Critical slowing down and fading away of the piston effect in porous media

We investigate the critical speeding up of heat equilibration by the piston effect (PE) in a nearly supercritical van der Waals (vdW) fluid confined in a homogeneous porous medium. We perform an asymptotic analysis of the averaged linearized mass, momentum and energy equations to describe the response of the medium to a boundary heat flux. While nearing the critical point (CP), we find two universal crossovers depending on porosity, intrinsic permeability and viscosity. Closer to the CP than the first crossover, a pressure gradient appears in the bulk due to viscous effects, the PE characteristic time scale stops decreasing and tends to a constant. In infinitly long samples the temperature penetration depth is larger than the diffusion one indicating that the PE in porous media is not a finite size effect as it is in pure fluids. Closer to the CP, a second cross over appears which is characterized by a pressure gradient in the thermal boundary layer (BL). Beyond this second crossover, the PE time remains constant, the expansion of the fluid in the BL drops down and the PE ultimately fades away

Radiation-cooled Dew Water Condensers Studied by Computational Fluid Dynamic (CFD)

Harvesting condensed atmospheric vapour as dew water can be an alternative or complementary potable water resource in specific arid or insular areas. Such radiation-cooled condensing devices use already existing flat surfaces (roofs) or innovative structures with more complex shapes to enhance the dew yield. The Computational Fluid Dynamic - CFD - software PHOENICS has been programmed and applied to such radiation cooled condensers. For this purpose, the sky radiation is previously integrated and averaged for each structure. The radiative balance is then included in the CFD simulation tool to compare the efficiency of the different structures under various meteorological parameters, for complex or simple shapes and at various scales. It has been used to precise different structures before construction. (1) a 7.32 m^2 funnel shape was studied; a 30 degree tilted angle (60 degree cone half-angle) was computed to be the best compromise for funnel cooling. Compared to a 1 m^2 flat condenser, the cooling efficiency was expected to be improved by 40%. Seventeen months measurements in outdoor tests presented a 138 % increased dew yield as compared to the 1 m^2 flat condenser. (2) The simulation results for 5 various condenser shapes were also compared with experimental measurement on corresponding pilots systems: 0.16 m^2 flat planar condenser, 1 m^2 and 30 degree tilted planar condenser, 30 m^2 and 30 degree tilted planar condenser, 255 m^2 multi ridges, a preliminary construction of a large scale dew plant being implemented in the Kutch area (Gujarat, India)

Analysis of low gravity tolerance of model experiments for space station: Preliminary results for directional solidification

It has become clear from measurements of the acceleration environment in the Spacelab that the residual gravity levels on board a spacecraft in low Earth orbit can be significant and should be of concern to experimenters who wish to take advantage of the low gravity conditions on future Spacelab missions and on board the Space Station. The basic goals are to better understand the low gravity tolerance of three classes of materials science experiments: crystal growth from a melt, a vapor, and a solution. The results of the research will provide guidance toward the determination of the sensitivity of the low gravity environment, the design of the laboratory facilites, and the timelining of materials science experiments. To data, analyses of the effects of microgravity environment were, with a few exceptions, restricted to order of magnitude estimates. Preliminary results obtained from numerical models of the effects of residual steady and time dependent acceleration are reported on: heat, mass, and momentum transport during the growth of a dilute alloy by the Bridgman-Stockbarger technique, and the response of a simple fluid physics experiment involving buoyant convection in a square cavity

Process modelling for Space Station experiments

Examined here is the sensitivity of a variety of space experiments to residual accelerations. In all the cases discussed the sensitivity is related to the dynamic response of a fluid. In some cases the sensitivity can be defined by the magnitude of the response of the velocity field. This response may involve motion of the fluid associated with internal density gradients, or the motion of a free liquid surface. For fluids with internal density gradients, the type of acceleration to which the experiment is sensitive will depend on whether buoyancy driven convection must be small in comparison to other types of fluid motion, or fluid motion must be suppressed or eliminated. In the latter case, the experiments are sensitive to steady and low frequency accelerations. For experiments such as the directional solidification of melts with two or more components, determination of the velocity response alone is insufficient to assess the sensitivity. The effect of the velocity on the composition and temperature field must be considered, particularly in the vicinity of the melt-crystal interface. As far as the response to transient disturbances is concerned, the sensitivity is determined by both the magnitude and frequency of the acceleration and the characteristic momentum and solute diffusion times. The microgravity environment, a numerical analysis of low gravity tolerance of the Bridgman-Stockbarger technique, and modeling crystal growth by physical vapor transport in closed ampoules are discussed

Un algorithme faible nombre de Mach pour la simulation des écoulements de fluides supercritiques par des méthodes spectrales

