Experimental investigation of filtered density functions in turbulent jets and turbulent partially premixed flames

Conditionally filtered conserved scalar (temperature) dissipation (CFD), diffusion (CFDIF), conditionally filtered velocity (CFV), the velocity filtered joint density function (VFJDF), the velocity-scalar filtered joint density function (VSFJDF), the velocity-scalar-scalar-dissipation filtered joint density function (VSDFJDF) and the transport equations for the VFJDF, VSFJDF and VSDFJDF are studied experimentally. Measurements are made in the fully developed region of an axisymmetric turbulent jet with a jet Reynolds number Re j = UjDj/nu of 40000 using an array consisting of three X-wires and resistance-wire temperature sensors. Filtering in the cross-stream and streamwise directions is realized by using the array and by invoking Taylor's hypothesis, respectively.; Our study of subgrid-scale (SGS) mixing in nonreacting turbulent jets has revealed two limiting SGS mixing regimes, the well-mixed regime with both small SGS variance and small SGS kinetic energy and the non-equilibrium regime with large values of the SGS variance and the SGS kinetic energy. The SGS scalar and the SGS velocity distribution and structure are qualitatively different in these two SGS regimes. In well-mixed regime the FJDFs are close to joint normal. For non-equilibrium regime the VFJDF has a uniform portion and both the VSFJDF and the VSDFJDF are bimodal-shaped, consistent with a combination of diffusion layers and plane strain in the SGS scalar and velocity, which is similar to the counter-flow model for laminar flamelets. The present study demonstrates the importance of including SGS velocity in mixing models.; In this work we also investigate SGS mixing in turbulent flames. The filtered mass density function (FMDF) of mixture fraction and other filtered variables are studied using measurement data obtained in turbulent partially premixed methane/air (Sandia) flames. Conditional FMDF conditional on the Favre filtered scalar (mixture fraction) and SGS scalar variance shows similar trends to the nonreacting results. For the measurement locations considered (up to 30 jet diameters downstream) the mixture fraction value jump across a diffusion layer is generally larger than the reaction zone width in the mixture fraction space; therefore the mixing field under such conditions can support flamelets. The conditionally filtered temperature near the stoichiometric mixture fraction decreases progressively with increasing SGS scalar variance. Furthermore, local extinction events appear to occur mostly when the SGS scalar variance is large, suggesting the possibility of flamelet extinction. The results suggest that the mixing regimes and the associated mixture fraction structure could potentially have strong influences on the combustion regime and extinction/reignition in turbulent nonpremixed flames.