Abstract
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.
Highlights
Large eddy simulation (LES) of bubbly flow in bubble column reactors adopts two approaches, which are Eulerian-Eulerian (E-E) and Eulerian-Lagrangian (E-L)
The results of Large-eddy simulations (LES) simulations clearly indicate that by employing the modified sub-grid scale (SGS) model with consideration of Stokes number, the bubble entrainment transient behaviour in the cylindrical bubble column that was observed in experimental work can be reasonably captured
The main conclusions reached as a result of the present study can be summarised as follows: (1) It can be observed from the simulation resulting from the modified SGS model that the gas hold-up and velocity profiles demonstrate a better agreement overall with the experimental results [34,35] compared with the standard Smagorinsky SGS model, but both gas hold-up and the streamwise gas velocity are slightly over-predicted in the vicinity of the bubble column wall
Summary
Large eddy simulation (LES) of bubbly flow in bubble column reactors adopts two approaches, which are Eulerian-Eulerian (E-E) and Eulerian-Lagrangian (E-L). The bubble size distribution can be calculated as part of the solution at each time step and models are required to account for break-up and coalescence of the bubbles This kind of approach is quite computationally intensive, it is still inhibitive for studying two-phase bubbly flows in large-scale bubble column reactors or at high void fraction system. Lakehal et al [2] pointed out that because Reynolds Averaging Navier-Stokes (RANS) models depend on time averaging, they appear to screen out the local fluctuations related to the turbulence as well as the fluctuations related to the interaction between the bubbles and surrounding eddies These local fluctuations are at least partially remained in LES modelling, instead of time averaging, the spatial filtering is employed. LES modelling only assume isotropy for unresolved scales instead of applying it to resolved scales at the same time
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