OH-PLIF Research On Radical Formation In Methane And N-heptane Laminar Diffusion Flames

With the progress of society and the development of industrial technology,human demand for fossil fuels is increasing.However,the fossil fuels currently in use,including coal,oil,natural gas and other energy sources,are formed through complex physical and chemical reactions and geological changes over billions of years.The exploitation and utilization of fossil fuels have directly led to energy crisis and environmental pollution.Exhaust gas recirculation is an effective means to control NOx and soot emissions.Therefore,the measurement of basic parameters closely related to the combustion characteristics of diluents is of great significance to the study of flame mechanism and the improvement of combustion efficiency.Methane and n-heptane were used as basic fuels to study the concentration distribution of OH radicals in laminar diffusion flame.In this paper,planar laser-induced fluorescence（PLIF）technology was used to collect OH radical fluorescence signal images of fuel under different working conditions by ICCD camera.The maximum fluorescence signal intensity and the total fluorescence signal intensity were calculated,and the influences of air flowrate,carrier gas flowrate,fuel flowrate,N₂ and CO₂ diluents on the formation and distribution of OH radical were analyzed.Experimental analysis showed that,for methane and n-heptane fuels,OH radical was mainly concentrated at the two wings of the flame,presenting symmetrical and uneven distribution.With the increase of fuel flowrate,the overall height of the fluorescence region increased,and the maximum OH radical fluorescence signal intensity and total intensity first increased and then decreased.With the increase of air flowrate,the formation of OH radical increased gradually.With the increase of carrier gas flowrate,internal flame height increased and the intensity of the maximum OH radical fluorescence signal and the total intensity showed a trend of first increasing and then decreasing.Both N₂ and CO₂ diluents reduced the maximum and the total OH radical fluorescence signal intensity,and when the dilution ratio increased to 5%,there would be promotion phenomenon.For the same proportion of diluents,the fluorescence region caused by CO₂ rised to a higher height.Compared with methane gas fuel,diluents had greater influence on the hydrodynamics and thermodynamic instability of n-heptane flame.