Experimental assessment of the sudden-reversal of the oxygen dilution effect on soot production in coflow ethylene flames II: soot radiation and flame transition analysis

Abstract

The current study focuses on the radiative characteristics of heavy smoking flames that are formatted by oxygen being added to the fuel stream of a steady laminar ethylene diffusion flame. In such a scenario, based on the experimental local soot temperature and volume fraction results, the soot local radiative fields are obtained by solving the Radiative Transfer Equation (RTE). Along with the diluted range (oxygen volume fraction XO2ax from 0 to 32%), the flame undergoes two significant transitions, i.e., smoking and non-smoking; however, only one remarkable radiative intensity transition between 30% and 32% is identified. The radiative loss due to the oxygen chemical effect is further assessed by comparing with that of N2 diluted flames. It is found that the chemical effect on radiative loss promotion is undermined by approximately 25%, compared with that on soot formation promotion. In contrast, the oxygen dilution effect results in a similar reduction level on soot formation propensity and soot radiative loss. Furthermore, the correlations between relative soot consumption rate and cumulated radiative loss below the flame tip region are explored, and the results indicate that the initial relative oxidation rate at the position of zFv,max (the peak location of flame cross-section mean soot volume fraction) and its residence time are two important factors that may affect the final flame smoking/non-smoking transition. Eventually, these local soot radiative intensity fields, together with the local soot temperature and volume fraction fields, provide a more comprehensive benchmark to refine the soot oxidation model as the ISF workshop required and understand the flame transition mechanism in the sooting flames.