Year: 2016
Type: dissertation
Abstract: Despite the extensive application of bubble columns and rigorous research efforts dedicated to understand their intricate behavior, detailed knowledge on the multiphase fluid flow and mass transfer as well as their interactions is currently very limited. Gas-liquid flow in bubble columns is exemplified by a blend of inherently unsteady complex processes with broadly varying spatial and temporal scales. The complex gas-liquid interactions involving liquid mechanics, chemical heat and mass transfer pose many demanding modeling problems to be solved. In this thesis, numerical investigations are carried out to study the gas-liquid flow in bubble columns. Efforts have been concentrated on the assessment of the suitable interfacial and turbulence closures. All the numerical simulations are performed in 3D transient Euler-Euler environment with the commercial CFD tool Ansys CFX-14.0 and all simulation results are compared with the already available experimental data. A key problem in bubble column simulations is to capture the physics involved. The interphase forces (e.g. drag, turbulent dispersion, wall lubrication, virtual mass and lift forces) and turbulence in the bubble columns can be affected due to interaction among gas and liquid phase. This work shows the comparison between different interfacial closure models reported in the literature. Moreover, the combined effects of the lift, virtual mass and other non-drag forces namely wall lubrication as well as the turbulent dispersion have been examined. Primarily, drag and lift forces are found to be dominating forces in action between two phases. The virtual mass effects on the bubble column flows are found to be insignificant. The addition of wall lubrication force mainly results in restriction of bubbles in the centre area of the column. Addition of turbulent dispersion force does not have any noticeable effect on average flow field; however it enhanced the spreading of bubble plume. A detailed analysis and study of different turbulence models namely Explicit Algebraic Reynolds Stress Model (EARSM), Re-Normalization Group (RNG) and Explicit Algebraic Reynolds Stress-Baseline (EARSM-BSL) is performed to evaluate the shear induced turbulence effects in the continuous phase. Impact of bubble-induced turbulence model of Sato and Sekoguchi is assessed as well. Furthermore, Bubble induced turbulence model of Morel, implemented in CFX and coupled with RNG model, is compared and evaluated against the Sato and Sekoguchi model. When EARSM turbulence model is employed, it is found that the EARSM is more successful in capturing the turbulence flow structures in continuous phase in comparison to RNG model, however, it failed to predict the average flow field. On contrary, RNG model has successfully predicted the averaged flow field, whereas the turbulent kinetic energy profiles at some locations have been slightly underestimated by the RNG model. The bubble induced turbulence model of Morel coupled with RNG model in comparison to Sato and Sekoguchi model, underpredicted the turbulent kinetic energy. In predicting the average flow field, both models showed comparable accuracy. Morel model in comparison to Sato and Sekoguchi model is found to be capable of capturing more dynamic bubbly flow. By combining BSL model with EARSM model (EARSM-BSL) not only improved the flow field predictions but also improved the turbulence flow structure prediction capabilities, especially near the sparger. With the addition of bubble induced turbulence, overall predictive performance of the EARSM-BSL model improved slightly, thus suggesting that bubble induced turbulence is an important factor and should be included in the investigations.
Author: R.M.A. Masood
Language: English
Topic: Fluid Dynamics and Mixing
Subfield: Fluid Flow and Transfer Processes
Field: Chemical Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3161094480