Ignition characteristics of premixed cool flames on a heated wall

This work examined the cool flame ignition and stabilization on heated walls with different surface reactivities. A premixed mixture of dimethyl ether (DME) and O₂ was impinged on a heated wall and the wall temperature was linearly ramped to determine the cool flame ignition temperature. Numerical analyses identify a hysteresis phenomenon in the cool flame ignition and extinction processes, which appears to be primarily governed by the low-temperature reaction kinetics. The formation of HCHO during the cool flame ignition process was monitored using the planar laser-induced fluorescence (PLIF) technique synchronized with instantaneous wall temperature measurements. The data indicated that the HCHO concentration near the wall surface abruptly increased at the ignition of the premixed cool flame. The predicted hysteresis behaviors for cool flame ignition and extinction with a nonlinear folded S-curve and a smoothly stretched S-curve were successfully observed experimentally for the first time ever for lean mixtures. Moreover, the results show that compared with the ignition limits on an inert SiO₂ surface, noticeable increases and decreases of the cool flame ignition temperature were observed on Fe and Pt surfaces, respectively. These findings suggest that significant wall chemical effects occur during the cool flame ignition process, based on interactions between the gas phase and surface reactions.