Wakes wall effects

Table 5.2 gives a new correlation, based on a critical examination of available data for spheres (N6). Results in which wall effects, compressibility effects, noncontinuum effects, support interference, etc. are significant have been excluded. The whole range of Re has been divided into 10 sub intervals, with a distinct correlation for each interval. Adjacent equations for match within 1% at the boundaries between sub intervals, but the piecewise fit shows slight gradient discontinuities there. The Re = 20 boundary corresponds to onset of wake formation as discussed above, the remaining boundaries being chosen for convenience. 

Wall Effect on Wake Volume for Large Bubbles in Viscous Liquids (Bl)"... 

So far, the influence of bubble wake on mean bubble velocity i b relative to the column wall has not been mentioned, since Eq. (5-3) has been formulated on the basis of uy,o, which already includes the effect of the wake (although it lacks a correction for wake fraction). In bubbling-bed models (FIO, F12, K24, L5, S18) an upward flow of solid carried by the bubbles and bubble wakes leads to a downflow of solid (that has been assumed uniform) in the remainder of the bed. Then the bubble velocity b relative to bed wall should be smaller than the slip velocity of the bubble Ms relative to emulsion, since the bubble phase is retarded by downflow of... 

Botros and Brzustowski [77] studied the velocity field of propane TDFCF experimentally and numerically. Their study revealed a pair of counter-rotating vortices in the flame. Gollahalli and coworkers [78-80] measured the flow field and turbulent characteristics of gas jet flames in crossflow. At very low values of R, the effects of crossflow stream are more dominant and the jet fluid burns in the wake of a model stack. A recirculation vortex is created and the flame stabilizes on the wall of the recirculation bubble. Figure 29.17a presents the velocity vector field and streamlines obtained by Huang and Wang [45] for down-wash... 

Oliver (02) observed that a sphere settling slowly near the wall of a tube in an otherwise stagnant fluid moved inwardly, towards the tube axis. Karnis (K4a, K5b), in a series of more detailed measurements, made simitar observations and followed the motion of the sphere all the way to the tube axis. It is on the basis of these observations that F, iP) is concluded to be negative. This conclusion accords with Oseen s (04) theoretical finding [also summarized in Berker (B4, p. 328)1 that, when small inertial effects are considered, a sphere moving parallel to a plane wall in a semi-infinite fluid experiences a repulsive hydrodynamic force, due to the sourcelike behavior of the flow at points distant from the sphere not lying within the wake [cf. Lamb (L5, p. 613)]. 

The liquid flow pattern in a bubble column of 15 cm diameter was investigated by Kojima et al. [63] at superficial liquid and gas velocites of the order of 1 cm/s. The liquid flow was found to be complicated and to vary continuously with time. The flow was mainly upward in the central part, but in other parts both upward and downward, although mainly downward near the wall. Radial flow was present across the entire cross section as a result of the pumping effect of the wakes of bubble swarms when the volume of liquid pumped per unit time by the wakes of the bubbles exceeds that of the liquid fed, downward flow has to occur, inducing radial flow. These observations confirm those of Towell and Ackerman [64] who, in addition, expressed the mixing in terms of axial effective diffusivities for the... 

Wear (also called wastage and erosion) of surfaces is a serious operational issue in some fluidized-bed reactors. Wear occurs when hard particles (e.g., silica-supported catalyst particles) continually strike fixed surfaces such as heat transfer tubes, reactor walls, or cyclone inner surfaces. The most damaging collisions tend to be those which are oblique (e.g., at 60 ) to the surface, for example, at about the 5 and 7 o clock positions, when bubble wakes slam into the underside of horizontal heat transfer tubes. If corrosion is also a factor, then the combined damage from erosion and corrosion can be considerably more extensive than estimated from the summation of the individual effects. 

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