LES of turbulent non-isothermal two-phase flows within a multifield approach (original) (raw)
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International Journal of Heat and Mass Transfer, 2020
The Eulerian-Eulerian Large-eddy simulations (LES) of gas-liquid two-phase flow in a cylindrical bubble column reactor have been conducted. When considering the turbulent eddy viscosity in LES, apart from the well-accepted contributions from shear turbulence and bubble induced turbulence (BIT), the effect of the interaction between entrained bubbles and eddies with a similar turbulence length scale to the sub-grid scale (SGS) cannot be neglected. With the consideration of the bubble response to the eddies on the induced sub-grid stresses, a modified SGS model, which incorporates the Stokes number, St, was proposed. The results of LES clearly indicate that the use of the modified SGS model can effectively capture the transient bubbly flow in the cylindrical bubble column. The power turbulent kinetic energy spectrum obtained in LES indicates that a slope similar to Komogorov-5/3 scaling law and the-3 scaling law can still be identified for a critical frequency f=10.70 Hz.
Towards large eddy simulation of isothermal two-phase flows: Governing equations and a priori tests
International Journal of Multiphase Flow, 2007
This article reports on the potential of application of LES in the calculation of turbulent two-phase flows, in the case where each phase is resolved and interfaces remain much larger than the mesh size. In comparison with single-phase flow, successful application of LES to resolve two-phase flow problems should account for the complex interaction between turbulence and interfaces. Non-linear transfers of turbulent energy across the interface have to be accurately modeled. The derivation of the complete filtered two-phase flow governing equations has been formulated to deal with turbulence at the interface in a comprehensive and practical way. Explicit filtering of 2D direct numerical simulations has been employed to evaluate the order of magnitude of the new subgrid contributions. A parametric study on the academic test case of two counter-rotative vortices and a more complex test case of phase inversion in a closed box have been utilized to perform an order of magnitude analysis of different transport mechanisms. Important features of turbulent energy transfer across the interface have been discussed. Analyses of the numerical results have been conducted to derive conclusions on the relative importance of the different subgrid scale contributions, and modeling issues and solutions are provided.
Large eddy simulation of a bubble column using dynamic sub-grid scale model
Chemical Engineering Journal, 2008
Euler-Euler simulations of the gas-liquid flow in a square cross-sectioned bubble column with LES (two sub-grid scale models) and the k-ε model are presented. The sub-grid scale modeling is based on the Smagorinsky kernel in both its original form and the dynamic procedure of Germano. The attempt has been made to assess the performance of these two sub-grid scale models. The Smagorinsky model with model constant C s = 0.12 performs quite well, and gives results almost identical to those given by the dynamic procedure of Germano. The SGS models are modified to account for bubble induced turbulence (Sato model) and it is observed that it does not change the results much. Predictions are also compared with the available experimental data. All the non-drag forces (turbulent dispersion force (only for RANS), virtual mass force, lift force) and drag force were incorporated in the model. An extended k-ε turbulence model has been used with extra source terms introduced to account for the interaction between the bubbles and the liquid. Though both LES models showed agreement in predictions, the Germano model still can be used to have estimates of C s value which are not known a priori. Moreover, if objective is to understand the steady and time averaged features, RANS can also perform well.
Chemical Engineering Science, 2008
In this work, we have presented a one-equation model for sub-grid scale (SGS) kinetic energy and applied it for an Euler--Euler large eddy simulation (EELES) of a bubble column reactor. The one-equation model for SGS kinetic energy shows improved predictions over the state-of-the-art dynamic procedure. With grid refinement, the amount of modelled SGS turbulent kinetic energy diminishes, as one would expect. Bubble induced turbulence (BIT) at the SGS level was modelled with two approaches. In the first approach an algebraic model was used, while in the other approach extra source terms were added in the transport equation for SGS kinetic energy. It was found that the latter approach improved the quantitative prediction of the turbulent kinetic energy. To the best of authors knowledge, this is the first use of a transport equation for SGS kinetic energy in bubbly flows.
