Michel Prevost

Study of a laminar falling film flowing over a wavy wall column: Part I. Numerical investigation of the flow pattern and the coupled heat and mass transfer

Abstract Flow pattern and heat and mass transfer characteristics for a film flowing over a vertical wavy column are numerically investigated in a laminar flow regime. In our approach, the heat and mass transfer coefficients are avoided in order to include hydrodynamics directly in the heat and mass transfer rates. As a consequence the numerical model is decomposed into two steps. Firstly, the flow pattern for a film with a free interface is developed. Secondly, heat and mass transfer are investigated with the incorporation of velocity fields. The heat and mass transfer coefficients increase in laminar flow.

Study of a laminar falling film flowing over a wavy wall column: Part II. Experimental validation of hydrodynamic model

Abstract The interface position of a film flowing over a wavy wall column is experimentally studied by an optical method composed of a charge coupled device (CCD) video camera. The results are compared with theoretical calculations and show a good agreement between results for both the film thickness and the vortex position. However, there exist some discrepancies because the interface is travelled by waves not accounted for in the mathematical model as it is supposed to be flat. Some characteristics of the waves are experimentally noted. Furthermore, the influence of the viscosity on the film thickness is established as well.

Simulation of velocity fields in a falling film with a free interface flowing over a wavy surface

A numerical model has been developed in order to simulate the flow behavior of a falling film over a wavy surface. The particularity of this model resides in the flow field which contains a mobile gas-liquid interface. This free interface induces a variable thickness in the film due to the vortex. After having written the equations, made a change of variables in space and discretised, we solve the non linear system of equations by a Newton type method. Our work is limited to steady flow. We study hydrodynamics, wall shear stress and thickness for various Reynolds numbers and various geometry. In this report we present the results for water.

Hydrodynamic behaviour and mass transfer performance of SiC foam

This work presents an experimental study of hydrodynamic behaviour and mass transfer performance of ceramic foam (SiC) as column packing. Voidage of the foam studied is of 91% with 640 m2/m3 specific area and 5 PPI. Hydrodynamics experiments are realised with air-water system in counter current in a 150 mm diameter column with a packing height of one meter. Parameters studied are pressure drop for dry and wet packing, flooding and liquid hold-up. Total reflux experiments with n-heptane/cyclohexane mixture were carried out in a distillation column at atmospheric pressure to study mass transfer by determination of HETP (Height Equivalent to Theoretical Plate). The performance in terms of pressure drop per unit height and flooding are quite lower in comparison with classical packings of distillation. The pressure drop obtained is about few millibars per meter (0-10 mbar.m-1) for F-Factor from the range of 0 to 1 and liquid superficial velocities from 1 to 10 m3m-2h-1. Liquid hold-up values, measured in the same range of flow, are in a range of 5 to 10%. The possible application of foam as a packing for distillation is so demonstrated.

Procede experimental de restitution de la partie imaginaire de l'amplitude complexe d'un objet de phase

Abstract An experimental process is described, capable of restoring the imaginary part of the complex amplitude of a phase object. It is based on a combination of phase-modulation interferometry and scanning the object by a flying-spot method. This experiment proves that it is possible to obtain the sign of the phase, and so the direction of surface relief.

Enhancement of absorption efficiency for a laminar film flow by hydrodynamic conditions generated by a new type of column wall

A numerical model is developed to quantify the effects of hydrodynamics on heat and mass transfer during an absorption, for a laminar film flowing over a wavy wall column. First of all, the modelling is written for a single wave of the wall shape. Then, an experimental set up, composed of a CCD video camera, validates this model. Finally, the model is extended to an entire column. The results include a comparison with the simulation of a smooth column having the same geometrical and operating conditions. The wavy column dissipates more heat through the wall (43%) due to the presence of a vortex in the furrows. This leads to an increase of the absorption rate at the interface (10%). Moreover, the wavy column reaches equilibrium more rapidly in spite of a lower mean film temperature.

Modelling of the coupling hydrodynamic transfer for a gas–liquid countercurrent flow on a wavy surface

This paper concerns laminar countercurrent gas–liquid flow over a wavy wall column, in the case of a falling liquid film. The modelling concerns the coupling of hydrodynamic and heat and mass transfer for an absorption as an example of application. The falling liquid film interacts, through the free interface, with the gas phase. The wavy surface generates particular hydrodynamic conditions with the presence of a vortex in both phases. The consequence of these vortices is an increase of transfers compared to the smooth wall.

Méthode de traitement des interférogrammes à deux ondes pour accroître leur sensibilité

Two-beam interference fringes are not always able to give sufficient information to determine the topography of very weakly deformed wave surfaces. The process described allows us to intercalate several intermediate levels, which vary linearly in terms of the phase, between the brightness extrema of a fringe. The interference pattern is submitted to an optoelectronics treatment where the photoelectric signal is compared with an adjustable electric reference signal.

Approche de traitement du probleme de la caracterisation des mouvements des objets microscopiques.

In the field of biology there are many problems that call for microscopic image analysis. Prominent among these is the study of the change of shape and of the motion of single cells. Such changes are difficult to analyze in real time; taking series of photographs, for example, and evaluating these photographs by microdensitometry and subsequent computer analysis are rather time-consuming processes. We present here another solution to this problem, using equipment that should be available even in the modestly equipped laboratory: a microscope, a scanning system such as a video monitor, and a microcomputer that can be interfaced with the scanner. The necessary procedures of data acquisition and data processing are discussed in detail.

Procedes d'analyse d'un objet de phase par balayage de son interferogramme

Combination of phase-modulation interferometry and scanning of the object is capable of restoring the sine of the phase everywhere in the object fields. Several processes are able to be employed to scan the object: flying-spot method, use of a vidicon tube or an image dissector tube. These processes are discussed and the problem of the plotting of the informations with a monitor device is studied.