Diagnostic Strategies Of Combustion Intermediate CH Based On Laser-Induced Fluorescence

CH is an important intermediate in hydrocarbon combustion,and the measurement of CH is of great significance.The most commonly used CH measurement technique in the combustion field is laser-induced fluorescence(LIF).Three excitation strategies of CH using LIF have been developed so far:A-X,B-X,and C-X,where X denote the electronic ground state of CH,and A,B,and C the electronically excited states.can be divided into three categories according to the excitation of different electron energy level transitions:CH A-X,B-X and C-X diagnostic strategies.In this paper,the three diagnostic strategies of CH LIF were systematically studied in flames.We believe that the research has important reference significance for the systematic understanding of CH diagnostic strategies;and plays an important role in promoting the practical application and optimization of CH diagnostic strategies LIF technique.Firstly,the images of CH are measured and analyzed by CH A-X,B-X,and C-X diagnostic strategies,and the relationship between the fluorescence intensity of CH and the laser energy was studied.Under the experimental conditions,the C-X diagnostic strategy of CH can realize the planar measurement of CH,while the B-X and A-X diagnostic strategies can only realize the one-dimensional line measurement of CH.The experimental results show that the C-X diagnostic strategy of CH is the most efficient among the three diagnostic strategies,which is 3-4 orders of magnitude higher than B-X and A-X diagnostic strategy,while the efficiency of B-X and A-X diagnostic strategy is basically the same.Secondly,the photolysis interference of CH C-X diagnostic strategy is studied in laminar premixed CH4/air flame.By measuring the fluorescence spectra,the sources of resonance fluorescence and photolysis fluorescence were explored,and the critical laser spectral irradiance for photolysis was obtained.The experimental results show that the fluorescence signal generated by laser excitation comes from the CH,OH and CH2O in the combustion field,and the OH signal intensity is one order of magnitude lower than the CH signal intensity,and the CH2O signal intensity is five orders of magnitude lower than the CH signal intensity.The signals generated by laser photolysis include C2 fluorescence and CH fluorescence.The C2 signal only exists in the product area of flame and does not interfere with CH measurement in the reaction area.CH radicals produced by photolysis mainly exist in the reaction zone of flame,which is the main interference of CH PLIF measurement.Photolysis interference can be ignored with lower laser energy.Finally,the C-X diagnostic strategy of CH is adopted to study the variation of CH PLIF intensity with different equivalent ratios in flames and the structure of flames at different Reynolds numbers.The experimental results show that the fluorescence intensity of CH increases first and then decreases with the increase of the equivalence ratio,and reaches the maximum value around the equivalent ratio of 1.2.