Physical space analysis of cross-scale turbulent kinetic energy transfer in premixed swirl flames

Abstract

Understanding the cross-scale kinetic energy transport in turbulent flames is critical for physically-accurate modeling. This paper presents an experimental analysis of cross-scale kinetic energy transport in turbulent premixed swirl flames at Karlovitz numbers between 10 and 20 via physical space filtering. High-resolution tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain 3D velocity fields, as well as estimates for the progress variable and density fields. Filtering these fields at scales in the range of the laminar flame thickness (0.9<Δ/δL0<2.2) allows the kinetic energy transfer across the filter scale to be quantified. Mean kinetic energy transfer from sub-filter scales to larger scales (i.e. back-scatter) occurs internal to the flame structure across the range of conditions studied, with the maximum back-scatter occurring towards the center of the flame. Compared to equivalent non-reacting cases, considerably more back-scatter occurred in the reacting flows across all cases studied. At lower Karlovitz number, the magnitude of the back-scatter monotonically increased with increasing filter scale. At higher Karlovitz number, the back-scatter magnitude saturated at filter scales around Δ/δL0≈1.8. A scaling is proposed that quantifies the relative significance of the swirl-induced pressure gradient relative to the flame-induced pressure gradient on the pressure-work source of kinetic energy, indicating that swirling flow can significantly affect the kinetic energy dynamics. Overall, the results demonstrate that combustion significantly impacts the cross-scale kinetic energy transfer at scales in the range of the laminar flame thickness for conditions and configurations of practical relevance. These conclusions have implications for LES modelling strategies, providing evidence that models should allow for up-scale energy transfer in the vicinity of the flame.