DNS of turbulence backscatter in a laboratory-scale swirl stabilized aero-combustor

Classical theories of turbulence suggest that the kinetic energy cascade in nonreacting flows has a net transfer of energy from large to small scales. However, previous studies demonstrated that the thermal expansion of premixed flames can reverse the flux of kinetic energy through the injection of kinetic energy at flame scales into larger scales, resulting in backscatter. These observations were confirmed experimentally for a premixed laboratory-scale aero-combustor presented in Kazbekov and Steinberg (2021). However, it is unknown whether backscatter also occurs in partially-premixed combustion in a swirl-stabilized combustor undergoing localized extinction/reignition near the lean blowoff limit. Near this limit, edge flame propagation was found to exist, and its flame structure is comprised of a fuel rich and fuel lean premixed leading edge that generates an order of magnitude larger thermal expansion due to heat release rate compared to the trailing non-premixed flames, acting as a source of flame-generated backscatter. Direct numerical simulation (DNS) of a laboratory-scale swirl stabilized aero-combustor is performed for a reacting Jet-A liquid spray flame and backscatter is quantified from the filtered and subgrid kinetic energy transport equations evaluated in physical coordinates. It is found that edge flames lead to an inversion of the turbulent kinetic energy cascade at scales close to the flame thermal thickness. The backscatter occurs within the flame brush and correlates with dilatation generated by thermal expansion. The total pressure gradient–velocity correlation term is also observed to produce kinetic energy at subgrid scales due to subgrid fluctuations in the pressure field caused by turbulent combustion. Hence, aero-combustors undergoing blow-off are more prone to backscatter compared to stable conditions due to the development of edge flames that propagate to reignite previously extinguished regions.