Quantitative imaging of multi-species fluid mixing in turbulent flows

The effects of disparate molecular transport properties in multi-species fluid mixing can be important in reacting flows. In simulations of turbulent combustion, details of the molecular diffusion process are often neglected under the assumption that strong turbulent 'stirring' renders any effects of varying molecular transport properties insignificant. However, there is evidence that simulations that neglect individual species diffusivities may have difficulty in reproducing observed flame behavior.;This work examines experimentally the effects of varying molecular transport properties, and in particular varying species diffusivities, on multi-scalar turbulent mixing. Planar imaging is used to measure various two-dimensional scalar fields in non-reacting axisymmetric turbulent jets. First, planar Rayleigh scattering is applied in a propane-helium jet issuing into a coflow of air. This is used to measure a variable, called xi, that quantifies the degree of differential mixing between the jet gases. Using a method proposed by Bilger & Dibble [Combust. Sci. Technol. 28, 161 (1982)], the differential diffusion variable xi is measured in low and moderate Reynolds number jets. For the lower Re, significant differential diffusion develops in the pre-transitional portion of the flow. Downstream of the turbulent transition, radial profiles of mean xi take on a characteristic form independent of Reynolds number, with an excess of the less-diffusive propane on the jet boundary. Evolution of the xi fields in the turbulent part of the flow is surprisingly consistent with the mixing of conventional scalar quantities. For example, power spectra of xi are monotonically decreasing, with a distinct k-5/3 inertial range. This spectral form is at odds with prior analytical and computational (DNS) results in isotropic turbulence, which predicted that the spectrum would show a peak intermediate between the diffusive cutoffs of the individual scalars. The results indicate that consideration of differential diffusion must account for the details of the flow configuration, particularly the uniformity of turbulence levels. This has important implications for reacting flows, where local laminarization by heat release can be significant.;In the second part of this work, the two-dimensional mole fraction fields of all species are measured in a turbulent acetone-helium jet issuing into air. These measurements are obtained by combining the Rayleigh scattering technique with simultaneous planar laser-induced fluorescence (PLIF) measurements of acetone mole fraction. Access to all of the relevant scalar concentration fields presents a broad picture of multi-species turbulent mixing. A thorough understanding of the mixing dynamics requires examination of differential diffusion within the context of individual scalar and scalar gradient fields. For example, the redistribution term in the transport equation for xi depends on the gradient of one of the underlying scalar quantities. Our results show a strong correlation between the redistribution term, w, and observed differential diffusion. This suggests that differential diffusion is strongest at the smallest flow scales, where w is known to be most significant. The spatial power spectra of each of the jet species, along with the power spectrum of the differential diffusion field, are also examined. The spectra of the differential diffusion field follows that of the less diffusive species, acetone, at high wavenumbers and dips well below the spectra of either of the jet gases at low wavenumbers. The acetone-helium cospectrum shows a sharp drop in the two-scalar correlation beginning at the scales when the constituent power spectra begin to diverge. A direct measurement of the dissipation scales also shows that the smallest flow scales are shared by both the acetone and differential diffusion fields. The dissipation thicknesses of helium are typically broader, although both jet gases may contain regions with dissipation thicknesses much larger than anything found in the differential diffusion field.