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.