HCHO PLIF Investigation of the Flame Shape in an Unsteady Swirling Jet Flow

This paper describes an experimental study of the spatial structure of the chemical reaction zone in turbulent swirling flames by planar laser-induced fluorescence of formaldehyde (HCHO). Combustion of the methane–air mixture at atmospheric pressure is considered for different values of the equivalence ratio φ: inverted cone flames for φ = 0.7 and 1.4 and lifted flames for φ = 2.5. Apart from small-scale deformations, the change in the chemical reaction zone shape is associated with two types of large-scale coherent structures, namely, an almost axisymmetric deformation mode, which appears to be due to the buoyancy effect on the combustion products, and rotation of an asymmetric mode due to the precession of the swirling flow.