Showing papers by "Yutaka Asako published in 2008"

Poiseuille number correlation for high speed micro-flows

Abstract: The Poiseuille number, the product of a friction factor and the Reynolds number (f Re) for a quasi-fully developed high speed flow in a micro-channel of Re < 2300 and Mach number Ma < 0.7, was obtained numerically. The numerical methodology is based on the arbitrary-Lagrangian–Eulerian (ALE) method. Two-dimensional compressible momentum and energy equations with no-slip and slip boundary conditions were solved for constant wall temperatures that are lower or higher than the inlet temperature. The channel height ranges from 10 to 100 µm and the channel aspect ratio is 200. The stagnation pressure, pstg, is chosen such that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure is fixed at atmospheric conditions. In the case of fast flow for both no-slip and slip boundary conditions, the value of f Re is higher than 96 due to compressibility effects. However, in the case of slow flow for slip boundary conditions (Maout < 0.3), the value of f Re is slightly lower than 96 due to rarefaction effects, even the flow is accelerated. The f Re correlation for slip boundary conditions is obtained from that for no-slip boundary conditions and incompressible theory as a function of the Mach number and the Knudsen number. The f Re correlation obtained for no-slip boundary conditions is also compared with that obtained for slip boundary conditions. The values of f Re obtained for no-slip and slip boundary conditions almost coincide within 3% for the channel height in the range 10–100 µm.

Spouting Enhancement by Addition of Small Quantities of Liquid to Gas-Spouted Beds

Abstract: The addition of liquid water to beds of polystyrene particles exceeding 3 mm in diameter enhanced spouting by causing a decrease in the minimum spouting velocity. While the degree of enhancement is quantitatively comparable to what we reported previously for fluidized beds of large light hydrophobic particles, the mechanism is quite different. Enhancement of spouting takes place because the liquid congregates at the spout-annulus interface, forming an effective nearly-impermeable draft tube which then increases the velocity of the gas up the spout. L'ajout d'eau liquide a des lits de particules de polystyrene de plus de 3 mm de diametre ameliore le jaillissement en causant une diminution de la vitesse de jaillissement minimale. Alors que le degre d'amelioration est du point de vue quantitatif comparable a ce que nous avons indique anterieurement pour des lits fluidises a larges particules hydrophobes legeres, le mecanisme est different. L'amelioration du jaillissement se produit parce que le liquide s'agglomere a l'interface jet-espace annulaire, formant un tube d'aspiration quasi impermeable efficace, ce qui a pour effet d'accroitre la vitesse du gaz en haut du jet.

Dynamic force reduction and heat transfer improvement for horizontal tubes in large-particle gas-fluidized beds

Abstract: The effects of tube bank configuration on forces and heat transfer were investigated for both two-dimensional and three-dimensional gas fluidized beds. Effective dynamic forces and heat transfer coefficients were measured for several tube bank configurations, and it was found that the average forces are smaller than for a single tube. The heat transfer coefficient can be increased by providing sufficient space for particles to descend around both sides of the tube bank. The results provide useful guidelines for optimizing the configuration of tube banks to achieve high heat transfer coefficients while reducing tube erosion due to dynamic forces.