Two-dimensional direct measurement of the turbulent flux in turbulent premixed swirl flames

An experimental investigation of the simultaneous determination of the two-dimensional flow and reaction progress field in a set of turbulent premixed swirl flames with varied stoichiometry is presented. With the conditioned particle image velocimetry (CPIV) technique, instantaneous planar velocity fields are evaluated with the simultaneously obtained instantaneous reaction progress variable fields, which allows the direct determination and statistical evaluation of density weighted turbulent flux terms in the two-dimensional domain. Consistent with former studies, it is confirmed that turbulent fluxes can be oriented against or with the gradient of the reaction progress variable. This behaviour is dependent on the spatial direction under investigation, the heat-release and the turbulence conditions. Moreover, a transition from gradient to counter-gradient turbulent diffusion along the isolines of the reaction progress variable for a fixed stoichiometry and turbulence level has been observed. This phenomenon is discussed with existing closure models for the turbulent flux term. However, none of the existing models is found to be capable to describe the experimental findings, especially, if also non-normal components of the turbulent flux are regarded.