D. Saury
ISAE-ENSMA, Thermique, Faculty Member
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This work investigates a natural convection flow occurring in a differentially heated cavity. The main purpose of this paper is to analyze the influence of the cavity depth variation on heat transfers and flow dynamics. Three-dimensional... more
This work investigates a natural convection flow occurring in a differentially heated cavity. The main purpose of this paper is to analyze the influence of the cavity depth variation on heat transfers and flow dynamics. Three-dimensional numerical simulations are conducted. The working fluid is air, the vertical aspect ratio (cavity height over width) is equal to 4, and the Rayleigh Number is equal to 108. The impact of the rear and front boundary conditions on the flow topology is highlighted. When the cavity depth increases, three-dimensional effects are encountered that enhance local heat transfer at the isothermal walls. In particular, for horizontal aspect ratio (cavity depth over width) greater than 1, an alternation of local maximum and minimum Nusselt numbers can be observed along the y-direction toward the center of the wall in the lower half part of the isothermal wall, which are similar to a wave-like behavior. It is shown that they are due to vortex structures generated ...
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... instantané en convection naturelle à grand nombre de Rayleigh en espace confiné. Didier SAURY *,1 , Francis DJANNA 1 , François PENOT 1 , Patrice JOUBERT 2 ... 304, 87118, (1995). [6] FX Trias, M. Soria, A. Oliva, CD Pérez-Segarra,... more
... instantané en convection naturelle à grand nombre de Rayleigh en espace confiné. Didier SAURY *,1 , Francis DJANNA 1 , François PENOT 1 , Patrice JOUBERT 2 ... 304, 87118, (1995). [6] FX Trias, M. Soria, A. Oliva, CD Pérez-Segarra, Direct numerical simulations of two ...
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A large-scale experimental differentially heated cavity was built and instrumented. Rayleigh numbers up to 1.2×1011 can be obtained with a temperature difference, ΔT = 20°C, between the hot and cold walls leaning in the range of validity... more
A large-scale experimental differentially heated cavity was built and instrumented. Rayleigh numbers up to 1.2×1011 can be obtained with a temperature difference, ΔT = 20°C, between the hot and cold walls leaning in the range of validity of the Boussinesq approximation. Previous data obtained locally for mean velocity by 2D LDV in the range give rise to questions regarding the general air flow circulation in the cavity. Particularly, a downstream flow along the vertical boundary layer was observed. This reverse flow caused by the temperature stratification outside this layer is not present in the upstream parts and was not previously observed in smaller cavities. The question of the global circulation in this cavity is thus posed. Evolution laws providing Nusselt numbers are also given and when possible compared to the literature for a large range of Rayleigh numbers.
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Buoyancy-induced cavity flows have been widely investigated for the last decades due to the contrast between their apparent simplicity, reflected by the conservation of symmetries and the restricted domain size, and the complexity of the... more
Buoyancy-induced cavity flows have been widely investigated for the last decades due to the contrast between their apparent simplicity, reflected by the conservation of symmetries and the restricted domain size, and the complexity of the flow structure (thin transitional boundary, gravity waves, recirculation regions) or the involved physical mechanisms, which result from the strong coupling between the flow and the temperature field through the heat transfers. Particularly recent studies have demonstrated the major influence of radiative transfer on these flows [2],[3]. However, performing direct numerical simulations for typical Rayleigh numbers (Ra~10) of the building industry remains strongly challenging [4],[5].
ABSTRACT A numerical study of natural convection with surface and air/H2O mixture radiation in a differentially heated cubic square cavity is presented. The coupled flow and heat transfers in the cavity are predicted by coupling a finite... more
ABSTRACT A numerical study of natural convection with surface and air/H2O mixture radiation in a differentially heated cubic square cavity is presented. The coupled flow and heat transfers in the cavity are predicted by coupling a finite volume method with a spectral line weighted sum of gray gase model to describe gas radiative properties. The radiative transfer equation is solved by means of the discrete ordinate method. Simulations are performed at Ra = 106, considering different combinations of passive wall and/or gas radiation properties and different cavity length. It was found that in presence of a participative medium representative of building, cavity length has a strong influence on temperature and velocity fields which affect the global circulation and heat transfers in the cavity. For each steady-state solution, the convective and radiative contributions to the global heat transfer are discussed. More specifically, boundary layer thickness, thermal stratification parameter, and three-dimensional effects are compared to pure convective case results. The results suggest that radiative effects, often considered as negligible in view of the relatively low optical thickness, may not be neglected when trying to predict regime transitions.
