An Experimental Multiparameter Investigation And Numerical Modeling On The Soot Of Counterflow Diffusion Flames

The formation of soot from incomplete combustion of fossil fuels is a major source of soot.The generation and emission of carbon soot not only reduce the efficiency of combustion on the device and causes serious pollution to the atmospheric environment on which human beings depend.Therefore,it is of great significance for people to reduce or even eliminate the soot in the process of combustion.In order to effectively suppress the formation of soot during combustion,it is necessary to deeply understand a series of complex physicochemical reactions from pyrolysis of fuel to transformation of soot.Soot is usually the result of a combination of factors such as the type of fuel,the type of flame,the pressure of the combustion environment,and the thermochemical structure of the flame.Also,the soot precursors in the flame largely determine the tendency of soot generation.Since the main combustion mode of the combustion device with high soot emission is diffusion combustion,the counterflow diffusion flame has the characteristics of a typical diffusion combustion system and quasi-one-dimensional characteristics,which is widely used in research related to chemical reaction kinetics.In order to further study the soot generation and evolution process,this paper adopts the counterflow diffusion flame as the research platform,and carries out multi-parameter experimental measurements as well as numerical simulations with ethylene and propane flames as the research objects.A particle image velocimetry（PIV）experimental system was built to quantitatively characterize the flow field of the flame under target conditions,and numerical simulations are used to reproduce the experimental results.It is shown that for the flow deviating from the ideal state at the nozzle exit,obtaining the actual nozzle exit velocity is the key to accurately characterize the thermochemical structure of the counterflow diffusion flame and the soot generation characteristics using quasi-one-dimensional simulations.Velocity boundary conditions quasi-one-dimensional numerical simulations can be determined by directly measuring the flow field at the nozzle outlet or by using the results of two-dimensional numerical simulations that include the internal channels of the combustor.Tunable Diode Laser Absorption Spectroscopy（TDLAS）was used to experimentally measure the temperature fields and carbon dioxide concentration distribution of ethylene and propane flames.The experimental data of the flow field and the corresponding numerical simulations were combined to accurately characterize the thermochemical structures of both flames.The results show that the temperature and concentration distributions of the main components obtained from the quasi-one-dimensional numerical simulations are in good agreement with the two-dimensional numerical simulations along the central axis,provided that the velocity boundary conditions are accurately provided.Moreover,both the quasi-one-dimensional and two-dimensional numerical simulation results agree well with the experimental data.A microprobe online sampling and analysis system was built based on a gas chromatograph（GC）and mass spectrometer（MSD）to qualitatively and quantitatively measure the gas-phase soot precursors（C₁-C₈）and polycyclic aromatic hydrocarbons with relatively large molecular weights（naphthalene）in ethylene and propane flame.Comparing the experimental data measured by the GC system and TDLAS system respectively,the presence of a sampling microprobe is shown to perturb the flame with the net effects being to shift the axial profiles of the species to the upper oxidizer nozzle.Quantitative correction of the probe effects can be achieved by comparisons against speciation data measured with non-intrusive optical techniques.At the same time,based on the data of the intermediate components of the flame measured experimentally by GC and MSD systems,combined with the quasi-one-dimensional numerical simulation results,the benzene and naphthalene in the soot precursor were analyzed by using detailed chemical reaction kinetic mechanisms,respectively.An optical diagnostic system was built,including laser extinction（LE）experimental system and laser-induced incandescent（LII）experimental system,to quantitatively characterize the sooting tendencies of the flame under target conditions.The experimental data measured by the LE and technique were also cross-validated to confirm the reliability of the soot volume fraction data.Meanwhile,the Multi-parameter measurements of ethylene and propane flames confirmed that aliphatic fuel flames with a low tendency to generate aromatic substances（benzene）may actually have higher sooting tendencies.The multi-field experimental data based on ethylene and propane flames measured in this paper can be used as a valid data set to validate and optimize the newly proposed gas-phase reaction kinetic model and soot model in the future.