Flame spread over solid fuels is controlled in part by gas-phase phenomena such as heat transfer, chemical kinetics, fuel-oxidizer mixing, and flame behavior. Experimental measurements resolving such multi-parameter phenomena at the gas-solid interface are limited which impedes the ability to predict flame spread accurately. This work presents a combination of laser diagnostics that resolve the gas-phase temperature and select species distributions associated with downward (opposed flow) flame spread over polymethyl methacrylate (PMMA) samples of 12 mm thickness. Simultaneous OH laser induced fluorescence (LIF) and CH* chemiluminescence are used to characterize the reactive species distribution that describe the wall-bounded flame. Hybrid fs/ps rotational coherent anti-Stokes Raman spectroscopy is employed alongside OH-LIF to characterize the flame structure in greater detail, providing valuable information of 2D gas temperature and O 2 /N 2 (molar ratio) fields alongside the reactive species distributions. Measurements are used to study gas-phase processes in the unburned region ahead of the flame as well as the enflamed region. Ahead of the flame, additional experiments measuring surface temperature describe differences in the preheat distance between the solid and gas phases. Depletion of oxygen and increase in gas temperature occur directly near the theoretical pyrolysis temperature on the solid, approximately 1–2 mm ahead of the leading edges of CH* and OH distributions. 2D gradient fields of gas temperature and O 2 /N 2 are calculated providing insight into the onset of chemical reactions and diffusion characteristics surrounding the flame. Average gas temperature in the flame’s leading edge is ∼1350 K and is ∼200 K colder on average than flame regions downstream of the leading edge. These measurements provide new insights into the gas-phase processes in flame spread. Novelty and significance statement Understanding of gas-phase processes governing downward (opposed flow) flame spread is limited due to a lack of non-intrusive experimental measurements. This work overcomes this limitation by using a suite of advanced laser diagnostic providing unprecedented access to gas temperature and selective species distributions during flame spread. Temperature and species gradient information provide fundamental insight into gas-phase heat transfer, diffusion processes, length scales, and general flame structure. Such information provides a contemporary view of gas-phase processes in downward flame spread, which has been largely missing in the literature.
Ojo et al. (Fri,) studied this question.