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Abstract In this experimental study, a natural convection flow in a differentially heated cavity has been disturbed in order to modify heat transfers. The disturbance is achieved by introducing a localized obstacle which acts as a small... more
Abstract In this experimental study, a natural convection flow in a differentially heated cavity has been disturbed in order to modify heat transfers. The disturbance is achieved by introducing a localized obstacle which acts as a small spatial extent passive system. The obstacle is placed inside the hot boundary layer of the cavity flow. Measurements have been carried out in terms of velocity fields, temperature profiles and heat transfers. The influence of the length and the vertical location for an insulating and a conducting obstacle have been analyzed. For the insulating obstacle, a part of the flow is deviated inside the colder core region in front of the obstacle, which leads to an increase of the downstream heat transfers as the deviated colder flow returns along the hot wall. For the conducting obstacle, a hot thermal plume is generated, which counters the obstacle effect observed for the insulating obstacle. In that case, the downstream heat transfer is increased or reduced depending on the vertical location of the obstacle. Relative changes on heat transfers compared to the case without obstacle are larger for longer obstacles and for higher vertical locations of the obstacle, for any conductivity. For instance, a relative heat transfer increase up to 83 % is observed downstream the insulating obstacle for the largest length and highest vertical location.
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Abstract The experimental work described in this paper presents a turbulent natural convection flow at high Rayleigh number (Ra H = 1.2 × 10 11 ) within a 4 m-high differentially heated cavity filled with air. Synchronized measurements... more
Abstract The experimental work described in this paper presents a turbulent natural convection flow at high Rayleigh number (Ra H = 1.2 × 10 11 ) within a 4 m-high differentially heated cavity filled with air. Synchronized measurements of velocity and temperature have been developed to describe the turbulent behavior of the flow. For this purpose, this coupled acquisition leads, among others, to the calculation of turbulent heat fluxes. These fluxes are provided in this paper and compared to other turbulent quantities (Reynolds stresses for instance). The synchronization of the acquisitions of velocity and temperature has been set up by the coupling of PIV and micro-thermocouple (O = 12.7 μm) measurements. Buoyant effects as well as velocity-temperature interactions on such a turbulent flow are discussed in this paper.
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ABSTRACT This article presents an experimental technique to identify the convective heat transfer coefficient inside an empty rotating cylinder with an inner axial airflow. This method is based on infrared thermography and consists in... more
ABSTRACT This article presents an experimental technique to identify the convective heat transfer coefficient inside an empty rotating cylinder with an inner axial airflow. This method is based on infrared thermography and consists in heating the outer face of the rotating cylinder using infrared lamps and acquiring the evolution of the external surface temperature versus time using an infrared camera. Heat transfer coefficients are then identified via three specific methods (an inverse model, a thermally thin wall model and an analytical method). The experiments were carried out for numerous rotational speeds and inner airflow rates.
ABSTRACT Dans le cadre du programme de recherche COCORACOPHA, soutenu par le PIE2 du CNRS, une expérience de convection naturelle a été mise en place pour aider à la connaissance des écoulements et à la maîtrise des transferts thermiques... more
ABSTRACT Dans le cadre du programme de recherche COCORACOPHA, soutenu par le PIE2 du CNRS, une expérience de convection naturelle a été mise en place pour aider à la connaissance des écoulements et à la maîtrise des transferts thermiques aux grandes valeurs du nombre de Rayleigh, de l'ordre de 1,5.10 puissance 11. Ceci doit se faire pour des écarts de température de l'ordre de la dizaine de degrés pour rester dans le cadre de l'hypothèse du fluide de Boussinesq et des applications auxx habitacles (bâtiments ou autres volumes). Il en résulte la réalisation d'une cavité de 3,84 m de hauteur et de 0,86 m² de section droite horizontale. Deux paroios verticales en vis-à-vis sont maintenues à température constante grâce à des bains thermostatés, régulés avec précision. Les quatre autres faces sont constituées d'isolant thermique, pouvant être recouvert intérieurement d'un mince film de faible émissivité. La stratification thermique, dans le coeur notamment, est comparée à celles déjà obtenues dans des cavités de plus petite taille et dans des cavités carrées ou de rapport de forme égal à 1. L'influence de l'émisivité des parois, donc des effets radiatifs entre parois, est examinée et comparée aux résultats antérieurs obtenus dans des configurations de plus petite taille.
