing the upper and the lower bounds of the flame radius range.A new empirical parameter,R_f,new,was defined according to the experimental results to represent the entire flame radius range.The random error is mainly affected by the number of points in the extrapolation.Second,the pressure and equivalence ratio effects on the flame chemistry were investigated using ethylene/air flames.The laminar flame speeds,with quantified uncertainties,were measured over a wide range of pressures and equivalence ratios.Sensitivity analysis shows that the laminar flame speed becomes more sensitive to flame chemistry at higher pressures,while the relative importance of chemical reactions is notably affected by the equivalence ratio and insensitive to pressure.Third,H₂/O₂/N₂ flames were used to reveal the separate and coupling effects of hydrodynamic and diffusional-thermal instabilities on the morphologies and critical flame radii of unstable flames,whose burned flame temperatures are controlled through manipulating the amount of N₂ in the air.It is shown that large cellular structures are dominated by the hydrodynamic instability while the smaller ones by the diffusional-thermal instability.The critical flame radius decreases significantly with increasing pressure or decreasing Lewis number,and is insensitive to the burned flame temperature.The normalized critical flame radius decreases as the Lewis number decreases and is insensitive to pressure and burned flame temperature.Meanwhile,the normalized critical flame stretch monotonously decreases as the Markstein number increases.Finally,three stages were observed in the self-acceleration of unstable flames,i.e.smooth expansion,transition and global pulsation stages.It is shown that the global pulsation frequency increases with increasing intensity of the flame-front instability,which is consistent with predictions from the hypothesis that the global pulsation behavior arises from the continuous cell growth and splitting during the flame acceleration.The normalized global pulsation frequencies of H₂/air flames,subjected to the coupling of hydrodynamic and diffusional-thermal instabilities,collapse under different pressures and decrease with increasing equivalence ratio.This indicates that the pressure and flame temperature effects are properly scaled out through the normalization.The acceleration exponents of the transition stage and global pulsation stage were also determined,with the latter slightly smaller than the critical value of 1.5 suggested for self-turbulization.