Spatially Resolved Laser-induced Breakdown Spectroscopy In Laminar Methane Flames

At present,the main source of power and heat for social production and life are still the burning of fossil fuels.So it is urgent to study the mechanism of combustion flame.The main method to study the mechanism of combustion is applying combustion diagnosis technology and discovering and mastering its law.LIBS(laser-induced breakdown spectroscopy)can realize simultaneous measurement of multi components,and the spectral information can provide a good qualitative and quantitative analysis for samples.The focused area of the laser pulse in the flame is small and the action time is short,so the interference to the flame flow field is also small.The work of this paper is applying LIBS in the study of the spatial distributions of methane laminar Bunsen flames.Firstly,the linear relationship between the flame equivalence ratio and the specific spectral line intensity ratio is analyzed theoretically.The experimental results also show that the relationship between the four sets of spectral intensity ratios(I_H/I_C?I_C/I_O?I_H/I_N?I_C/I_N)and the equivalence ratio of the premixed laminar flame is linear.And linear relations of the H/O intensity ratio and the equivalence ratio is calibrated at three different points on the flame axis.Then,LIBS is applied in the study of spatial distributions of the laminar premixed methane–air flames.According to the temperature dependency of the breakdown threshold laser pulse energy,a new approach was proposed to characterize the laminar premixed flame temperature distributions based on the plasma energy distributions.Combination of the analysis of the spatial distributions of spectrum intensity and plasma energy,the laminar premixed flames structure was investigated deeply,including the distribution of the flame temperature,the width and distribution of different flame region(e.g.premixed combustion regions,high temperature regions.),as well as the location of the flame front.The distributions of the equivalence ratios of different heights in different flames are obtained by linear relations of the H/O intensity ratio and the equivalence ratio.Finally,LIBS is applied in the study of spatial distributions of the laminar diffusion methane–air flames.The experimental results shows that the plasma energy distributions can also be used to characterize the temperature distributions of the laminar diffusion flame.The spatial distributions of the flame temperature and equivalence ratio are analyzed by the distributions of the plasma energy and the H/O intensity ratio.the positions of the flame fronts and the width of secondary combustion regions are also be determined.According to the relevant measurement,the linear equation for H/O intensity ratio and equivalence ratio can be determined approximately.By this linear equation,the axial distribution of local equivalence ratio and the flame length is obtained.