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

摘要

Understanding the cross-scale kinetic energy transport in turbulent flames is critical for physicallyaccuratemodeling. This paper presents an experimental analysis of cross-scale kinetic energy transportin 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 fluorescencemeasurements are used to obtain 3D velocity fields, as well as estimates for the progressvariable and density fields. Filtering these fields at scales in the range of the laminar flame thickness(0.9 <△/δ_L~0 < 2.2) allows the kinetic energy transfer across the filter scale to be quantified. Mean kineticenergy transfer from sub-filter scales to larger scales ( i.e. back-scatter) occurs internal to the flamestructure across the range of conditions studied, with the maximum back-scatter occurring towards thecenter of the flame. Compared to equivalent non-reacting cases, considerably more back-scatter occurredin the reacting flows across all cases studied. At lower Karlovitz number, the magnitude of the backscattermonotonically increased with increasing filter scale. At higher Karlovitz number, the back-scattermagnitude saturated at filter scales around △/δ_L~0 ≈ 1.8. A scaling is proposed that quantifies the relativesignificance of the swirl-induced pressure gradient relative to the flame-induced pressure gradient on thepressure-work source of kinetic energy, indicating that swirling flow can significantly affect the kineticenergy dynamics. Overall, the results demonstrate that combustion significantly impacts the cross-scalekinetic energy transfer at scales in the range of the laminar flame thickness for conditions and configurationsof practical relevance. These conclusions have implications for LES modelling strategies, providingevidence that models should allow for up-scale energy transfer in the vicinity of the flame.