Écoulement des particules surfaciques des lits granulaires dans les cylindres rotatifs : une approche originale

La simulation des écoulements polyphasiques est actuellement un enjeu scientifique, industriel et économique, important. L’étude des systèmes gaz-solide fait appel à des modèles qui mettent en œuvre l’influence des particules et les effets des collisions sur le transfert de quantité de mouvement. L’objectif principal de ce travail est d’améliorer la compréhension, via des simulations, des écoulements de matière dans les cylindres rotatifs et de proposer une contribution à la caractérisation de leur hydrodynamique. Les simulations réalisées avec le code de calcul FLUENT ont permis de confirmer la faisabilité de l’approche CFD pour les études hydrodynamiques des systèmes granulaires. La mise en œuvre de l’outil de simulation dans son état actuel a permis de réaliser des études de validation des modèles et de comparer les résultats numériques aux données expérimentales. Pour mener cette validation de façon satisfaisante, on aborde des cas de simulations clés sur des cylindres rotatifs à l’échelle de laboratoire pour lesquels il existe une base de données intéressante et diversifiée. De cette validation émane une nouvelle approche sur la configuration de l’écoulement des particules de surface dans les lits granulaire à l’intérieur des cylindre rotatif ; un événement nouveau et important est mis en évidence : les vecteurs de vitesse des particules ne sont pas toujours parallèles à la surface libre, comme il est fait état dans les travaux antérieurs ; à la limite supérieure de la couche active elles forment un angle avec la surface libre, ce qui représente une situation typique des particules qui sautillent (avance par petits sauts) pour se déplacer

Numerical study to predict optimal configuration of wavy fin and tube heat exchanger with various tube shapes

This paper aims to investigate the influence of tube shapes on thermal-flow characteristics of sinusoidal wavy finned-tube heat exchangers. Two row staggered bundle with six geometries of tubes (four flat tube geometries, one oval tube and a circular tube) are analyzed for a range of (1600 Re 4800). The inspection revealed that the heat flux and the pressure drop decrease with the tube flatness for all Reynolds values. However, the oval tube O1 reaches, for all Reynolds values, the lowest values of heat flux and pressure drop. Regarding the global performance criterion, the sinusoidal wavy fins with O1 shaped tubes reached the highest global performance values, being 14.8–24.4% and 31.6–36.3% higher than the fin with F1 and O2 tube geometry, respectively

EFFECTS OF AXIAL MAGNETIC FIELD AND THERMAL CONVECTION ON A COUNTERROTATING VON KARMAN FLOW

International audienceThe effects of thermal convection and of a constant axial magnetic field on a von Karman flow driven by the exact counter-rotation of two lids are investigated in a vertical cylinder of aspect ratio Gamma(= height/radius) = 2 at a fixed Reynolds number Re(= Omega R-2/v) = 300. Direct numerical simulations are performed when varying separately the Rayleigh and Hartmann numbers in the range [0, 1800] and [0, 20], respectively, in the limit of the Boussinesq approximation and of a small magnetic Reynolds numbers, Re-m << 1. Without a magnetic field, the base flow symmetries of the von Karman flow are broken by thermal convection that becomes dominant in the range of Ra [500, 1000]. Three-dimensional solutions are characterized by the occurrence of a steady, m = 1, azimuthal mode exhibiting a cat's eye vortex in the circumferential plane. When increasing the Rayleigh number in the range [500, 1000], the vortex pulsates in an oscillatory manner, due to variations of the flow intensity. Otherwise, increasing the axial magnetic field intensity stabilizes the flow, and the oscillatory motion can be inhibited. Numerical solutions show that the critical Rayleigh number for transition increases linearly with the Hartmann number. Finally, results show that when varying the Rayleigh number, the structure of the electric potential can be strongly modified by thermal convection. Such an observation suggests new induction mechanisms in the case of small nonzero values of the magnetic Reynolds number

Numerical Investigation of Thermal-Flow Characteristics in Heat Exchanger with Various Tube Shapes

In this study, eight configurations of oval and flat tubes in annular finned-tube thermal devices are examined and compared with the conventional circular tube. The objective is to assess the effect of tube flatness and axis ratio of the oval tube on thermal-flow characteristics of a three-row staggered bank for Re (2600 ≤ Re ≤ 10,200). It has been observed that the thermal exchange rate and Colburn factor increase according to the axis ratio and the flatness, where O1 and F1 provide the highest values. O1 produces the lowest friction factor values of all the oval tubes at all Re, and F4 gives 13.2–18.5% less friction than the other tube forms. In terms of performance evaluation criterion, all of the tested tubes outperformed the conventional circular tube (O5), with O1 and F1 obtaining the highest values. The global performance criterion of O1 has been found to be 9.6–45.9% higher as compared to the other oval tube geometries at lower values of Re, and the global performance criterion increases with the increase in flatness. The F1 tube shape outperforms all the examined tube designs; thus, this tube geometry suggests that it be used in energy systems

Influence of counter-rotating von Kármán flow on cylindrical Rayleigh-Bénard convection

The axisymmetric flow in an aspect-ratio-one cylinder whose upper and lower bounding disks are maintained at different temperatures and rotate at equal and opposite velocities is investigated. In this combined Rayleigh-Benard/von Karman problem, the imposed temperature gradient is measured by the Rayleigh number Ra and the angular velocity by the Reynolds number Re. Although fluid motion is present as soon as Re not equal 0, a symmetry-breaking transition analogous to the onset of convection takes place at a finite Rayleigh number higher than that for Re = 0. For Re 95, it is a Hopf bifurcation to a limit cycle. The steady states and limit cycle are connected via a pair of saddle-node infinite-period bifurcations except very near the Takens-Bogdanov codimension-two point, where the scenario includes global bifurcations. Detailed phase portraits and bifurcation diagrams are presented, as well as the evolution of the leading part of the spectrum, over the parameter ranges 0 <= Re <= 120 and 0 <= Ra <= 30 000