Simultaneous imaging of soot volume fraction, PAH, and OH in a turbulent n-heptane spray flame

Abstract

Soot formation and oxidation in a turbulent n-heptane jet spray flame are investigated through simultaneous imaging of quantitative soot volume fraction (fv), flame-front (OH), and soot-precursor marked by smaller (2–4 ring) polycyclic aromatic hydrocarbons (PAH). Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques are used. The spray flame is composed of a dual branch axisymmetric structure. The inner composite branch (B1) consists of fuel-lean and non-premixed reaction zones, whereas combustion in the outer branch (B2) occurs in a non-premixed mode. Frequent local flame extinctions occur along B1, while no extinctions are observed in B2. Consequently, the soot inception occurs near B2. In the onset of soot detection (fv ≥ 0.03 ppm) region, soot layers are thinner than those from downstream regions where soot occupies nearly the same area as of PAH. Soot generally oxidizes across the flame-front as evidenced from the absence of soot-OH overlap. The soot structures are intermittent with peak soot probability of 60%. The most probable peak fv measures 1 ppm, while the conditional mean is 0.42 ppm which is 2.5 times that of time-averaged fv. Conditional mean fv shows monotonically increasing trend with height. The flame lift-off height and local extinctions do not instantaneously influence the soot concentration, possibly due to larger convective and soot formation timescales. Soot-PAH correlations are explored. However, no strong instantaneous correlations are noted, most likely due to different chemical timescales of soot and the imaged smaller-PAH. Nevertheless, a few distinct trends are noted in soot-PAH correlations. fv weakly increases with the PAH-LIF intensity. The PAH intensity from soot onset to peak region decreases, while both soot and PAH are nearly consumed at a local flame-tip (oxidation) region. The soot-PAH overlap decreases consistently from soot onset to oxidation region. The reported database and deduced insights can aid in the development and validation of soot models in two-phase reacting flows.