Experimental Measurement And Simulation Of Radicals And Carbon-to-oxygen Atom Ratio In Partially Premixed Flames

Partial premixed combustion is widely used in practical combustion facility because of its cleanness and stability.As the equivalence ratio of partial premixed flame increases,the gas-phase PAHs will gradually transform to condensed-phase soot particles,resulting in the soot inception.And the local carbon-to-oxygen atom ratio in the flames is an important condition for the soot inception.In addition,the chemiluminescence from the excited radicals is always appear around the soot inception zone,so it has important significance for understanding and controlling the soot formation to study the local carbon-to-oxygen atom ratio and related excited radicals in the flames.In this paper,the carbon-to-oxygen atom ratio and excited radicals in ethylene partially premixed flames were experimental measured and simulated.The details are summarized as follows:Firstly,the detailed USC-Mech version II mechanism combined with CH*radicals reaction mechanism was used in the numerical model,and the semi-empirical soot model was also used.the CH*distribution and its relationship with flame structure was simulated and analyzed at different equivalence ratios,and the influence of the carbon-to-oxygen atom ratio and temperature distribution to the soot formation was further studied.Results show that CH*is mainly distributed at the inner and outer flame front,which is the marker of reaction heat release.The downstream of the inner flame front is the soot precursor and soot enrichment zone.and the outer flame front is the oxidant enrichment zone.With the increase of equivalence ratio,the peak concentration of CH*and the flame temperature decreases,the soot volume fraction increases.The C/O ratio firstly increases and then decreases along the axial direction,and the peak appears at the soot inception zone where the temperature is relatively low.The outer flame front has the higher temperature and the lower C/O ratio,so it has strong oxidizability and it is unfavorable for soot formation.Then,the hyperspectral image technology combined with the inverse Abel transform reconstruction algorithm were used to detect and reconstruct the CH*and C₂*radicals in the flame,and the simulation results were also analyzed.The results show that at the inner flame front,the intensity of excited radicals decreases along the flame height.With the equivalence ratio increases,the height of the inner flame front increases,the intensity of CH*and C₂*decreases,while at the outer flame front,the intensity change is not obvious,the soot radiation intensity increases at the inter-conal zone.The CH*distribution from the simulation is in good agreement with the reconstruction results.At the inner flame front,CH*can be produced by the oxidation reaction of C₂H and O or O₂,while at the outer flame front,C₂H can only be oxidized by oxygen atoms.C₂H₂ is the intermediate product of both excited radicals and soot formation,with the increase of equivalence ratio,the oxygen mole fraction in the premixed gas decreases,resulting in the inhibition of the oxidation from C₂H₂to CH*and C₂*,meanwhile,the soot formation is promoted.Finally,the laser-induced breakdown spectroscopy（LIBS）was used to measure the local C/O ratio in the ethylene partially premixed flame,The C/O ratio along the axial direction at different equivalence ratios was studied,and the measured results also compared with the simulation results.The results show that the measured spectral intensity ratio has a linear relationship with their C/O atom ratio.Along the flame axial direction,the carbon atom spectral intensity decreases,the oxygen atom spectral intensity decreases firstly and then increases,and C/O atom ratio decreases.The trend of the detection results is consistent with the simulation results.