Velocity wake region

Zirconium alloys, especially Zircaloy-2, have shown extremely good corrosion resistance relative to other materials. Fairly severe attack in a high-velocity wake region by a very abrasive thoria slurry has been noted. 

It is possible that ambient room air can reach the critical region in the bench due to created wakes. Such a wake region is depicted in Fig. 10.60 tor a hon-zontal flow bench with small bottles and an air velocity of 0.45 m s . ... 

Bhaga (B3) determined the fluid motion in wakes using hydrogen bubble tracers. Closed wakes were shown to contain a toroidal vortex with its core in the horizontal plane where the wake has its widest cross section. The core diameter is about 70% of the maximum wake diameter, similar to a Hill s spherical vortex. When the base of the fluid particle is indented, the toroidal motion extends into the indentation. Liquid within the closed wake moves considerably more slowly relative to the drop or bubble than the terminal velocity Uj, If a skirt forms, the basic toroidal motion in the wake is still present (see Fig. 8.5), but the strength of the vortex is reduced. Momentum considerations require that there be a velocity defect behind closed wakes and this accounts for the tail observed by some workers (S5). Crabtree and Bridgwater (C8) and Bhaga (B3) measured the velocity decay and drift in the far wake region. 

Figure 16 An example of PIV/LIF/SIT taken turbulent velocity fields in the wake region of two bubbles together with the bubble shadows at four instants (Tokuhiro...

The structure of wakes behind the gas bubbles affects several aspects (such as holdup, gas-liquid mass transfer, etc.) of three-phase fluidized-bed behavior. The magnitude and composition of such wakes are still not known with any certainty. Wake holdups have been estimated from experimental measurements of gas and solid holdups. It is commonly assumed that the bed can be divided into three regions a liquid fluidized region, a gas-bubble region, and a bubble-wake region and that the bubbles and their wakes travel at the same velocity. Different investigators have, however, assumed different values of hws the ratio of solids holdup in the wake to the solids holdup in the liquid fluidized region. Different methods have been used to calculate wake holdups from the experimental... 

COLOR FIGURE 3.3 Velocities in the wake region for a cylindrical vegetative canopy with a 5 m ground clearance immersed in a logarithmic upstream velocity profile with effective foliage element diameter = 0.001 m and a porosity of 0.99995. 

For outer region ylh > 0.2), it is generally believed that velocity deviates from log-law distribution and wake region exists, its velocity distribution can be expressed as follows ... 

The equation for turbulent dispersion was based on the classical development of Chen and Middleman (1967) (see Section 12-2), with the energy dissipation term calculated for drag on a cylinder. Two cases were assumed for the dissipation volume in the wake region behind the cylindrical impeller blade. The first was that an eddy length proportional to the cylinder diameter determined the dissipation volume. The second was that this volume was proportional to the velocity of the cylinder (tip speed) and a characteristic eddy decay time. Equation (12-74) results from the second case. It showed reasonable agreement with data taken at higher speeds. 

Interestingly, the shape of the wake is similar to that developed behind a hypersonic blunt body where the flow converges to form a narrow recompression neck region several body diameters downstream of the rear stagnation point due to strong lateral pressure gradients. The liquid material, that is continuously stripped off from the droplet surface, is accelerated almost instantaneously to the particle velocity behind the wave front and follows the streamline pattern of the wake, suggesting that the droplet is reduced to a fine micromist. 

Ranade and Joshi (1987) have developed a criterion for small bubbles. The small bubbles rise upward without any oscillations. The liquid carried upward in the bubble wakes is released at the top liquid surface, which then flows downward in the bubble-free region. The downward liquid flow hinders the bubble rise. It was proposed that the transition will occur when the bubble rise velocity equals the downward liquid velocity. Under this condition, the bubble rise velocity with respect to the column wall is zero and the gas phase accumulates in the column, leading to transition. 

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