Ultra-lean blow-off dynamics of a holder-stabilized premixed flame in a preheated mesoscale combustor near laminar critical condition

Abstract

The present work experimentally illustrates and analyzes the ultra-lean flame blow-off dynamics near laminar critical condition under the synergistic effect of heat and flow recirculation which exists in many practical combustors (such as aero-engine). The results indicate that the flame blow-off process can be divided into three stages at Reynolds number Re = 240. In the first stage, the flame initially becomes thinner over time. In the second stage, the residual flame with repetitive local extinction and re-ignition (RFRER) appears (the second stage), which is experimentally found for the first time for the methane/air premixed mixture of Lewis number Le = 1.0. The analysis indicates that the local extinction of the flame is mainly caused by the stretch effect on two sides. In the final stage, the residual flame with oscillation appears. The oscillating residual flame is finally carried convectively upstream and extinguishes within the recirculation zone as the present Lewis number effect is small. The analysis also indicates that the residual flame of hydrocarbon fuel cannot be maintained, but this issue can be solved via adding enough fast-diffusing species (such as hydrogen). This mechanism for suppressing or preventing the local extinction of the residual flame may be also suitable for the turbulent flames in many practical combustors. The present study provides the theoretical basis for predicting the flame status and suppressing the unstable flames at extreme operating conditions.