- Title
- Numerical investigation of particle dynamics in the presence of unconfined and confined gas bubbles
- Creator
- Ge, Linhan
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The thesis aims to improve the fundamental understanding of particle dynamics in the flotation process and gas-liquid-solid (GLS) Taylor flow microreactors. In both systems, bubbles play an essential role in determining the overall performance by influencing the dynamics of other phases. In the flotation process, small and unconfined bubbles are used as carriers to capture hydrophobic particles while large and confined Taylor bubbles are introduced to microreactors to suspend and transport particles. These two processes involve complex phase dynamics and interactions with a wide spectrum of spatial and temporal scales. The difficulties in experimentally investigating the two systems lead to a demand for the establishment of efficient and accurate computational frameworks for modelling and design of these systems. In this study, Computational Fluid Dynamics and Discrete Element Method were coupled (CFD-DEM) to analyse the particle-bubble interactions in the flotation process. Specifically, CFD was used to resolve the fluid flow surrounding a rigid spherical bubble, while DEM is used to treat the particle phase. In contrast, the Volume of Fluid, which can account for bubble deformation, is coupled to DEM (VOF-DEM) to simulate a GLS Taylor flow microreactor. Although CFD/VOF-DEM coupling has been widely applied for the modelling of multiphase flow systems, it is still a challenge to select a single length scale to resolve the continuous phase and the dispersed phase simultaneously in a coupling model. Conflicts arise when there is a need to resolve fluid features at different length scales such as for systems in the presence of bubbles and particles with large size differences. In this work, a special coupling scheme was proposed to resolve the conflicts to realise the so-called multi-scale modelling. The CFD-DEM model proposed was firstly applied to simulate the interaction and approach of a particle swarm to a stationary bubble for various solid fractions (0.01 ≤ εp ≤ 0.25) and bubble Reynolds numbers (50 ≤ Re>sub>b ≤ 200). It was observed that the collision efficiency decreased with increasing solid fraction until achieving a plateau value. This plateau was attributed to the increase in the lateral expansion of the swarm and the increase in particle velocities. The former effect decreased the number of collisions by interception, while the latter increased the inertial effects. However, when the bubble Reynolds number increased, the particle swarm did not have enough time to accelerate or deform before reaching the bubble, and thus, the collision efficiency became insensitive to the solid fraction. Following the study of the particle-bubble collision, the particle dynamics in the bubble wake for Reb = 100, 160, 220 and 280 were investigated. The results showed that as the bubble Reynolds Number increased, the discharge rate, i.e. the rate of particles escaped from the bubble wake, increased. The maximum number of particles that can recirculate in the bubble wake, namely the bubble carrying capacity, was affected by both particle density and particle size. The mechanism of particle ejection by the wake was explained by analysing particle trajectory and dispersion pattern. A preferential concentration behaviour was discovered for high inertia particles and was used to understand the varying carrying capacities and discharge rates under different flow conditions. Finally, a further force analysis successfully explained the mechanism of preferential concentration and particle dispersion pattern in the bubble wake. For the confined bubble system, the emphasis was on particle dispersion patterns in the liquid slugs of a vertical GLS Taylor flow microreactor. VOF-DEM simulations were conducted to simulate a cylindrical channel under three operating conditions, as reported in the literature (Liedtke, 2014). The coupling scheme of VOF-DEM guaranteed the full resolution of the fluid field with a mesh finer than the particle size. Therefore, the simulated bubble shapes were consistent with the description in the literature. The liquid film thickness and the bubble velocity computed by VOF-DEM were in good agreement with the results calculated by reported empirical and analytical models. It was found that both the axial and radial liquid velocity profile in the liquid slugs were closely related to the two-phase velocity and the length of the slugs. The simulated particle dispersion pattern qualitatively agreed with reported experimental observations. The particle flow patterns were found to transit from relatively homogeneous distribution to preferential concentration in both the axial and radial directions, which are hard to observe from the experiment. The simulation results also showed that the particle trajectory could form a steady zig-zag path and be highly controllable by tuning the operating conditions. In summary, the developed numerical models are demonstrated to be a powerful and reliable tool to investigate particle dynamics in complex GLS three-phase flow systems. The CFD-DEM simulations revealed that the particle-fluid interactions are essential to account for the influence of particle swarm dynamics on the particle-bubble collision efficiency. In addition, it was shown that the bubble wake entrainment phenomenon is a complex function of bubble Reynolds number and particle properties. In the GLS Taylor flow microreactors, though the particle dynamics is severely interfered by the bubble behaviours/properties, organised flow patterns do appear under a given operating condition. This is a desirable feature for the design of microreactors, which can also substantially benefit from the abundant information available from VOF-DEM simulations.
- Subject
- particle; bubble; flotation; Taylor flow; microreactor; CFD-DEM; VOF-DEM
- Identifier
- http://hdl.handle.net/1959.13/1411257
- Identifier
- uon:36319
- Rights
- Copyright 2019 Linhan Ge
- Language
- eng
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