Direct Optical Measurement of Fluid Dynamics and Dispersed Phase Morphology in Multiphase Flows

Abstract

Multiphase flows are present in numerous industrial processes. Knowledge of multiphase flow phenomena makes it possible to control and optimize the process parameters. Process optimization saves energy costs, improves product quality, increases productivity and allows the treatment of new, non-optimal raw materials. However, it is difficult to obtain knowledge on the turbulent multiphase flows that take place in industrial processes. The flow phenomena cannot be predicted by numerical simulations, and experimental investigations have several physical limitations. Comprehensive experimental studies can be carried out using a laboratory-scale process and the results can be exploited in the development of a numerical model that hopefully may provide satisfactory estimates of the flow phenomena in the real, industrial-scale process. To fulfil the eventual goal of process optimization, the appropriate numerical model must be validated with reliable and detailed experimental data. The aim of this study is to develop direct optical measurement methods that provide reliable and accurate experimental data on multiphase flow systems and to extend measurements to denser and more complex multiphase flows.This thesis consists of eight papers that focus on various aspects of measuring the fluid dynamics and dispersed phase morphology in multiphase flows. It concerns the development of direct optical measurement techniques for studying the dynamics of turbulent multiphase flows that are of fundamental importance in industrial processes. It describes direct optical measurement methods for simultaneously measuring the size, shape and velocity of dispersed phase particles, and the fluid flow field surrounding the particles in dispersed multiphase flows, namely in bubbly flows, in sprays, in the flow of solid particles and in the three-phase flow of micro-bubbles and flocculate particles in a liquid. The bubble-particle-fluid interactions are studied and the distributions and concentrations of all phases are measured. Several methods for image acquisition, signalprocessing and data analysis are developed. The overlapping object recognition (OOR) methods presented in Papers I and VIII allow a unique, reliable analysis of experimental images of overlapping irregularly-shaped dispersed particles. The proposed methods are studied extensively and applied to experimental cases. Five processes are included in the study: 1. the dissolution process in the gas-liquid reactor with a Rushton turbine, 2. mixing processes in the mixing tank and in a centrifugal pump, 3. the flocculation process of organic contaminants, 4. the sedimentation process of flocs and 5. the dissolved air flotation process of flocs. The processes are successfully studied on a laboratory scale. The process parameters are automatically monitored and detailed data for process optimization is provided. In the future, the developed methods will be applied in the in-line measurement of industrial-scale processes.