Flux-based measurement of local scalar source terms in turbulent reacting flows: Theory and application to soot formation in an aero-engine model combustor

Abstract

Local rates of formation and consumption of scalar quantities in turbulent reacting flows are an important information for physical understanding and model validation, but have generally been difficult to measure. The present work describes a novel approach for spatially resolved measurements of scalar source terms in a 2D domain. The method relies on the analysis of scalar fluxes obtained from simultaneous planar measurements of flow velocity and scalar concentration. A theoretical derivation based on the classical convection-diffusion equation shows that mean local scalar sources and sinks can be estimated from the divergence of the scalar fluxes if three conditions regarding stationarity, rate of molecular transport and gradients in out-of-plane direction are satisfied. The method is then applied to estimate for the first time the rates of soot formation and oxidation in a turbulent swirl-stabilized flame in a pressurized aero-engine model combustor. The estimation is based on simultaneous measurements of velocity field and soot distribution using high-speed stereo-PIV, and it is shown that the required conditions are valid here. The results reveal a large zone with elevated rates of soot formation in the inner recirculation zone (IRZ) of the flame, from where soot is transported towards the flame base and inner shear layers where it is subsequently oxidized. Additional insights are obtained from a decomposition of source terms and fluxes into their mean and turbulent parts, and estimations conditioned on the gas composition in the IRZ. The newly determined source terms and fluxes provide an additional, essential information about the mechanisms of soot production in the flame that can also be valuable for model validation. The results thus demonstrate the potential of this approach, which can in principle be applied to a wide class of reacting flows and scalar quantities.