The chemical structure of triple flames in laminar blue whirls

Abstract

The blue whirl is a conical, near-limit flame with a bright, blue ring, occurring at low heat-release rates that can stabilize over a pool of liquid fuel. This unique structure forms following suppression of soot formation in a laminar fire whirl under the influence of vortex breakdown. Recent literature on the blue whirl has hypothesized and predicted the existence of a triple flame at the blue ring. In this work, we explore the distribution of various chemical species in the flame to quantitatively establish a map of equivalence ratio around the flame. Using high-speed chemiluminescence and planar laser-induced fluorescence, the distribution of OH, PAH and CH radicals were measured. The OH*/CH* ratio was used to estimate the local equivalence ratio. Results show that the blue ring is mostly stoichiometric, but there is a small rich region below it, towards the fuel layer, and a lean region above it, towards the wake of the vortex breakdown bubble. This structure provides experimental evidence that a triple flame exists in the blue whirl. The time scales of flow within the recirculation zone are estimated using soot traces that are observed occasionally, and then used to estimate a range of Damköhler numbers that can lead to stable blue whirl formation, providing an important scaling factor to design clean, practical burners using the blue whirl regime.