The probabilistic nature of ignition of turbulent highly-strained lean premixed methane-air flames for low-emission engines

Abstract

The probabilistic nature of ignition of turbulent highly-strained lean premixed flames has been investigated. Two flow configurations have been considered in this study; counter-flow and bluff-body flames. Different successful and non-successful ignition events in both flows have been observed directly with high-speed imaging. In addition, the flame front structure during the flame propagation following the ignition has been observed using OH-PLIF technique. Moreover, the ignition probability has been measured in both flows with different equivalence ratios and flow velocities. It has been found that high bulk velocities decrease the ignition probability in all locations and for all flames. For counter-flow flames, ignition is sometimes possible even in locations where there is negligible probability of finding flammable mixture and is sometimes impossible in locations with high probability of flammable flow. The edge flame propagation speed has also been detected in this type of flames. For bluff-body flames, the flame propagation behavior following the spark ignition depends mainly on the location of the spark in respect to the recirculation zones. Furthermore, the flame inside the side recirculation zone (SRZ) quenches soon after the whole flame lights-up. Failed ignition has been viewed close to the extinction flow conditions of the flame. Igniting the flame away from the extinction conditions results in 100% ignition probability regardless of the ignition location. However, close to extinction, ignition probability decreases gradually and achieving ignition is not possible at all outside the central recirculation zone (CRZ). The obtained ignition probability contours of both flows have been compared with the previously-studied ignition probability contours of turbulent non-premixed flames of the same burners.