Alain Liné

Effect of bubble deformation on stability and mixing in bubble columns

Abstract The paper deals with hydrodynamics in bubble columns. The objective of the paper is to study stability and mixing in a bubble column. The modeling of parameters such as stationary drag and added mass is addressed. In addition, the effect of bubble deformation in terms of eccentricity is highlighted. In a previous paper, the transition between homogeneous and heterogeneous regimes in bubble column without liquid flow has been shown to be driven by the deformation of the bubbles associated to drag and added mass. In the present paper, this work is generalized to bubble column with liquid flow and to the transition from bubble flow to slug flow in a vertical pipe. Numerical simulations of gas–liquid reactors are presented. The numerical simulations are validated in the case of gas plume after the Becker et al. data (Becker, S., Sokolichin, A., & Eigenberg, G. (1994) Gas–liquid flow in bubble columns and loop reactors: Part II. Comparison of detailed experiments and flow simulations. Chemical Engineering Science, 49 (24B), 5747–5762. The numerical simulations are finally applied to a bubble column. The simulations of residence time distribution coupled to transient hydrodynamics are shown to be very sensitive to the modeling of interfacial transfer of momentum from the bubbles to the liquid in terms of drag and added mass, including the effect of bubble deformation.

Forces on Spherical Particles in Terms of Upstream Flow Characteristics

The paper is devoted to the estimation of hydrodynamic forces induced by different flows around an isolated solid sphere. The goal of the study is to relate the stresses exerted by the flow around the particle to the upstream flow characteristics, in particular in terms of velocity gradient in the case of pure shear flow. Three-dimensional numerical simulations have been performed. Fluent® code has been used. The computational flow dynamics (CFD) code has been first validated in classical cases of (i) uniform flow around a fixed spherical particle at low and finite Reynolds numbers (1 to 300) and (ii) linear shear superimposed to a uniform flow past a fixed spherical particle at finite Reynolds numbers (10 to 100). After the validation step of the study, the CFD code has been applied to pure linear shear flow around a fixed spherical particle. The effect of free rotation of the sphere has been analysed. The magnitude of the forces exerted on each hemisphere of a spherical particle is derived in this paper compared to forces inducing floc breakup.

Influence of the nature of hydrodynamic constraints on aerobic biofilms

The aim of this paper is to relate biofilm detachment to local hydrodynamics. Biofilm was submitted to erosion tests in three devices: a Couette Taylor Reactor (CTR) and two Stirred Reactors (SR) Without Baffles (WOB) and With Baffles (WB). Each device involved different hydrodynamics and different impacts on the biofilm detachment. In CTR, the detachment was significant in one specific zone on the biofilm. In SR WOB, poor detachment was observed. In SR WB, major detachment was observed only on the external face. Baffles in SR induced a transient flow and a temporal non-uniformity of shear stress improving biofilm detachment.

Turbulent Macroscale in the Impeller Stream of a Rushton Turbine

Publisher Summary Particle Image Velocimetry (P.I.V.) technique is used in mechanically agitated tank equipped with a Rushton turbine. This chapter focuses on the analysis of the flow field in terms of mean flow and fluctuating motion. The fluctuations are expressed in terms of turbulence (random fluctuation) and pseudoturbulence (fluctuation induced by the periodic motion of the blades). From instantaneous velocity field taken in a plane with at given angles relative to the position of the blade, it is possible to derive spatial two-point velocity correlation functions in order to deduce integral length scales of turbulence and local dissipation rate of turbulent kinetic energy. P.I.V. technique is based on the following steps—seeding the fluid flow volume under investigation, illuminating a slide of the flow field with a pulsing light sheet, recording two images of the fluid flow with a short time interval between them, using a numerical CCD camera, and finally, processing these images by dividing the whole images into interrogation areas and using inter correlation techniques to get the instantaneous velocity field. The kinetic energy of each component of these fluctuations is determined. P.I.V. is used to estimate integral length scale of turbulence and local dissipation rate of turbulent kinetic energy. The state of the turbulence is also analyzed to estimate the influence of the blade motion in terms of anisotropy.