Effects of oxygen addition to the fuel stream on the soot formation in an ethylene coflow partially premixed flame based on the orthogonal decoupling method

In this work, an orthogonal decoupling method based on the orthogonal design and decoupling method was used to simulate the effects of adding oxygen (O-2) to the fuel stream on the soot formation characteristics in an ethylene (C2H4) coflow partially premixed flame. The dilution effect, thermal effect and chemical effect of the added O-2, as well as the interaction between these effects were decoupled and analyzed. Results show that the peak soot volume fraction (SVF) in the flame decreases first and then increases as the O-2 mole fraction gradually increases to 30%, and is lowest at an addition of 20%. The orthogonal decoupling calculations were performed for two O-2 concentration ranges of 5%-10% and 25%-30%, and the differences on soot formation at these two ranges were found to be dominated by the chemical effects of O-2 through the extremum difference analysis of the calculated peak SVF, number density of primary particle and particle aggregation degree. The chemical effect of O-2 inhibits the formation of soot at an O-2 addition of 5%-10%, while promotes the formation of soot at an O-2 addition of 25%-30%, by affecting the inception, hydrogen-abstraction-carbon-addition (HACA) reaction, and polycyclic aromatic hydrocarbon (PAH) condensation rates of soot formation. When the O-2 concentration is 25%-30%, the chemical effect of O-2 promotes the soot surface growth rate and therefore increases the soot particle size, which in turn retards the complete oxidation process, leading to an increase in SVF.