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.