  This paper presents an experimental analysis of flame-induced enstrophy
transport in premixed swirl combustion at Karlovitz numbers between 20-50. Such
flames posses 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. 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.
