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Boundary Layer Thickness (boundary + layer_thickness)
Selected AbstractsMechanisms affecting the dissolution of nonaqueous phase liquids into the aqueous phase in slow-stirring batch systemsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2001Mathias Schluep Abstract Understanding the kinetics of the exchange processes between nonaqueous phase liquids (NALs) and water is important in predicting the fate of anthropogenic compounds such as petroleum hydrocarbons, i.e., benzene, toluene, ethylbenzene, and xylene (BTEX) as well as polynuclear aromatic hydrocarbons (PAHs). Exchange processes occurring in the environment resemble the experimental setup of the slow-stirring method (SSM) designed to determine solubilities and octanol-water partition coefficients. Data obtained from SSM experiments for diesel fuel compounds are interpreted by a linear transfer model that is characterized by an aqueous molecular boundary layer and the water/NAPL equilibrium partition coefficient. For the chosen experimental setup, the boundary layer thickness is 2.42 × 10,2 cm. Typical equilibration times lie between 1 and 2 d. Due to the temperature dependence of the aqueous diffusivity, this time increases with decreasing temperature. Transport within the NAPL phase can slow down the exchange process for the more water-soluble compounds (e.g., benzene) provided that the stirring rate exceeds a critical value. [source] Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hillHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2009Hideki Yanaoka Abstract This study presents a three-dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn-shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn-shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high-turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20261 [source] Heat transfer for Marangoni-driven boundary layer flowHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 2 2002David M. Christopher Abstract Marangoni convection induced by variation of the surface tension with temperature along a surface influences crystal growth melts and other processes with liquid,vapor interfaces, such as boiling in both microgravity and normal gravity in some cases. This paper presents the Nusselt number for Marangoni flow over a flat surface calculated using a similarity solution for both the momentum equations and the energy equation assuming developing boundary layer flow along a surface. Solutions are presented for the surface velocity, the total flow rate, and the Nusselt number for various temperature profiles, Marangoni numbers, and Prandtl numbers. For large bubbles, the predicted boundary layer thickness would be less than the bubble diameter, so the curvature effects could be neglected and this analysis could be used as a first estimate of the effect of Marangoni flow around a vapor bubble. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 105,116, 2002; DOI 10.1002/htj.10019 [source] Practical techniques for a three-dimensional FEM analysis of incompressible fluid flow contained with slip walls and a downstream tube bundleINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2001Yuzuru Eguchi Abstract Two practical techniques are proposed in this paper to simulate a flow contained in a plenum with a downstream tube bundle under a PC environment. First, a technique to impose slip wall conditions on smooth-faced planes and sharp edges is proposed to compensate for the mesh coarseness relative to boundary layer thickness. In particular, a new type of Poisson equation is formulated to simultaneously satisfy both such velocity boundary conditions on walls and the incompressibility constraint. Second, a numerical model for a downstream tube bundle is proposed, where hydraulic resistance in a tube is imposed as a traction boundary condition on a fluid surface contacting the tube bundle end. The effectiveness of the techniques is numerically demonstrated in the application to a flow in a condenser water box. Copyright © 2001 John Wiley & Sons, Ltd. [source] Effect of Altitude on Energy Exchange Characteristics of Some Alpine Medicinal Crops from Central HimalayasJOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 1 2004S. Chandra Abstract To explore the conservation and cultivation of endangered alpine medicinal crops at comparatively lower altitudes, a study on variations in morphological parameters and energy exchange characteristics was conducted on five herbaceous medicinal crops from the alpines of Central Himalayas. Plants of same age were selected from the alpine medicinal crop nursery, Tungnath (3600 m), and were planted at the nurseries at 2100 and 550 m altitudes. After well acclimatization at lower altitudes, plants were examined for morphological and energy exchange studies during their active growth period. The energy balance sheet of these plant species indicates that most of the energy absorbed by the leaves dissipates by re-radiation, transpiration and thermal conductance across leaf surfaces. All species maintained leaf temperature below the surrounding air temperature at all altitudes and therefore gained energy by convection of heat as well as by boundary layer thermal conduction. Leaf-to-air temperature difference, gain of energy by convection of heat and boundary layer thermal conduction was maximum at an altitude of 2100 m in all the species. Boundary layer thermal conductivity, boundary layer thickness, thermal conductivity of the leaf and therefore, total energy absorbed by the leaves of these species increase significantly with decreasing altitude. Leaf thickness significantly decreases with decreasing altitude, which in turn enhances total energy absorption (r = ,0.975, P < 0.005) of the leaves in all the species. The results indicate that all these species absorb higher amount of energy at lower altitudes, which indicates their adaptability to warm temperatures at low altitudes (up to 550 m). Therefore, these species can be cultivated at relatively lower altitudes. However, a proper agronomic methodology needs to be developed for better yields. [source] Effect of filament drawdown on aerodynamic drag and heat transfer in fiber spinningAICHE JOURNAL, Issue 5 2004C. Miller Abstract The momentum and thermal boundary layer equations are solved numerically to assess the effects of filament drawdown on aerodynamic drag and heat transfer in melt fiber spinning. It is found that, relative to the case without drawdown (constant filament velocity and diameter), the aerodynamic drag on the filament increases substantially, but the heat transfer rate is suppressed. Moreover, the air velocity profile eventually becomes fully developed (unlike the zero drawdown case), and the (velocity) boundary layer thickness, rather than continuing to grow with distance beneath the spinneret, eventually becomes proportional to the filament diameter, and thus decreases with increasing axial distance. Quantitative results are presented for the dimensionless drag force per unit length along the filament (the Drag number) and the Nusselt number as functions of the dimensionless axial distance and a new dimensionless parameter, the Drawdown Reynolds number. © 2004 American Institute of Chemical Engineers AIChE J, 50: 898,905, 2004 [source] A Simple Index to Restrain Abnormal Protrusions in Films Fabricated Using CVD under Diffusion-Limited Conditions,CHEMICAL VAPOR DEPOSITION, Issue 4 2004Y. Kajikawa Abstract Cauliflower-like protrusions formed in CVD processes under diffusion-limited conditions have been studied both experimentally and theoretically. Both approaches indicate that the difference in diffusion fluxes to the film and to the protrusions controls the growth of such protrusions. However, direct comparisons of these two approaches have never been done, probably due to the complexity of the theoretical models. To simplify model protrusion growth, we developed a one-dimensional (1D) analytical model by hypothesizing the diffusion of growth species in the boundary layer above a growing film. Based on this model, we propose a non-dimensional quantity, ksf/D, as an index of protrusion growth (D is the diffusion coefficient of the growth species, ks is the surface reaction-rate coefficient, and f is film thickness). This index represents more directly the protrusion growth than does the previously proposed index, the Damköhler number, Da,=,ks,/D, where , is boundary layer thickness. To obtain smooth, protrusion-free films, D/ks should be kept larger than the desired film thickness. By controlling the process conditions to satisfy this index, we successfully fabricated protrusion-free films with SiC deposition from dichlorodimethylsilane (DDS). [source] | ||||||||||