ABSTRACT Nowadays, design of electrical motors runs up more and more against the problems of heatings during operation. These excessive heatings result from a weak heat dissipation. The improvement of cooling system passes thought the... more
ABSTRACT Nowadays, design of electrical motors runs up more and more against the problems of heatings during operation. These excessive heatings result from a weak heat dissipation. The improvement of cooling system passes thought the improvement of convective heat transfers within the motors. However, these heat transfers strongly depend on the flow structure. This structure is particularly significant for the enclosed motors. The air is completely enclosed inside the machine and consequently, it is never renewed. Indeed, for this kind of configuration, the flow structure is only regulated by the geometry. It is thus not possible to modify it from the outside as for the open motors. That is why, the numerical study presented below analyzes the flow structure in an enclosed induction motor of railway traction. The sliding mesh method has been carried out in order to keep a significant accuracy on the flow nature. Then, some air spaces in the motor have been proved to be particularly sensitive for this kind of enclosed motor.
ABSTRACT This paper presents an analyse of the heat transfer coefficient downstream a backward facing step with various upstream airflow conditions: uniform flow outside a laminar boundary layer, uniform flow outside a turbulent boundary... more
ABSTRACT This paper presents an analyse of the heat transfer coefficient downstream a backward facing step with various upstream airflow conditions: uniform flow outside a laminar boundary layer, uniform flow outside a turbulent boundary layer, and a fully developed flow. The local heat fluxes are obtained from temperature determination by infrared thermography on an assumed thermally thick wall used as boundary conditions of a numerical model. In this article we mainly focus on the maximum heat flux point position determined experimentally and numerically, and also on the influence of the expansion ration on the value of the maximum Nusselt number.
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... Document Type: Research article. Affiliations: 1: Aerospace Engineering and MechanicsDepartment,The University of Alabama ... In this: publication; By this: publisher; In this Subject: Heat; By this author: Sharif MAR ; Liu W. You are... more
... Document Type: Research article. Affiliations: 1: Aerospace Engineering and MechanicsDepartment,The University of Alabama ... In this: publication; By this: publisher; In this Subject: Heat; By this author: Sharif MAR ; Liu W. You are signed in as: Google (Institutional account). ...
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ABSTRACT The goal of this paper is to develop simultaneous measurements of the temperature and velocity in order to contribute to the development of models adapted to natural convection flows and better apprehend their turbulent... more
ABSTRACT The goal of this paper is to develop simultaneous measurements of the temperature and velocity in order to contribute to the development of models adapted to natural convection flows and better apprehend their turbulent characteristics. The experimental setup consists in a vertical open channel whose sidewalls are kept at ambient temperature. A heated square bar is placed into the lower part of this channel close from one of its wall. It drives air in the whole channel. The temperature of the bar is chosen to obtain a turbulent air flow. This study focuses on the measurement technique developed to realize synchronized temperature and velocity measurements in a turbulent natural convection flow. This technique permit to measure turbulent heat fluxes (<u'T'> or <v'T'>). Measurement strategies will be presented and discussed in the paper. Some experimental care is needed to avoid disturbing the airflow. The challenge is to choose two complementary measurement techniques which have to be synchronized but which cannot be carried out at the same location when a laser is used. In this study, the velocity and the temperature measurements are respectively carried out using PIV technique and a specific K type micro-thermocouple (12.7 μm in diameter). The location of the thermocouple with respect to the laser sheet has been investigated as well as the influence of the laser on the temperature measurements. The criterion used for finding the best location is the correlation coefficient between the temperature and the velocity data. Some preliminary results of coupled velocity-temperature measurements are provided showing the feasibility of this kind of measurements.