Soot formation from n-heptane counterflow diffusion flames: Two-dimensional and oxygen effects

Abstract

Soot formation of n-heptane is experimentally and numerically studied in low-strain rate (K = 50 s−1) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO2=0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (Tmax) and peak soot volume fraction (fv,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and fv,peak increase from xO2=0.3 to xO2=0.45 due to the related increase of flame temperature, which fosters n-heptane pyrolysis near the fuel nozzle leading to higher concentrations of C1C4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO2. However, the higher flame temperature in the sooting region at xO2=0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO2=0.3.