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.
Measurement of phase interaction in dispersed gas/particle two-phase flow
Abstract For simultaneous measurement of size and velocity distributions of continuous and dispersed phases in a two-phase flow a method is proposed and applied to increase the sensitivity of a phase-Doppler-anemometry (PDA) system. Design considerations are presented to increase the detectable size range of the PDA system to approximately 1:200 (in diameter) for simultaneous detection of tracers and particles. For this the optical properties for light scattering of the particles are properly adjusted to the measurement problem by using homogeneously coloured particles and a special signal processing procedure, which is developed to guarantee reliable signal processing of the tracer signals even with poor signal-to-noise ratios (SNR). The application of the experimental configuration is described by simultaneous measurements of gas and particle velocity and velocity fluctuation profiles in a two-phase jet arrangement with a particle diameter range from 1 (tracers) up to 160 μm (particles). In this two-phase flow at high Stokes numbers ( St ≫1) different turbulence structure modification effects are identified. The height of influence of these effects depends on the local position in the jet. Near the nozzle exit high gas velocity gradients exist and therefore high turbulence production in the shear layer of the jet is observed. Here the turbulence structure in the jet mainly depends on lateral turbulence transport. Due to a changed turbulence structure with reduced intensity of large eddies, this lateral transport in the two-phase jet is decreased in comparison to the single-phase jet in this area. In the area at greater nozzle distances where the jet is nearly developed, the velocity gradient in the shear layer is lower and due to this lateral turbulence transport effects become less important. Here axial turbulence transport along the jet dominates and turbulence intensity reduction is higher.
Comportement de particules solides polydispersées dans un jet d'air turbulent
Cette etude a ete realisee en vue d'acquerir une meilleure comprehension des phenomenes d'interaction lors du processus d'injection de carburant entre une phase dispersee liquide et une phase continue gazeuse. Le but est d'ameliorer la qualite du melange air-carburant afin de reduire les emissions de polluants. Nous avons realise une experimentation modele generant un jet de tube turbulent charge en particules solides polydispersees. Les fortes evolutions du temps caracteristique de l'ecoulement ainsi que du temps de relaxation des particules nous permettent d'envisager un grand nombre d'interactions particules-ecoulement. Des mesures simultanees de vitesse des phases continue et dispersee ont ete realisees avec une technique laser a phase doppler. Parallelement des mesures de flux de particules nous ont permis de caracteriser la dispersion des particules dans le jet. Une analyse dimensionnelle par nombre de stokes permet bien de caracteriser l'intensite des interactions longitudinales entre les particules et l'ecoulement. Nous avons pu observer une forte anisotropie entre les vitesses fluctuantes longitudinale et transversale. L'analyse des mesures transversales a permis de mettre en evidence le mouvement radial sortant des particules plus marque que celui du fluide traduisant un processus d'ejection des particules du jet. Dans une derniere partie, nous avons pu, par reconstruction du signal de vitesse de la phase continue, calculer la vitesse du fluide vue par les particules. Cette information, inaccessible avec les modeles numeriques eulerien-eulerien des ecoulements diphasiques, nous a permis de mesurer la vitesse de glissement instantanee entre les particules et le fluide. De plus, ces mesures ont mis en evidence l'existence de zones de plus forte concentration des particules au sein du jet. Cette etude nous a permis, au travers de mesures realisees dans un ecoulement modele, d'acquerir une meilleure comprehension des interactions profondes entre une phase dispersee solide et une phase continue gazeuse