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Results are reported for an experimental study directed at investigating the fine scale structure of turbulent jet flows using the simultaneous imaging of the concentration and velocity fields. The measurements are obtained in an axisymmetric coflowing jet using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) imaging of acetone vapor seeded into the jet gas. The measurements resolve the Kolmogorov scale, η, of the flow. These measurements are used to investigate the relationships among the vorticity, strain rate, kinetic energy dissipation, and scalar dissipation fields. The data are also used to analyze the physical size of the structures in the scalar and kinetic energy dissipation fields. The results show good correlation between regions of high compressive principal strain, scalar dissipation, and kinetic energy dissipation. Structures in the scalar dissipation field are seen to be sheet-like with thicknesses ranging between 1η and 6η, while those in the kinetic energy dissipation field are more topologically complex and range in scale from 1η to 10η. The mean sizes of scalar and kinetic energy dissipative structures are 3η and 4η, respectively. The constant Λ relating mean scalar dissipative structure size to outer scale variables () is measured to be 7.8. These results indicate that scalar dissipative structures are approximately a factor of two smaller than has been suggested in previous studies.

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Gas-phase production of carbon nanotubes in presence of a metal catalyst with a continuous wave CO 2 laser is investigated by combining coherent anti-Stokes Raman scattering ͑CARS͒, laser-induced fluorescence ͑LIF͒, and laser-induced incandescence ͑LII͒. These in situ techniques provide a unique investigation of the different transformation processes of the primarily carbon and metal vapors issued from the vaporization of the target by the laser and the temperature at which these processes occur. Continuous-wave laser provides with stable continuous vaporization conditions very well suited for such in situ investigations. Temperature profiles inside the reactor are known from CARS measurements and flow calculations. Carbon soot, density, and size of carbon aggregates are determined by LII measurements. LIF measurements are used to study the gas phases, namely, C 2 and C 3 radicals which are the very first steps of carbon recombination, and metal catalysts gas phase. Spectral investigations allow us to discriminate the signal from each species by selecting the correct pair of excitation/detection wavelengths. Spatial distributions of the different species are measured as a function of target composition and temperature. The comparison of LIF and LII signals allow us to correlate the spatial evolution of gas and soot in the scope of the different steps of the nanotube growth already proposed in the literature and to identify the impact of the chemical nature of the catalyst on carbon condensation and nanotube nucleation. Our study presents the first direct evidence of the nanotube onset and that the nucleation proceeds from a dissolution-segregation process from metal particles as assumed in the well-known vapor-liquid-solid model. Comparison of different catalysts reveals that this process is strongly favored when Ni is present.