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One-dimensional turbulence (ODT) simulations of a spatially developing planar shear layer are used to obtain the probability density function (PDF) of the concentration of a passive scalar. An asymmetry of the PDF with respect to transverse location in this self-similar flow appears in the ODT results and in experimental results obtained with planar laser-induced fluorescence of acetone. Using the ODT simulations, estimates are presented of the effect of measurement volume upon the resulting PDFs. Variable density effects on the concentration of the denser fluid within incompressible flow are also examined.

Quantitative passive scalar measurements were performed in an incompressible planar mixing layer at Re δ up to 104 using planar laser-induced fluorescence of acetone seeded into one side of the layer. Probability density functions compiled from sets of images showed a preferred mixture composition, favoring the high-speed fluid, which extended across the layer. This preferred composition produced non-marching PDFs and an inflection in the average mixture fraction profile. The spatial resolution of the experiment was found to be sufficient to accurately measure the fraction of mixed fluid within the layer. The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re δ ≈ 5,000, the approximate location of turbulent transition, in contrast to high Schmidt number experiments which show minimal mixing before the transition point.

The effects of injection pressure and orifice diameter on soot in diesel fuel jets were investigated under quiescent, direct-injection (DI) diesel engine conditions. These investigations were carried out in a constant-volume combustion vessel with complete optical access. The injector used was an electronically-controlled, common-rail injector. Injection pressures between 40 MPa and 190 MPa and orifice diameters between 501.1m and 180 µm were considered. Soot was measured with a line-of-sight laser extinction technique and visualized with planar laser-induced incandescence. Flame lift-off lengths used in the analysis of the soot measurements were determined with time-averaged OH chemiluminescence imaging. The results show that the peak soot in a fuel jet decreases with increasing injection pressure and decreasing orifice diameter. The decrease in soot with increasing injection pressure was linear with increasing injection velocity (i.e., the square-root of the pressure drop across the injector orifice). The decrease in soot with decreasing orifice diameter was such that for the smallest orifice diameter considered (50 μm), no soot was detectable for the ambient gas and injector conditions considered. These direct measurements of soot within a fuel jet, coupled with estimates of the amount of air entrained relative to the amount of fuel injected, confirm that air entrainment and fuel-air mixing upstream of the lift-off length have an important role in determining the soot levels within a diesel fuel jet.