FLOW VISUALIZATION OF INTERACTIONS BETWEEN PARTICLE WAKES AND JET VORTICES
The interactions between vortices of a round free shear jet and particle’s wakes are visualized by means of smoke-gas flow visualization technology. Particles in different diameters are fixed at different positions along the jet axial central line. The typical flow structural compositions of the gas jet at the different flow developing stages are presented. Particle wakes occur obviously for the larger particle when the particle Reynolds number is larger than 100, and they change the overall flow structural pattern. More ejections in the vortex rings are accused in the radial direction at the initial flow status. The large-scale structures are broken up because of the presence of the wakes behind large particles. The local turbulence anisotropy is then attenuated in the downstream. Small particles without wakes do not change the flow structure pattern. The results are valuable to develop accurate turbulence modulation models including wake effects.
Measurements of fluid/particle correlated motion in the far field of an axisymmetric jet
Abstract Typical features of fluid-particle interaction in the far field of an axisymmetric polydispersed particle laden tube jet were measured and analysed in the present study. Measurements up to 45 jet diameters were obtained by using a phase Doppler anemometer. The statistical properties of four particle size classes were obtained in order to cover a wide range of particle relaxation times. The downstream evolution of the mean longitudinal particle velocity field aandof the particle radial and longitudinal turbulent components is first displayed. We show that a Stokes number based on the time scale of the large “structural” eddies is a relevant parameter to describe the selective influence of the jet flow on the particle mean velocity field. The anisotropy between axial and radial fluctuating velocity of the particles is found to increase with increasing particle relaxation time scale. A method is proposed and validated to determine the statistics of the velocity of the fluid seen by the particles. Measurements are made in the jet far field at x D = 30 . Ejection of solid particles in regions of high outward velocity is detected statistically. The fluid-particle correlations are presented and used to analyse the evolution of particle kinetic stresses. The experimental results confirm that the radial particle turbulent velocity is mainly controlled by the dragging by the fluid turbulence and decreases with increasing particle relaxation time compared to the fluid turbulence integral time scale. In contrast, production by the mean particle velocity gradient and transport terms are important mechanisms affecting the streamwise particle turbulence velocity. These mechanisms are responsible for the increase in the streamwise particle turbulence velocity with increasing particle relaxation time.
High-resolution data processing for phase-Doppler measurements in a complex two-phase flow
Measurements of the particle size and velocity in a gas-solid two-phase flow using the phase-Doppler technique based on transient recorder data processing and the cross-spectral density (CSD) function are improved in accuracy by introducing new complex interpolation functions to the discrete frequency lines. The improvements of the accuracy for size and velocity measurements are additionally based on the use of an external electronic unit, which is allowed to trigger the transient recorder near the peak of the envelope of the band-pass filtered bursts. This procedure could improve the signal-to-noise ratio of the processed part of the signal and hence the accuracy and data rate. A simultaneous measurement of the gas velocity is achieved by seeding the flow with small spherical glass beads which have the same refractive index as the dispersed phase particles. The identification of signals from these seeding particles-with a diameter less than 4 mu m-is based on the phase between the two signals.