Near-field turbulence structure of coaxial particle-laden jets.

摘要

The effects of solid particles on the flow structure in the near field region of a coaxial water jet, 0 ≤ x/d_i ≤ 6, having inner to outer diameter ratio d_i/ d_o = 0.39 are investigated experimentally for four annular to central jet velocity ratios in the range 0.11 ≤ U_o/ U_i ≤ 1.15. Glass beads of 0.24 mm are used at three volume loadings of 0.03%, 0.06%, and 0.09% in the central jet with a Reynolds number of 4.1 × 104. Measurements are acquired nonintrusively using Molecular Tagging Velocimetry (MTV) at downstream distances up to 6 inner jet diameters. High spatial resolution of MTV measurements afforded detail examination of the axial velocity radial gradient fluctuation (∂u/∂r)′. It is found that the addition of particles does not affect the mean fluid velocity profile in the region explored. The results also indicate a small enhancement of axial turbulent velocity and radial gradients of velocity fluctuations due to the presence of particles. The degree of enhancement is proportional to the particle volume flow ratio. The results of the particle dispersion measurements indicate that, in the range of Stokes number examined here, the spread of particles is very similar to that of a passive scalar in the near field region of coaxial jets.; High spatial resolution profiles of mean axial velocity, axial turbulent intensity, skewness, kurtosis, radial gradients of mean velocity, and velocity gradient fluctuations of single-phase flow at the four velocity ratios are presented and discussed. In the near exit region of the inner mixing layer, evidence suggests the existence of two trains of vortices shed from the inner jet wall for velocity ratios greater than 0.18. The results also indicate that for the flow configuration examined the length of the annular potential core is a function of velocity ratio. Turbulence characteristics of the flow appear to be influenced by both the velocity ratio and absolute velocity of the annular jet. Integral length and Taylor microscales in the inner shear layer are shown to grow monotonically with downstream distance with their ratio reaching a constant value.; Applications of spline-based data reduction techniques for molecular tagging velocimetry (MTV) are explored for the case in which two cameras are employed. Two camera, MTV systems are required in cases where the initial tagged region may exhibit variations from instant to instant (e.g., particle-laden flows). To accommodate two cameras, an affine warp mapping scheme that correlates the two camera views is introduced, and the associated uncertainties were quantified. Spline based techniques are presented for both multiple line and grid based images. Comparison with previous multiple line methods indicates that even with the additional uncertainties associated with two cameras, the present spline technique improves subpixel resolution by about a factor of three.