Thermal diffusivity identification by 2 nde derivative analysis of transient temperature profile

International audienceThis paper deals with a new method to identify the thermal diffusivity of isotropic materials. Contrary to the flash method, a continuous and constant heating is applied on the rear surface of the sample. An analytical solution of the 1-D transient equation is developed based on Green's function and gives the temperature profile on the opposite surface at the first moments. Regarding the 2 nde derivative of this equation, we found that it reaches a maximum value at a characteristic time tc 2nde which is proportional to the thermal diffusivity α and the sample thickness es. Experiments are performed on a well-known material, ARMCO Iron, to check the feasibility and the accuracy of the identification with noisy temperature measurements. The method is also applied to insulation materials. Uncertainties on the identified values are discussed and rules are given t

Etude expérimentale du changement de phase liquide/vapeur dans un milieu poreux d'une boucle diphasique à pompage capillaire

Cet article présente une étude expérimentale sur le transfert de masse et de chaleur avec changement de phase dans un milieu poreux en mousse de cuivre. Les phénomènes de changement de phase en milieux poreux occupent une place importante dans un grand nombre d'applications, en particulier les systèmes de transfert thermique tels que les boucles fluides diphasiques à pompage capillaire. Les boucles fluides diphasiques à pompage capillaire sont des dispositifs qui utilisent la circulation d'un fluide et les propriétés du changement d'état liquide-vapeur dans un corps poreux pour transférer d'importantes quantités de chaleur sur des distances qui peuvent atteindre quelques mètres. L'objectif de cet article est d'étudier l'effet des pertes de charge gravitaires et les propriétés du matériau (porosité, diamètre moyen de pores) sur le transfert de masse et de chaleur dans la mèche poreuse. Les résultats expérimentaux montrent qu'une augmentation des pertes de charges gravitaires entraîne une augmentation de la température du bâti métallique, ce qui fait chuter le rendement de l'évaporateur. L'effet de la porosité et du diamètre moyen des pores sur le phénomène de changement de phase dans le milieu poreux a été aussi étudié. Nous avons montré que la variation de ces deux paramètres a un impact direct sur le rendement de l'évaporateur. En effet, ces paramètres interviennent à la fois au niveau de la dynamique du fluide dans la mèche (perméabilité et pression capillaire maximale) et au niveau des transferts thermiques (la porosité intervient dans la conductivité thermique effective)

Review of fluid flow and convective heat transfer within rotating disk cavities with impinging jet

International audienceFluid flow and convective heat transfer in rotor-stator configurations, which are of great importance in different engineering applications, are treated in details in this review. The review focuses on convective heat transfer in predominantly outward air flow in the rotor-stator geometries with and without impinging jets and incorporates two main parts, namely, experimental / theoretical methodologies and geometries/results. Experimental methodologies include naphthalene sublimation techniques, steadystate (thin layer) and transient (thermochromic liquid crystals) thermal measurements, thermocouples and infra-red cameras, hot-wire anemometry, laser Doppler and particle image velocimetry, laser plane and smoke generator. Theoretical approaches incorporate modern CFD computational tools (DNS, LES, RANS etc). Geometries and results part being mentioned starting from simple to complex elucidates cases of a free rotating disk, a single disk in the crossflow, single jets impinging onto stationary and rotating disk, rotor-stator systems without and with impinging single jets, as well as multiple jets. Conclusions to the review outline perspectives of the further extension of the investigations of different kinds of the rotor-stator systems and their applications in engineering practice

PIV measurements of an air jet impinging on an open rotor-stator system

International audienceThe current work experimentally investigates the flow characteristics of an air jet impinging on an open rotor-stator system with a low non-dimensional spacing, G = 0.02, and with a very low aspect ratio, e/D = 0.25. The rotational Reynolds numbers varied from 33000 to 532000, while the jet Reynolds numbers ranged from 17200 to 43000. Particle image velocimetry (PIV) measurements were taken along the entire disk diameter in three axial planes. From the obtained PIV velocity fields, the flow statistics were computed. A recirculation flow region, which was centered at the impingement point and possessed high turbulence intensities, was observed. Local peaks in root-mean-square fluctuating velocity distributions appeared in the recirculation region and near the periphery, respectively. Proper orthogonal decomposition analysis was applied to the cases of the jet impinging on the rotor with and without rotation to reveal the coherent structures in the jet region

