Effects of Flow Fluctuation and Variation of Global Flame Structure on Extinction of Counterflow CH4-N2/O2-N2 Nonpremixed Flames

Abstract

Extinction limits of counterflow CH4-N2/O2-N2 non-premixed flames were investigated experimentally by manipulating the global flame structure, that is flame location in counterflow, by varying stoichiometric mixture fraction : Zst, and under fixed stoichiometric adiabatic flame temperature by keeping molar stoichiometry : Mst constant. Furthermore, the effects of two fluid dynamical fluctuations that were oscillated flow at frequency : 20 Hz and perforated plate turbulent flow on the extinction characteristics of the counterflow flames were also examined. As a result, the extinction limits of the laminar flame increase with increase in Zst. Amount of oxygen in oxidizer flow increases with Zst and oxygen promotes chain-branching reaction : O2+H→OH+O. Therefore the laminar flames stability becomes higher with increase in Zst under the constant adiabatic flame temperature. For the oscillatory flow, the extinction character indicates the similar tendency to the laminar flame, that is, the oscillated flame shows quasi-steady extinction phenomena and the extinction limits always depend on the flow condition of oxidizer even if the flame is located in the fuel side. On the other hands, the perforated plate turbulent flame stability always depends on both the flame location and the streamside with turbulence. The extinction limits of the counterflow turbulent flame are more influenced by the turbulent fluctuation of the flow in which the flame is located than that of the flow without the flame. It is thought to be that the difference for the extinction character between the oscillatory flame and the turbulent flame is due to unsteadiness of the fluctuation in the flow.