Numerical simulation of small bubble-big bubble-liquid three-phase flows

Numerical simulations of the small bubble-big bubble-liquid three phase heterogeneous flow\ud in a square cross-sectioned bubble column were carried out with the commercial CFD\ud package CFX-4.4 to explore the effect of superficial velocity and inlet dispersed phase\ud fractions on the flow patterns. The approach of Krishna et al. (2000) was adopted in the\ud Euler-Euler framework to numerically simulate the gas-liquid heterogeneous flow in bubble\ud columns. On basis of an earlier study (Zhang et al. 2005), the extended multiphase k - ε\ud turbulence model (Pfleger and Becker, 2001) was chosen to model the turbulent viscosity in\ud the liquid phase and implicitly account for the bubble-induced turbulence. The obtained\ud results suggest that, first of all, the extended multiphase k - ε turbulence model of Pfleger and\ud Becker (2001) is capable of capturing the dynamics of the heterogeneous flow. With\ud increasing superficial velocity, the dynamics of the flow, as well as the total gas hold-up\ud increases. It is observed that with increasing inlet phase fraction of the big bubbles, the total\ud gas holdup decreases while the dynamic nature of the flow increases, which indicates that the\ud small bubble phase mainly determines the total gas holdup while the big bubble phase\ud predominantly agitates the liquid

Detailed modeling of hydrodynamics mass transfer and chemical reactions in a bubble column using a discrete bubble model

A 3D discrete bubble model is adopted to investigate complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas¿liquid bubble column reactor. In this model a continuum description is adopted for the liquid phase and additionally each individual bubble is tracked in a Lagrangian framework, while accounting for bubble¿bubble and bubble¿wall interactions via an encounter model. The mass transfer rate is calculated for each individual bubble using a surface renewal model accounting for the instantaneous and local properties of the liquid phase in its vicinity. The distributions in space of chemical species residing in the liquid phase are computed from the coupled species balances considering the mass transfer from bubbles and reactions between the species. The model has been applied to simulate chemisorption of CO2 bubbles in NaOH solutions. Our results show that apart from hydrodynamics behavior, the model is able to predict the bubble size distribution as well as temporal and spatial variations of each chemical species involved

Recent progress towards hydrodynamic modelling of dense gas-particle flows

In this paper a state-of-the-art review will be presented on hydrodynamic modeling of dense gas-particle flows as encountered in the fluid\ud bed family of gas-solid contactors. After a brief introduction the different classes of fundamental hydrodynamic models will be discussed together with their physical basis and mutual advantages and disadvantages. Thereafter some typical results will be presented on first principles modeling of dense\ud gas-fluidized beds. Finally the conclusions will be presented and areas which need substantial further attention will be indicated

Comparison of Eulerian hydrodynamic models with non-intrusive X-ray measurements in pressurized dense gas-fluidised beds

The objective of much fluidisation research has been the prediction of industrial beds, which are often large and operated at high pressure and temperature. This may be achieved through the use of Eulerian two-fluid models. Two such models have been compared with experimental results of the structure of a jet in a bubbling fluidised bed. In the first two-fluid model the particles are treated as a Newtonian fluid with a constant viscosity; in the second, kinetic theory for granular flow is used to describe the rheological properties of the particulate phase. The experiments were examined non-intrusively using X-ray equipment. The comparison of the experiments with the models revealed significant and systematic differences. These are described and the implications they have for the modelling and scaleup of bubbling fluidised beds are discussed

Comparison of PIV measurements and a discrete particle model in a rectangular 3D spout-fluid bed

Particle image velocimetry and a 3D hard sphere discrete particle model were applied to determine particle velocity profiles in the plane around a spout in a spoutfluid bed for various initial bed heights, spout and background fluidization velocities. Comparison between experimental and numerical results revealed that the particle velocities are underestimated by particle image velocimetry, which is probably caused by steep velocity gradients, and overestimated by the model, which is\ud probably caused by the closure for fluid-particle drag

Detailed 3D modelling of mass transfer processes in two phase flows with dynamic interfaces

We developed a method that will enable us to determine mass transfer coefficients for a\ud large number of two phase flow conditions based on numerical simulation. A three-dimensional\ud direct numerical simulation based on the Front Tracking technique taking into account the mass\ud transfer process was chosen for this purpose. The dissolved species concentration in the liquid\ud phase is tracked using a scalar mass balance while the value of the concentration at the interface\ud is determined via an immersed boundary technique. In the present study, simulations are carried\ud out to investigate the effect of the bubble shape on the dissolved species concentration fiel

The effect of gas-phase density on bubble formation at a single orifice in a two-dimensional gas-fluidised bed.

In this study the effect of the gas-phase density on the process of bubble formation at a single orifice in a two-dimensional gas-fluidized bed has been studied experimentally and theoretically. Specifically, a detailed comparison between experimentally observed and theoretically calculated bubble growth curves has been made in the case where the density of the gas injected through the orifice (He and SF6) differs significantly from the density of the primary fluidizing agent (air). The calculations have been carried out using an earlier developed, first principles hydrodynamic model of gas-fluidized beds which has been extended with a species conservation equation to calculate the composition of the fluidizing gas in the vicinity of the evolving bubbles. Besides, the present experimental and theoretical results were compared with predictions obtained from adapted versions of approximate bubble formation models previously reported in the literature. The advanced hydrodynamic model appears to predict the experimentally observed diameters satisfactorily. In addition, the model correctly predicts the effect of the gas-phase density on the experimentally observed bubble growth. This effect can be explained satisfactorily in terms of the dependence of the interphase momentum transfer coefficient on gas-phase density. Finally, calculations with a three-dimensional version of our hydrodynamic model have been carried out to account for the effect of the front and back wall of the pseudo two-dimensional gas-fluidized bed used in our experiments. Our preliminary computational results indicate that the magnitude of the wall effect strongly depends on the boundary condition enforced for the gas-solid dispersion at these walls. In the case that the no-slip boundary condition was enforced in the calculations for the solid phase, the wall effect was significant and a considerable deviation between computed and experimentally observed bubble growth curves was found. However, when a more realistic partial slip boundary condition for the solid phase was implemented the agreement between theory and experiment could be improved by altering the slip parameter in the partial slip boundary condition expression

Modeling of mass transfer and chemical reactions in a bubble column reactor using a discrete bubble model

A 3D discrete bubble model is adopted to investigate complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas-liquid bubble column reactor. In this model a continuum description is adopted for the liquid phase and additionally each individual bubble is tracked in a Lagrangian framework, while accounting for bubble-bubble and bubble-wall interactions via an encounter model. The mass transfer rate is calculated for each individual bubble using a surface renewal model accounting for the instantaneous and local properties of the liquid phase in its vicinity. The distributions in space of chemical species residing in the liquid phase are computed from the coupled species balances considering the mass transfer from bubbles and reactions between the species. The model has been applied to simulate chemisorption of CO2\ud bubbles in NaOH solutions. Our results show that apart from hydrodynamics behavior, the model is able to predict the bubble size distribution as well as temporal and spatial variations of each chemical species involved