2007
This article reports on the potential of application of LES in the calculation of turbulent two-phase flows, in the case where each phase is resolved and interfaces remain much larger than the mesh size. In comparison with single-phase flow, successful application of LES to resolve two-phase flow problems should account for the complex interaction between turbulence and interfaces. Non-linear transfers
Large eddy simulation of the Gas–Liquid flow in a square cross-sectioned bubble column
Chemical Engineering Science, 2001
In this work the use of large eddy simulations (LES) in numerical simulations of the gas-liquid flow in bubble columns is studied. The Euler-Euler approach is used to describe the equations of motion of the two-phase flow. It is found that, when the drag, lift and virtual mass forces are used, the transient behaviour that was observed in experiments can be captured. Good quantitative agreement with experimental data is obtained both for the mean velocities and the fluctuating velocities. The LES shows better agreement with the experimental data than simulations using the k-epsilon model.
CFD simulation of bubble column—An analysis of interphase forces and turbulence models
Chemical Engineering Journal, 2008
3D transient CFD simulations of bubble column have been performed for a wide range of superficial gas velocity on an industrially relevant cylindrical column and the CFD predictions have been compared with the experiments of Menzel et al. [T. Menzel, T. Weide, O. Staudacher, U. Onken, Reynolds stress model for bubble column reactor, Ind. Eng. Chem. Res. 29 (1990) 988-994]. Simulations have also been performed to understand the sensitivity of different interphase forces (drag, lift, turbulent dispersion and added mass). This work highlights the importance of choosing the C L value and the drag law in accordance with the bubble size. Further, a laboratory scale bubble column with three different spargers (perforated plate, sintered plate and single hole) has been simulated using three different turbulence closure (k-ε, RSM and LES) models, with the purpose of critically comparing their predictions with experimental data [M.R. Bhole, S. Roy, J.B. Joshi, Laser doppler anemometer measurements in bubble column: effect of sparger, Ind. Eng. Chem. Res. 45 (26) (2006) 9201-9207; A.A. Kulkarni, K. Ekambara, J.B. Joshi, On the development of flow pattern in a bubble column reactor: experiments and CFD, Chem. Eng. Sci. 62 (2007) 1049-1061].
Eulerian–Lagrangian based large-eddy simulation of a partially aerated flat bubble column
Chemical Engineering Science, 2008
Gas-liquid bubbly flow in a flat bubble column ("Becker" case with a gas flow rate of 1.6 l/min) is studied by means of large-eddy simulation (LES) combined with Lagrangian particle tracking with two-way coupling. The unsteady two-phase flow considered is relatively dilute in a global sense, but has higher gas clustering locally. The bubble size, of the order of millimeter, is relatively large compared to the smallest liquid fluctuation scales. It is demonstrated that, in such a setting, a single-phase LES along with the point-volume treatment of the dispersed phase can serve as a viable closure model, even though its application assumptions are not fully met. For the backward momentum coupling we used a "particle-source-in-ball" (PSI-ball) concept, which in essence is a generalization of the conventional particle-source-in-cell (PSI-cell) method as well as template-function based treatment. A high prediction accuracy is achieved in an extensive comparison with classical experimental data, covering not only the mean feature of the flow and transient bubble dispersion patterns, but also the second-order statistics of the liquid which is vital in assessing a closure model and has not been enough addressed in the past RANS-based studies. ᭧
International Journal of Chemical Reactor Engineering, 2015
In the present work, we formulate a simplistic two-fluid model for bubbly steam-water flow existing between fuel pins in nuclear fuel assemblies. Numerical simulations are performed in periodic 2D domains of varying sizes. The appearance of a non-uniform volume fraction field in the form of meso-scales is investigated and shown to be varying with the bubble loading and the domain size, as well as with the numerical algorithm employed. These findings highlight the difficulties involved in interpreting the occurrence of instabilities in two-fluid simulations of gas-liquid flows, where physical and unphysical instabilities are prone to be confounded. The results obtained in this work therefore contribute to a rigorous foundation in on-going efforts to derive a consistent meso-scale formulation of the traditional two-fluid model for multiphase flows in nuclear reactors.