Flame- and flow-conditioned vorticity transport in premixed swirl combustion

Abstract

This paper presents an experimental analysis of flame-induced enstrophy transport in premixed swirl combustion at Karlovitz numbers between 20 and 50. Such flames possess a large-scale pressure field that – in addition to the pressure fields associated with small-scale turbulent vortices – can interact with density gradients to produce baroclinic torque. Simultaneous tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain high-resolution velocity and progress-variable fields. This allows statistical evaluation of the various terms in the enstrophy transport equation. The impact of small- and large-scale pressure gradients is assessed by conditioning the baroclinic torque on the position of the fluid within the instantaneous flame front, within the flame brush, and axially within the combustor. At all conditions studied, the baroclinic torque was a significant contributor to enstrophy transport, with a comparable magnitude to vortex stretching and viscous diffusion. Enstrophy attenuation and production by baroclinic torque tended to occur towards the reactant and product sides of the instantaneous flame surface, respectively. However, the value of the baroclinic torque also depended equally strongly on the position in the combustor. Hence, both small- and large-scale pressure fields can result in significant enstrophy changes through baroclinic torque. A scaling is proposed to quantify the significance of swirl-induced pressure field on baroclinic torque enstrophy production; it is shown to increase with increasing swirl number and decreasing Damköhler numbers. This is evidence both that flame-induced vorticity dynamics are significant in swirl combustion, and that large-scale geometry-dependent flow fields can impact flame-generated turbulence.