Behavior of Low Strain Rate Flame Disks in Counterflow Diffusion Flame

Abstract

A study was conducted to clarify flame characteristics through the evaluation of critical mole fractions at flame extinction and edge-flame oscillation of low strain rate flames using the global strain rate, velocity ratio, and burner distance as experimental variables. The transition from a shrinking flame disk to a flame hole was verified through gradient measurements of the maximum flame temperature. Evidence of edge-flame oscillation in flame disks was also found using numerical simulations in zero and normal gravity. The main mechanisms of flame extinction and edge-flame oscillation were analyzed by comparing the energy fractions in the energy equation. For low strain rate flame disks, radial conduction heat loss rather than flame radiation was a significant contributor to flame extinction and even edge-flame oscillation. This was experimentally demonstrated by evaluating the critical mole fraction at flame extinction and edge-flame oscillation, as well as measurements of the flame temperature gradient along the flame disk surface. These results suggest that low strain rate flame responses are determined not only by one-dimensional flame responses, but also by multi-dimensional flame responses such as radial conduction heat loss. The results also show that extinction of low strain rate flames is more probably due to multi-dimensional heat losses than to radiative extinction.