Étude numérique d'un écoulement turbulent dans une jonction en T avec transfert de chaleur

Ce travail concerne l’étude d’un écoulement turbulent d’air dans une conduite rectangulaire présentant une bifurcation perpendiculaire à l’écoulement principal. Le changement brusque de la direction de l’écoulement à l’entrée de la jonction crée une zone de recirculation qui peut s’avérer néfaste dans les installations industrielles. Nous nous intéressons ici à la caractérisation dynamique et thermique de l’écoulement à travers cette branche de la conduite dont l’une des parois latérales est chauffée. La simulation numérique de l’écoulement est basée sur la modélisation statistique du second ordre (RSM) en formulation de faibles nombres de Reynolds (Reynolds-stress-ω). La modélisation aux tensions de Reynolds permet en effet de tenir compte des fortes anisotropies présentes dans les zones de recirculation. Nous avons considéré l’effet du rapport de débits entre l’entrée de la conduite principale et la sortie de la branche sur le comportement dynamique et thermique de l’écoulement. Ainsi, pour trois rapports de débits choisis, l’analyse est effectuée à travers l’évolution des profils des vitesses moyennes, du champ turbulent et du nombre du Nusselt

Analysis of Fluid Dynamics and Heat Transfer in a Rectangular Duct with Staggered Baffles

This computational fluid dynamic analysis attempts to simulate the incompressible steady fluid flow and heat transfer in a solar air channel with wall-mounted baffles. Two ꞌSꞌ-shaped baffles, having different orientations, i.e., ꞌSꞌ-upstream and ꞌSꞌ-downstream, were inserted into the channel and fixed to the top and bottom walls of the channel in a periodically staggered manner to develop vortices to improve the mixing and consequently the heat transfer. The analyses are conducted with the Commercial CFD software FLUENT using the finite volume method for Reynolds number varying from 12,000 to 32,000. The numerical results are presented in terms of streamlines, velocity-magnitude, x-velocity, y-velocity, dynamic pressure coefficient, turbulent kinetic energy, turbulent viscosity, turbulent intensity, temperature field, coefficient and factor of normalized skin friction, local and average numbers of normalized Nusselt, and thermal performance factor. The insertion of the S-shaped baffles in the channel not only causes a much high friction loss, f/f0 = 3.319 - 32.336, but also provides a considerable augmentation in the thermal transfer rate in the channel, Nu/Nu0 = 1.939 - 4.582, depending on the S-baffle orientations and the Reynolds number. The S-upstream baffle provides higher friction loss and heat transfer rate than the S-Downstream around 56.443 %, 55.700 %, 54.972 %, 54.289 % and 53.660 %; and 25.011 %, 23.455 %, 21.977 %, 20.626 %, and 19.414 % for Re = 12,000, 17,000, 22,000, 27,000, and 32,000, respectively. In addition, the result analysis shows that the optimum thermal performance factor is around 1.513 at the highest Reynolds number and S-downstream

Integrated drive and reliabilities: fault tolerant architectures and supply

One major challenge of e-aerospace motor is high densities (kW/kg, Nm/kg) keeping a high functional reliability. With high switching frequency Voltage Source Inverter (VSI) using Wide-Band Gap (WBG) components, the DC-bus capacitor mass can be reduced. On contrary, because of very short commutation time (<50ns), the requirements for the connexion between VSI and machine terminals are higher. For a better proficiency, the integration of VSI inside the machine is then desirable. Besides, with WBG components, the temperature limit of the VSI is becoming higher than 150°C and consequently close to those of machines, favouring a common cooling and so, mass reduction. Finally, in integrated drive, the major drawback of fault-tolerant multiphase machines due to numerous external cables is alleviated. In order to benefit of this evolution, new topologies of integrated drive can be imagined. The paper examines different candidates of multiphase machines for integration and proposes an original topology taking into account simultaneously different constraints and opportunities