Influence of the oxygen index on acoustically forced laminar ethylene non-premixed flames

Soot concentration plays a crucial role in the radiative emission of combustion processes. While its effects are better comprehended in laminar flames generated in controlled laboratory settings, the exact implications on practical flames with turbulent behavior are still unclear. Forced laminar flames provide valuable insights to bring closer laminar and turbulent combustion. This work presents measurements of local radiative heat flux on transient laminar flames, allowing the study of its relationship with soot concentration and temperature, and providing new data for model calibration on time-dependent numerical simulations. Acoustically forced ethylene coflow non-premixed flames were experimentally studied using the line-of-sight attenuation (LOSA) technique and two-color emission measurements to determine the soot volume fraction, temperature, and local and global measurements of the radiative flux for the time-dependent flames, oscillating at a frequency of 10 Hz. First, the flames studied by Shaddix and Smyth [Combust. Flame 107 (1996)] were reproduced, and then new flames were analyzed varying the oxygen molar concentration in the oxidizer flow, denoted as Oxygen Index (OI), ranging between 21% and 33%. Results show that the forced condition generates significant changes in soot production, distribution of soot temperature, and radiative properties compared to the steady flame. The divergence of the radiative heat flux, computed from soot temperature and volume fraction measurements, is increased compared to the steady flame for OIs from 21% to 29%, whereas for 33%, it is reduced. Furthermore, due to the increase in temperature and soot content, the flame radiation is enhanced between 40% and 70% of the phase cycle for all the OIs studied. Finally, the total radiant fraction from soot was found to increase with the flame forcing regardless of the oxygen doping.