Les simulations des écoulements de fluides supercritiques ont toujours été menées avec des méthodes de volumes finis. Les algorithmes itératifs qui doivent alors être utilisés pour résoudre les équations, fortement couplées en raison des comportements critiques, entraînent des temps de calcul très longs. Nous proposons un algorithme, associé à une méthode spectrale, pour découpler les équations d’énergie et d’état des équations de Navier-Stokes permettant ainsi de réduire les temps de calcul

Dew, fog, and rain as supplementary sources of water in south-western Morocco

International audienceThis study reports on one year (May 1, 2007 - April 30, 2008) of dew, fog and rain measurements carried out in the dryland area of Mirleft, Morocco in order to be used as alternative or supplemental sources of water. Four standard dew condensers and a passive fog net collector of 1 m² surfaces were used. Meteorological data were collected. 178 dew events (18.85 mm), 31 rain events (48.65 mm) and 7 significant fog episodes (1.41 mm) occurred, corresponding to almost 40% of the yearly rain contribution (48.65 mm, 31 events). Chemical and biological analyses were carried out. Dew and rain pH were neutral (close to 7) and the total mineralization was considerable (dew: 560 mg/L; rain: 230 mg/L). Ca²+, K+, SO42- and NO3- are found of continental origin; Cl-, Na+ and Mg2+ are of sea origin. The ions concentration agrees with the World Health Organization recommendations for potable water. The biological analysis shows harmless vegetal spores and little contamination by animal/human bacteria. A cost analysis shows that, with little investment, the population of the arid and semi-arid coastal areas of south-western north Africa could make dew water an interesting supplementary alternative water resource

Evaporation d'une goutte de liquide pur dans sa vapeur près du point critique : pertinence de l'hypothèse quasi-stationnaire et corrections au premier ordre

International audienceAbstract : The quasi-steady hypothesis applied to the vaporization process of a pure critical droplet in its vapor at the critical pressure leads to theoretical results that differ quite significantly from the numerical ones. Two kinds of unsteadyness can be involved in order to explain such results : the presence of an unsteady area far from the droplet and the occurrence of initial data relaxation. Corrections to the quasi-steady results that take into account those two kind of unsteadiness are worked out. These analytical results are in good agreement with the numerical results.L'hypothèse quasi-stationnaire appliquée au processus d'évaporation d'une goutte de liquide pur critique dans sa vapeurà la pression critique conduità des résultats qui diffèrent assez significativement des résultats numériques. Deux sources d'instationnarités peuvent être invoquées pour expliquer ces écarts : la présence d'une zone insta-tionnaire loin de la goutte et la relaxation des conditions initiales. Des corrections aux résultats quasi-stationnaires prenant en compte ces deux effets sont déterminées. La comparaison avec les résultats numériques s'avère satis-faisante

Low-frequency vibrations in a near-critical fluid

The response of a near-critical fluid to low-frequency vibrations is investigated by means of numerical simulations. Its characteristics are first established by one-dimensional analysis. It is shown that the strong thermo-mechanical coupling occurring in the boundary layers tends to make the fluid oscillate homogeneously at low frequencies, and with a larger amplitude than in a normal gas. The numerical results obtained in this first part are found to confirm earlier predictions made in pioneering theoretical work. Then, the study is extended to a two-dimensional configuration. In a square cavity, the wall shear stresses developing along the longitudinal boundaries do not affect the one-dimensional regime, since the viscous layer present in these areas behaves like the Stokes boundary layer. By contrast, thermostatting these boundaries, like the others, generates local curvature of the stream lines. The fluid response to the homogeneous acceleration field then takes some more pronounced two-dimensional patterns, but remains driven by the strong alternating expansions and retractions of the fluid in the thermal boundary layers, which are specific to near-critical fluid

L'effet piston en milieu poreux

Ce travail vise à étudier les mécanismes de transfert de masse et de chaleur au sein d'un fluide pur, au voisinage de son point critique, lorsque celui-ci sature une matrice poreuse. Hors milieu poreux. L'étude hydrodynamique et thermique des fluides supercritiques en absence de gravité et maintenus à volume constant, a mis en évidence une « accélération critique » du transport de chaleur par effet piston. Cet effet a pour conséquence une thermalisation très rapide et homogène du volume de fluide. L'impact d'un milieu poreux homogène et indéformable sur le devenir de l'effet piston est l'objet de ce travail. Une partie simulation numérique porte sur la vérification d'un modèle théorique proposé pour décrire les régimes de propagation de la chaleur. Une partie expérimentale présente, quant à elle, la réalisation d'une cellule instrumentée correspondant à la situation d'étude et des mesures tests réalisées au sein de celle-ci