Defect and Diffusion Forum Vol. 348

Abstract: We consider in the present work the fusion laser cutting of stainless steel sheets under a nitrogen laminar gas jet. The molten metal is treated as a laminar and steady viscous incompressible fluid. The mathematical model describing our problem is set in terms of Navier-Stokes equations, solved numerically using the finite differences method, where the effect of the gas jet velocity on the molten boundary layer is considered. The generated shear stress occurring on the gas-liquid interface and its contribution in the momentum is carried out, and it is found that when the skin friction and the shear stress decrease, the thickness and the velocity at the edge of the molten boundary layer increase along the kerf surface. The layer thickness reduces when the assisting gas velocity is increased.

Abstract: In this paper it is presented a mathematical model in the context of physics/transport/diffusion/dispersion about a contaminant that is a product of wood fiber cooking in a pulp cellulose plant, which is currently being thrown in a lagoon that does not have any water treatment process. The objective is improving the environmental conditions prevailing according to an environmental technology program of water treatment process required by the Government of Mexico through regulations and certifications in water treatment industries, and the prediction of the involvement of the contaminant in fishes from the Lagoon.

Abstract: The pioneering works in the area of mass transport in porous media go back to the end of last century. The partial differential equations governing the mass and heat transfer can be solved using numerical techniques, and in this paper we solve them analytically under different boundary conditions including time-periodic boundary conditions. The nature of these solutions is discussed. Analytic solutions provide valuable physical insight and are usually easier to compute. In addition, these solutions may help to experimentally determine the parameters in a setting where both the mass and temperature gradients are present, without resorting to a simplified set of equations that govern heat and mass transfer separately.

Abstract: The evaporation from open water remains a difficult process to measure or estimate. The major source of difficulty is the fact that the required meteorological parameters are rarely measured over the water. In this study, we calculate the evaporation rate from the pan using the energy budget method which is widely considered to be the most accurate method of estimating evaporation and A CFD-based methodology to estimate monthly evaporation for the two dams. The evaporation rates calculated with the energy budget and the application of CFD were in good agreement with monthly evaporation measurements in dams located in areas of different climates in Algeria.

Abstract: An experiment-calculated investigation of forced convection of nanofluids based on Al₂O₃ nanoparticles was carried out. The hydrodynamic description and a model of homogeneous nanofluids were used. The homogeneous nanofluids model assumes that the hydrodynamics and heat transfer can be described by conventional Navier-Stokes and heat transfer equations with the physical parameters corresponding to nanofluids. The results showed that this model very well described the experimental data in some cases. However, in some other cases, there are discrepancies between experiment and theory that can be explained by the real heterogeneity of nanofluids and the errors in the experimental determination of thermal conductivity and viscosity of nanofluids.

Abstract: This work presents an experimental study of the effect of a magnetic field on laminar forced convection of a ferrofluid flowing in a tube filled with permeable material. The walls of the tube are subjected to a uniform heat flux and the permeable bed consists of uniform spheres of 3-mm diameter. The ferrofluid synthesis is based on reacting iron (II) and iron (III) in an aqueous ammonia solution to form magnetite, Fe₃O₄. The magnetite is mixed with aqueous tetra methyl ammonium hydroxide, (CH₃)₄NOH, solution. The dependency of the pressure drop on the volume fraction, and comparison of the pressure drop and the temperature distribution of the tube wall is studied. Also comparison of the wall temperature distribution, convection heat transfer coefficient and the Nusselt numbers of ferrofluids with different volume fractions is investigated for various Reynolds numbers (147 < Re < 205 ). It is observed that the heat transfer is enhanced by using a porous media, increasing the volume fraction had a similar effect. The pressure coefficient decreases for higher Reynolds number. The effect of magnetic field in four strategies, named modes, on ferrofluid flow through the porous media is presented.

Abstract: Asymptotic solution for the shear stress distributions and velocity profiles of steady electroosmotic (EO) and magnetohydrodynamic (MHD) flows are obtained in a parallel flat plate microchannel. A fully-developed flow is considered and the fluid obeys a constitutive relation based in a simplified Phan-Thien-Tanner model. The effect of the following dimensionless parameters on the fluid flow control is predicted: the viscoelastic parameter and the Hartmann number. The momentum equation, boundary conditions and the constitutive rheological model are combined to formulating a nonlinear differential equation to solve the shear stress, which is expanded in a regular expansion series in powers of small Hartmann numbers. This limit of small Hartmann numbers and low electrical conductivity in the buffer solution correspond to the range where the electric and magnetic effects can be used to move a charged solution in the flow control and sample handling in biomedical and chemical analysis.

Abstract: Consideration is given to twin inline elliptical fume jets issuing within an oncoming cooler environmental crossflow. Jets are emitted from similar nozzles, characterized by a variable injection height. Such a configuration is found at large scale, in the industrial urban zones, and more particularly in multiple chimney power plants. It is found at small scale as well like in cooling in electronic devices. The present study is carried out numerically by means of the finite volume method together with the Reynolds Stress Model (RSM) second order turbulent closure model and non uniform grid system particularly refined around the emitting nozzles. Emphasis is put on the temperature distribution around the emitting nozzles in order to highlight the joint effect of the jets elevation and temperature. It was mainly found that both parameters are complementary and help straitening the discharged jets, leading their thermal mixing away from the injection ground. Nomenclature

Abstract: Impingement heat transfer between a circular jet and a semi-spherical concave surface with or without coverage of porous material is investigated experimentally and numerically. For cases with coverage of the porous material on the target plate, a trapping hole for the jet fluid is fabricated. Measured local Nusselt number distributions along a meridian are documented. The flow and temperature fields at the conditions similar to that of experiments were computed with CFD software to support the experimental results and help to explain the physics. Varying parameters include Reynolds number, nozzle-to-plate distance, relative curvature, and a target surface with or without the covered porous material. Results show that the attachment of a porous material increases Nusselt number, with more influence at the stagnation zone than the far field. Increasing Reynolds number usually increases Nusselt number unless it is too high. Although an increase in the nozzle-to-plate distance decreases stagnation Nusselt number, the influence in heat transfer is small in the far field. The trapping-hole diameter should be the same as that of the jet diameter for best heat transfer enhancement.

Abstract: The flow structures and their interactions behind side-by-side cylinders of unequal diameters at small gap ratios are studied by dye-flow visualization and particle image velocimetry at Reynolds number 1000. The whole-field mean and fluctuating velocity distributions and evolutions in the wakes are studied by the particle image velocimetry. As the gap ratio decreases, the mutual interaction of the wakes behind side-by-side cylinders pushes the recirculation region behind the large cylinder farther downstream at the expense of deterioration of the wake behind small cylinder. This change is important and may be relevant to the applications of passive flow control strategy.