  This paper experimentally analyzes the simultaneous influence of heat release
and coherent vortex structures on the transfer of kinetic energy across scales
around the laminar flame thickness $\delta^0_\mathrm{L}$ in a turbulent
premixed swirl flame and a non-reacting swirl flow. High-resolution tomographic
particle image velocimetry and formaldehyde planar laser induced fluorescence
measurements are used to obtain 3D velocity fields and estimates of the
progress variable and density fields. The kinetic energy transfer across a
filter scale of $\Delta = 1.5 \delta^0_\mathrm{L}$ was then quantified using
physical space analysis. Coherent flow structures were identified using the
swirling strength and proper orthogonal decomposition was used to identify the
dominant periodic flow structure. While non-reacting regions of the flow show
mean down-scale energy transfer (forward-scatter), mean back-scatter is
observed internal to the flame. Importantly, the back-scatter magnitude in the
flame increases in regions undergoing flame/vortex interaction. That is, the
mean back-scatter magnitude at locations simultaneously inside the flame and a
large-scale coherent vortex is higher than regions in the flame and not in a
vortex, with the mean back-scatter magnitude increasing with the swirling
strength. This increased back-scatter may be due to locally higher heat release
rates in locations of flame/vortex interaction. Overall, the results
demonstrate the importance of kinetic energy back-scatter at scales around the
flame thickness and the complicated relationship between back-scatter and local
flame/flow structure; these results should be considered when modeling
turbulence in flames.
