Flows at high velocity

Motionless inline mixers obtain energy for mixing and dispersion from the pressure drops developed as the phases flow at high velocity through an array of baffles or packing in a tube. Performance data on the Kenics (132) and Sul2er (133) types of motionless mixer have been reported. 

The catalysts used in this CCR commercial service must meet several stringent physical property requirements. A spherical particle is required so that the catalyst flows in a moving bed down through the process reactors and regenerator vessel. These spheres must be able to withstand the physical abuse of being educated and transferred by gas flow at high velocity. The catalyst particles must also have the proper physical properties, such as particle size, porosity, and poresize distribution, to achieve adequate coke combustion kinetics. 

Consideration will now be given to the various flow regimes which may exist and how they may be represented on a Flow Pattern Map to the calculation and prediction of hold-up of the two phases during flow and to the calculation of pressure gradients for gas-liquid flow in pipes. In addition, when gas-liquid mixtures flow at high velocities serious erosion problems can arise and it is necessary for the designer to restrict flow velocities to avoid serious damage to equipment. 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

Gas-continuous impinging streams involve flows at high velocity, and so power consumption naturally becomes an important concern [62]. As is well known, the theoretical or minimum work per unit time for fluid transportation is equal to the product of the pressure drop and the volumetric flow rate of the fluid ... 

Fig. 13.41 The Keuerleber and Pahl (1970) mixhead. In the closed or recirculation position, reactants recirculate through grooves (c) along the cylindrical cleanout piston (b). In the open position, reactants flow at high velocity through circular orifices (a), impinge in the chamber (d), and flow out to the mold cavity (diagram from G. Oertel, 1985 (80)). [Reprinted by permission from C. W. Macosko, RIM Fundamentals of Reaction Injection Molding, Hanser, Munich, 1989.]...

Gas and liquid must be separated within the reactor so that the liquid can be pumped back to the spraying nozzle. The gas-liquid flow at high velocity that hits a liquid surface below the monolith bed will in many cases cause a foam problem. Lowering the velocity and providing a large contact area with the gas bulk, by directing the liquid from the... 

In contrast to packed catalyst beds, however, countercurrent flow of gas and liquid is in principle possible in internally finned monoliths at realistic fluid velocities that are of interest for large-scale industrial applications. The main limitation to countercurrent flow at high velocities is at the outlet of a channel, rather than in the channel itself. With a suitable design of the outlet geometry, however, this problem can be alleviated so that countercurrent operation becomes possible in the velocity range of interest. 

Double Refraction of Flow. I. An Apparatus for the Study of Double Refraction of Flow at High Velocity Gradients. Rev. Sci. Insts. IS, 243 (1944). 

Fig. 3 Secondary flow in the cross-section of a curved tube (A) radial flow at moderate velocity, (B) radial flow at high velocity, and (C) pressure gradient between a maximum pressure at the outer wall and a minimum pressure at the inner wall.

Figure 1. Sketch of reactor configuration used for catalytic oxidation on monolith reactors at millisecond contact times. Gases slightly above atmospheric pressure flow at high velocities through porous ceramic monolifiis coated with Rh or Pt.

Table 7.5 does not give any clear information about critical velocities, but it indicates that such thresholds exist for the copper alloys in the velocity range represented in the table (1.2-8.2 m/s). More specifically, both Figure 7.46 and Table 7.6 show examples of critical velocities for erosion corrosion. The values are not absolute they depend on the composition of the environment, the temperature, geometrical conditions, the exposure history, the exact composition and treatment of the material etc. In connection with Figure 7.46 it can be mentioned that austenitic stainless steels show excellent resistance to erosion corrosion in pure liquid flow at high velocities, while some ferritic [7.42] and ferritic-austenitic steels are attacked less than the austenitic ones if the liquid carries solid particles. The data in Table 7.6 originate from work by Efird [7.43], who interpret his results as follows for each alloy in a certain environment, there exists a critical shear stress between the liquid and the material surface. When this shear stress is exceeded, surface films are removed and the corrosion rate increases markedly. 

In order to predict the direction of corrosion and mass transfer, it is essential to have data on thermodynamic properties of chemical compositions and steel components as a function of temperature. If the liquid metal is flowing at high velocity, the material is subject to erosion. Formation of the film (consisting of both steel and liquid metal coolant components) on the structural metal surface is another type of corrosion, since this is not protective film. Due to the difference in chemical activity between sodium and lead, technologies of these coolants are quite different, although some methods share a number of common features. 

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