Nous considérons un film liquide de mélange binaire mince ruisselant sur la face interne de lune des deux plaques planeset parallèles. Le film en évaporationen présence dun écoulementco-courant dair est soumis à une densité surfacique de flux thermique constante et uniforme à traversla plaque mouillée. On étudie lestransferts de chaleur et de masse dans cette configuration en résolvant les équations couplées dansle liquide et le gaz. Les simulations présentées concernent deux plaques verticales dont la plaque sèche est adiabatique et deux mélanges binaires. Nous montronslimportance de la prise en compte de lépaisseur du film ainsi quecelle de la composition du mélange sur les transferts. Si les résultats obtenus pour le mélange éthanol–eau sont prévisibles, il nen estpas de même pour le mélange eau–éthylène glycol.

The results of numerical calculations for the downward flow of pure vapour condensing on horizontal tubes are presented. An implicit finite difference method is used. The equality of sheer stress at the liquid-vapour interface is used as the coupling condition between the two phases. The inertia and convection terms are retained in the analysis. The vapour phase velocity is obtained from potential flow. The method of source density distribution on the body surface is used for the determination of the vapour flow in tube banks. The effect of flooding produced by condensation on upper tubes is taken into account by assuming that the vapour velocity field is not affected by the condensate flow From one tube to another. The results presented show that the vapour boundary layer separation depends on the Froude number. The heat transfer in the intertube space is analyzed and compared with the theoretical and experimental results of other authors. Good agreement is shown.

We measured the acoustic Frequencies emitted by an argon plasma torch. At constant intensity, acting on the anode - cathode distance, indirectly monitored the are voltage. The results on the evolution of the observed acoustic frequencies suggest that their position is mainly a function of the are temperature. The poorness of the are temperature estimation from enthalpy balance does not allow to ascertain that it is the case. Direct measurements on the are column must be performed to demonstrate this hypothesis.

The authors present the study of vaporization, in forced convection, of two or three binary fuel mixture droplets. The droplets are situated one behind the other and are evaporated by forced convection in a thermal wind tunnel equipped with a video recording system. The image processing provides the radius regression of the droplets in accordance with the elapsed time, which is compared with results from a calculated model based on the concept of the gaseous e lm surrounding the droplets and the source of heat and mass transfer. During the period of suspension of the droplets, their vaporization begins in natural convection. From the opening of the obturator that allows the arrival of hot aire ow, the forced convection sequence begins, as does the interaction between droplets. Average Nusselt and Sherwood numbers used for the calculation of gaseous e lm thicknesses surrounding droplets come from established correlations in natural and forced convection. In the case of interaction, these Nusselt and Sherwood numbers are corrected. The experiments and calculations that are undertaken for several mixtures of heptane and decane, and for several distances between the droplets, are in good agreement. Numerical results of the validated model are analyzed using a systematic parametric study.

The vaporization of multicomponent fuel droplets was studied experimentally in a heated flow and the results were compared to the model proposed by Abramzon and Sirignano. The droplet was suspended on a permanent holder which was set up in a thermal wind-tunnel. This wind-tunnel was fitted with a video recording system and an infra-red camera. The period during which the droplet was suspended on the holder before the opening of the hot air flow damper was recorded. This first sequence corresponds to the droplet vaporization in natural convection, whose initial experiment conditions, especially diameter, temperature, composition of the droplet, are well known. Then the damper was turn on, and the sequence of forced convection begun. The initial diameter of the droplet was recorded by the video system. The other initial conditions of this second sequence cannot be determined experimentally. The distribution of temperature in the droplet and the surface temperature, the mass fraction distribution in the droplet and the surface mass fraction were unknown. These unknown parameters were determined by coupling our experiment with a model using "the film concept" in natural convection. Experimental results were compared with the calculations and found satisfactory, in natural convection as well as in forced convection initiated by this method. The method was tested in the case of a fuel mixture droplets (heptane-decane) for different initial concentrations and variable durations of the sequence in natural convection. (C) 1999 Elsevier Science Inc. All rights reserved.

In order to characterize the working regime of an argon plasma torch we measured and analysed by fast Fourier transform (FFT) the four following parameters: torch voltage, torch current, net radiative power emitted by the plasma jet, and noise (acoustic pressure) emitted by the torch. At high flow rate we observed a turbulent behavior characterized by a high correlation voltage-acoustic pressure or at lower flow rate a broad band spectrum whose global amplitude decreases with flow rate. These behaviors are attributed to the fluctuations of the are root position. At low flow rates we observed in the acoustic pressure spectrum a narrow peak whose frequency decreases with flow rate and whose maximum amplitude coincides with turbulent laminar transition. We suggest these acoustic pressure fluctuations could result from the electric self oscillations of the are. We give some results for other types of gases.

In this article we propose a numerical analysis of the heat and mass transfer in a binary thin film flowing on an inclined plane. This analysis rests on the resolution of transfer equations in liquid and vapour phases by using an implicit finite difference method. These equations are coupled and the mass diffusion in the liquid film is taken into account. The most interesting results are obtained in forced convection, particularly in the case of ethylene glycol-water mixture. In fact, results obtained show that it is possible to increase the accumulated evaporation rate when the molar fraction of ethylene glycol is less than 40%. In addition, even if the heat transfer coefficient decreases generally when the ethylene glycol composition in the mixture increases, it was found that is is possible to stabilise perhaps to increase the heat transfer coefficient. This remark has already been observed by other authors. (C) 1998 Elsevier Science Ltd. All rights reserved.

