Study of the flow in a hydrocyclone using positron emission particle tracking and computational fluid dynamics simulation

Abstract

This thesis presents my work of studying the flow in a hydrocyclone using positron emission particle tracking (PEPT) and computational fluid dynamics (CFD) simulation. Hydrocyclone separators are a type of cyclone separators with liquid as the carrier phase. Applications of the hydrocyclones can be found in numerous diverse industries. The thesis starts with an introduction of cyclone and hydrocyclone separators (Chapter 1). Theoretical models are described to enunciate the physics of the separation process. The anterior experimental Lagrangian studies and numerical studies of the flow in (hydro)cyclones, both of which are related to the research approaches used in this work, are reviewed. In Chapter 2 the ‘end of the vortex’ (EOV) phenomenon, which is observed by PEPT and analyzed in this work, is introduced. Most prior researches about the EOV, including experimental and theoretical studies, targeted the gas cyclones. Here, implicit and direct experimental observations of EOV in gas cyclones and swirl tubes are reviewed, followed by more detailed analysis of the flow field at the EOV area, the influencing factors and the consequences. In Chapter 3 the positron emission particle tracking (PEPT) technique used to track the particles and quasi-fluid elements is explicated. The essentials of this technique— the camera, the tracer and labelling technique, and data processing algorithms, are expounded. Chapter 4 gives a summary of papers included in this thesis and some unpublished results. Concluding remarks and suggestions for further work on the cyclone and EOV as well as PEPT technique are given in Chapter 5. The papers are displayed in Chapter 6. Paper I reports the first set of particle trajectories obtained using PEPT with the first version of the refined data processing algorithms. The tracks and some preliminary analyses have indicated the occurrence of the EOV phenomenon in the hydrocyclone. This is the first time that the trajectories in a hydrocyclone are obtained with such high temporal and spatial resolutions. The clearly observed influence of EOV on the particles is also unprecedented. In Paper II the flow field of the EOV phenomenon in a hydrocyclone is successfully simulated for the first time using the LES (large eddy simulation) turbulence model. Paper III gives a detailed account of the development and verification of the algorithm used thus far, which is the foundation of the further developed variations of algorithm. In Paper IV PEPT was utilized to compare the tracks and flow patterns in the hydrocyclone operated under different conditions. An abnormally long residence time of the particle in the hydrocyclone under certain operational conditions was observed. An algorithm finding the cyclone axis and transforming the particle Cartesian positions to cylindrical coordinates was developed. The cylindrical positions and velocity components as a function of axial position and time show in detail the peculiar flow pattern caused by the EOV. The section where the anomalous flow field appeared and the time the particle wandered in the EOV area were specifically determined. In Paper V the 3-D particle trajectory was simulated using Lagrangian tracking in the fluid field calculated using the LES turbulence model. The characteristics of the simulated and PEPT-obtained particle tracks conform to each other very well. The superposition of simulated particle trajectories on the static pressure shows how the particle behavior is influenced by the EOV. The goal of Paper VI is to follow neutral-density particles to study the flow of the fluid elements. The flow condition was modified to render the tracer neutrally buoyant, which, however, created a difficult tracking condition where radioactivity dispersed into the fluid due to ion exchange. A new tracking strategy was developed to cope with the decreasing signalto- noise ratio. Several smoothing strategies were also employed to de-noise, and their effects and applicability were scrutinized. The tracks of neutral-density tracers reveal the pathlines of quasi-fluid element. The cylindrical velocity components derived using the aforementioned coordinate-transformation algorithm makes it possible to study the flow patterns close to the tangential inlet and in the cyclone body. The tangential velocities as a function of radius were fitted using theoretical formulae, which yielded the parameters necessary for describing the tangential velocity profile. Cylindrical velocity components as a function of time show the correlations between the different velocity components, which elucidates the separation process and principle. The unpublished results in Section 4.2 include CFD simulations of a hydrocyclone using the LES turbulence model and Lagrangian particle tracking for understanding the effect of the underflow discharge on the ‘end of the vortex’ phenomenon and the associated flow field and particle tracks. The difference between 20% and no underflow discharge is significant. The characteristics of simulated particle tracks under conditions of different underflow discharge conform to experimental particle tracks obtained using the PEPT technique. During the series of studies, the algorithm for positioning the tracer has been continuously developed, improved and verified, which means that the temporal and spatial resolutions, as well as the data processing efficiency continually improved.