OH-PLIF study on the mechanism regulating flame-wall interaction with catalytically active CeO2-ZrO2 coatings

Abstract

Technically, using catalytically active coatings regulate flame-wall interactions to enhance flame stability in small-scale burners, such as the common ZrO2-based ceramics with doping CeO2 which benefit to improve the high-temperature mobility and reactivity of lattice oxygen as an oxygen source to promote gas phase combustion. Probing by OH-PLIF laser diagnosis, the aim of this study was to investigate the dopant content effect (0–20 wt.% CeO2) on the flame characteristics of methane-air mixtures of varying equivalence ratio (0.9, 1.0, and 1.2) in an adjustable-gap narrow channel (2–12 mm) at a wall temperature range of 373 K to 973 K, including the dynamic flame morphology, flame quenching performance, and near-wall spatial distribution of OH radicals. Results show that the flame-wall distances are strongly correlated with wall temperature, equivalence ratio, and channel scale. After doping CeO2, the measured quenching distances can significantly drop, but do not show a linear downward trend with increasing CeO2 content. The case of 5%CeO2-ZrO2 coating demonstrates a small critical quenching Peclet number at high wall temperatures. The maximum OH fluorescence intensity in flame core areas increases with CeO2 doping into CeO2-ZrO2 at 973 K when the channel clearance is less than 4 mm. However, the absolute concentration of near-wall surviving OH radicals in the presence of 20%CeO2-ZrO2 coating is substantially lower than in the 5%CeO2-ZrO2 case due to differences in the redox properties. In situ diffuse reflectance infrared Fourier transformations spectroscopy tests further indicate that the adsorption quantities of C2H4 and C2H6 species on the coating surface also show a non-monotonous CeO2 dependence, as they are important intermediates in the low-temperature C2 chain reaction of methane.