Les auteurs résolvent les équations de la couche limite laminaire et permanente qui entoure des corps à symétrie de révolution (sphère, ellipsoïdes allongé et aplati), au nez orienté vers le bas, portéà 500°C, en rotation uniforme autour d'un axe vertical dans un écoulement axial ascendant de gaz d'hydrogène à 25°C. La variabilité des propriétés physiques du fluide en fonction de la température est prise en compte ainsi que la convection naturelle qui en résulte. Un code de calcul reposant sur une méthode aux différences finies permet de retrouver des résultats de la littérature. les auteurs montrent qu'il est possible de contrôler la croissance de la couche limite thermique en agissant sur les conditions opératoires, notamment sur le profil des corps.

This study concerns the evaporation of a finite liquid film flowing on one of the walls of a vertical canal, 2m high. The canal consists of two parallel plates separated by the distance e. The wall on which the liquid flows is submitted to a constant heat flux, the other wall is thermally isolated (an adiabatic wall). To the forced convection due to the flow of air which enters the canal at uniform speed, is added the force of natural convection by the flow of heat imposed on the heated wall. The influences of inlet liquid temperature and imposed wall heat flux on the liquid Nusselt number at the liquid-wall interface are examined in detail.

In this study, we are interested in heat and mass transfers around a sphere in a gas environment. The sphere is porous and saturated with a single constituent liquid. The study is envisaged in natural convection by taking into account the variability of the physical properties according to the temperature and the pressure. To describe the liquid-vapor equilibrium and to determine the molar fraction at the surface of the sphere in a high pressure gas we use the fugacity and the Redlich-Kwong equation. In addition, mass and heat transfer equations are linked, due to the fact of taking into account the enthalpic diffusion in the heat equation. The resolution of transfer equations (impulsion, heat and mass) around the sphere allow us to determine Nusselt and Sherwood local numbers. These numbers values are used to define, by integration, their average magnitude as a function of dimensionless numbers qualifying transfers.

Voltage, current, acoustic pressure and light fluctuations from a plasma torch were recorded and analysed by fast Fourier transform (FFT). These measurements were performed for three nozzle diameters, and varying the argon plasma gas flow rate up to 30 l/min. At high flow rates the results confirm the observations depicted in the litterature concerning the good correlation between these different signals. At low gas now rate a peak in the acoustic pressure spectra which maximum intensity coincide with the laminar-turbulent transition could be used to characterize this transition.

The authors solve the equations for a steady-state laminar boundary layer around a revolving body (sphere, extended and flattened ellipsoids). Hydrogen gas at 25 degrees C flows upwards around the body. Its nose faces downwards, the temperature at its surface is 500 degrees C, and it rotates about its vertical axis at a constant velocity. The variation in the physical properties of the fluid as a function of the temperature is considered as well as the resulting natural convection. The results of the calculation using the finite difference method show a good agreement with those in the literature. It is revealed that it is possible to control the development of a thermal boundary layer using the operational conditions, especially the contour of the body. Copyright (C) 1996 Elsevier Science Ltd.

A thin-film evaporation from a surface of a axisymmetrical body in a humid air stream was studied using a finite difference method associated with Thomas' algorithm. The surface of the body is supposed isothermal. The variability of physical properties is taken into account, and a steady-state and laminar regime is supposed. The influence of the main parameters of the system on the evaporation is determined, especially the wall profile. It is shown that, for certain conditions of calculation, it is possible to find a flat ellipsoid of revolution offering a uniformly accessible wall between the pole and the equator.

A plasma torch allows two how regimes: either a laminar (silent mode) or a turbulent mode (noisy mode) which have different properties. The transition of regime is controlled by parameter change (gas how rate, current are and anode diameter) and followed by a noise degree change. This study deals with the characterization of this transition by acoustic methods. Wave and spectral analysis of control information, radiative energy and acoustic pressure show a frequency peak. The amplitude and frequency of which vary with the running conditions. This frequentail component of acoustic pressure characterizes accurately the regime transition.

The main purpose of this research is the interest in gas-liquid interfacial heat and mass transfer. The investigation is of a much broader interest because it relates to the basic problem of the interaction of a gas stream and a thin liquid film, a situation that is frequently encountered in engineering. A numerical method is presented which allows to treat the film running on a plane or cylindrical plate with an exact localisation of the gas-liquid interface. The application of this model in the case of an inclined plate shows the effectiveness by a liquid film of protecting an insulated plate from injurious effects of the hot gases. When an important hot flux is imposed to the plate, the temperature elevation can be controlled. Comparisons with numerical results of other authors are satisfying.

In this paper, we present a model for pure fuel droplet evaporation at high pressure with natural convection. The principle of the method uses the film theory : heat and mass transfer between droplet surface and ambiant gas takes place in a thin film around the droplet. In addition, in order to take natural convection into account, we first of all write and resolve local equations for a limited gas phase around a porous sphere. With this calculation, we able to estimate the corresponding mean Nusselt and Sherwood numbers in terms of mass and thermal Grashof numbers and thus determine the thickness of the gas film. In the first section of this paper, we present the numerical method and the correlation obtained for taking natural convection into account. In the last section of this paper, we present a comparison between our numerical results and experimental and numerical results in the appropriate literature.

The present work concerns a steady-state, laminar, established and unwavy water flow in thin film on an iclined surface which is isothermal or subject to uniform heat flux

The authors solve using an finite difference method the equations of steady-state laminar boundary layer around a body of revolution (sphere, extended and flattened ellipsoids). Hydrogen gas at 25 degrees C flows upward around the body. Its nose faces downward, the temperature of its surface is 500 degrees C and it rotates about its vertical axe at a constant velocity. The variability of the physical properties of the fluid as a function of temperature is considered as well as the resulting natural convection. It is revealed that flow, heat and mass transfers depend of the physical properties of the gas. It is possible to control the development of thermal and diffusional boundary layers through the operational conditions, specially the contour of the body.