The cross-scale physical-space transfer of kinetic energy in turbulent premixed flames

Multi-scale interactions and kinetic-energy transfer between turbulence and flames are fundamental to understanding and modeling premixed turbulent reacting flows. In order to investigate these phenomena, direct numerical simulations of a turbulent premixed flame are analyzed in this study. The results reveal a flux of kinetic energy that involves a cross-scale transfer through the turbulence cascade and whose prevailing direction in the flame brush is from subgrid to resolved scales. The root cause of this reversal in energy transfer, termed subgrid-scale (SGS) backscatter, is the effect of thermal expansion in the subgrid scales, by which small amounts of enthalpy created by combustion heat release are transformed into small-scale kinetic energy by means of the SGS pressure-gradient velocity correlation. The resulting overload of SGS kinetic energy is transferred to the resolved scales through SGS backscatter. This cross-scale flux of energy, along with a larger one that relies on large-scale quantities only and does not involve the energy cascade, describes the transformation of combustion heat release into kinetic energy in the turbulent premixed flame. Based on scaling analyses, it is theorized that the contribution of the cross-scale flux to the total kinetic-energy augmentation vanishes in combustion regimes in which the flame-transit time is too short to allow for activation of the non-linear convective mechanisms of the energy cascade. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.