High-Rayleigh-Number Convection in a Vertical Channel

See Also * Phys. Rev. Focus 17, story 9International audienceWe measure the relation between convective heat flux and temperature gradient in a vertical channel filled with water, the average vertical mass flux being zero. Compared to the classical Rayleigh-Bénard case, this situation has the advantage of avoiding plates and, thus, their neighborhood, in which is usually concentrated most of the temperature gradient. Consequently, inertial processes should control the convection, with poor influence of the viscosity. This idea gives a good account of our observations, if we consider that a natural vertical length, different from the channel width, appears. Our results also suggest that heat fluxes can be deduced from velocity measurements in free convective flows. This confers to our results a wide range of applications

Natural cross-ventilation of buildings, an experimental study

International audienceNatural cross ventilation can be a promising passive solution for summer thermal comfort in buildings. It takes advantage of the night temperature of the air to cool the walls of the building. Although this technique is well-known in hot climate, its use in new buildings requires being able to predict the quantity of heat that can be dissipated. There is indeed a lack of experimental data either to build design rules for engineers or to validate numerical code dedicated to the design. In this study, experimental research is carried out on a full-scale dwelling and a small-scale model, and evidence is given on the relevance of such a comparison. The flow has been estimated with spatially discrete local sensors in both systems, and flow visualizations have been additionally performed in the small-scale controlled model. For windy periods, the indoor airflow is found to be driven by the wind, as expected. For calm periods which are critical for summer comfort, the flows look much more complex with the observation of unsteady flow reversals

Convection naturelle dans un canal vertical chauffé asymétriquement : stratification thermique externe et écoulements de retour (S23).

Le refroidissement passif d'une façade double-peau photovoltaïque est étudié expérimentalement par l'analyse de la convection naturelle dans un canal vertical chauffé ouvert dans une cellule en eau. Une paroi est chauffée à une densité de flux de chaleur uniforme, l'autre étant adiabatique. L'étude est réalisée pour un nombre de Rayleigh modifié élevé (Ra* = 2, 75.10^7 ). Les mesures de température de la paroi chauffée et de vitesse en sortie du canal par PIV (Particle Image Velocimetry) ont mis en évidence deux états dits "mélangé" "et "stratifié" qui sont caractérisés par la position du maximum de température de paroi situé, respectivement, à mi-hauteur et en sortie. Les mesures de vitesse montre que l'état stratifié est toujours associé à l'existence d'un écoulement de retour. Les trois expériences, réalisées pour différentes stratifications thermiques de la cellule en eau, montrent trois dynamiques différentes de changement d'état

Heat convection in a vertical channel : Plumes versus turbulent diffusion

11 pagesInternational audienceFollowing a previous study [Gibert , Phys. Rev. Lett. 96, 084501 (2006)], convective heat transfer in a vertical channel of moderate dimensions follows purely inertial laws. It would be therefore a good model for convective flows of stars and ocean. Here we report new measurements on this system. We use an intrinsic length in the definition of the characteristic Rayleigh and Reynolds numbers. We explicit the relation between this intrinsic length and the thermal correlation length. Using particle imaging velocimetry, we show that the flow undergoes irregular reversals. We measure the average velocity profiles and the Reynolds stress tensor components. The momentum flux toward the vertical walls seems negligible compared to the shear turbulent stress. A mixing length theory seems adequate to describe the horizontal turbulent heat and momentum fluxes, but fails for the vertical ones. We propose a naive model for vertical heat transport inspired by the Knudsen regime in gases

Comparison between rough and smooth plates within the same Rayleigh-Bénard cell

International audienceIn a Rayleigh-Bénard cell at high Rayleigh number, the bulk temperature is nearly uniform. The mean temperature gradient differs from zero only in the thin boundary layers close to the plates. Measuring this bulk temperature allows to separately determine the thermal impedance of each plate. In this work, the bottom plate is rough and the top plate is smooth; both interact with the same bulk flow. We compare them and address in particular the question whether the influence of roughness goes through a modification of the bulk flow

Convection naturelle dans un canal vertical en eau avec chauffage pariétal : influence de la stratification

Pour des applications liées à l'intégration de l'énergie solaire sur les bâtiments, nous étudions la convection naturelle qui se développe dans un canal vertical ouvert en haut et en bas avec un chauffage pariétal. Le fluide est de l'eau pour s'affranchir des problèmes de rayonnement entre parois. Nous présentons des mesures expérimentales de température, de flux de chaleur et de vitesse dans le canal. Outre des corrélations globales à l'échelle du canal, nous présentons des résultats issus de mesures en deux points très proches de température et de vitesse

Mise en évidence expérimentale d'une instabilité convective dans un écoulement de Poiseuille-Rayleigh-Bénard

PARIS-BIUSJ-Thèses (751052125) / SudocNANCY/VANDOEUVRE-INPL (545472102) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Early-stage natural convection in a vertical channel

International audienceThis work deals with transient natural convection in a vertical channel with wall heating, the channel being located inside a cavity. This experimental work is focused on the first hours after the onset of the heating. The aim of this study is to investigate the link between the heat transfer in the channel with respect to the flow outside the channel. A thermal model of the fluid above the channel is performed and the result fits well with the experimental data. Moreover, the model leads to an expression for the ascending mass flow rate in the channel by using temperature measurements which is an important result because direct measurements remain a tricky problem. This study confirms the approach developed by the same team in a previous wor