Axial gas velocity

Moujaes, S. F., and R. S. Dougall, 1990, Experimental Measurements of Local Axial Gas Velocity and Void Fraction in Simulated PWR Steam Generator Rod Bundles, Can. J. Chem. Eng 68 211.(3) Moxon, D., and P. A. Edwards, 1967, Dryout during Flow and Power Transients, UK Rep. AEEW-R-553, UK AEEW, Harwell, England. (5)... 

U2 Gas velocity component immediately behind the shock Ww Axial gas velocity in the wall region... 

At higher gas velocities in the channels, the effective intrabed diffusivity increases substantially, reaching values as high as or even higher than the diffusivity in the bulk gas. This enhancement of intrabed diffusivity cannot be ascribed solely to parallel flow as calculated by the Eigun equation, since a 2.5-9-times-higher axial gas velocity is needed to account for the experimentally found enhancement of the intrabed diffusivity. The substantial enhancement must be attributed to momentum transport through the screen [6]. 

For turbulent flow the diffusivity D is a function of the velocity gradients, hence of the lateral position for laminar flow it is the constant free molecular diffusivity Di. The axial gas velocity follows from the momentum balance over a small element of the fluid domain, as discussed in the previous section. 

However, concerning the influence of a solid phase on the radial distribution of the axial gas velocity, the reported findings are matching The overall... 

Figs 8.12 and 8.13 show simulated and experimental results with superficial gas velocity, Vg = 8 (cm/s). Fig 8.12 shows the axial and radial liquid velocity components, the axial gas velocity component and the gas fraction 2.0 (m) above the column inlet. Fig 8.13 shows the number density in each class 2.0 (m) above the inlet. 

Effective dispersion is affected by both axial and radial dispersion, as well as by Taylor dispersion due to the radial profile of axial gas velocities. Schiigerl (1967) related the effective dispersion coefficient, Dge, axial dispersion coefficient, Dg, radial dispersion coefficient, Z)gr, and the radial velocity profile by... 

Secondly, the gas flow, and therefore the particle flow or vector in turbulent flow, is highly Irregular due to three-dimesional flow fluctuations. Further, the magnitude of the instantaneous velocity fluctuations in the y- and x-directions are easily 10-30% of the fluctuating axial gas velocity Such velocity fluctuations, especially in the y-direction mask the small y-directional migration velocity due to the electrical field. Only if such turbulent-flow fluctuations bring the small particle close to the collector plates does the electrical field take over and the particle is captured, since the gas flow velocities are much smaller in the wall region. 

Fig. 7. Axial density profiles in the (—) bubbling, (------) turbulent, and (----) fast and ( ) riser circulating fluidization regimes. Typical gas velocities for...

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Siemes and Weiss (SI4) investigated axial mixing of the liquid phase in a two-phase bubble-column with no net liquid flow. Column diameter was 42 mm and the height of the liquid layer 1400 mm at zero gas flow. Water and air were the fluid media. The experiments were carried out by the injection of a pulse of electrolyte solution at one position in the bed and measurement of the concentration as a function of time at another position. The mixing phenomenon was treated mathematically as a diffusion process. Diffusion coefficients increased markedly with increasing gas velocity, from about 2 cm2/sec at a superficial gas velocity of 1 cm/sec to from 30 to 70 cm2/sec at a velocity of 7 cm/sec. The diffusion coefficient also varied with bubble size, and thus, because of coalescence, with distance from the gas distributor. 

Measurements of axial mixing in a hydrogen-a-methylstyrene bubble-column have been reported by Farkas (FI). Measurements of axial heat diffusion were carried out, the diffusion coefficient being of the order of 7.5 cm2/sec at a nominal gas velocity of about 4 cm/sec, a value that is in good agreement with some of the results of Siemes and Weiss. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

The interaction of parametric effects of solid mass flux and axial location is illustrated by the data of Dou et al. (1991), shown in Fig. 19. These authors measured the heat transfer coefficient on the surface of a vertical tube suspended within the fast fluidized bed at different elevations. The data of Fig. 19 show that for a given size particle, at a given superficial gas velocity, the heat transfer coefficient consistently decreases with elevation along the bed for any given solid mass flux Gs. At a given elevation position, the heat transfer coefficient consistently increases with increasing solid mass flux at the highest elevation of 6.5 m, where hydrodynamic conditions are most likely to be fully developed, it is seen that the heat transfer coefficient increases by approximately 50% as Gv increased from 30 to 50 kg/rrfs. 

Figure 29. Calculated axial and radial gas velocity profiles for Run GJ18.

Figure 35. Typical axial tracer concentration profiles—tracer gas injected via air tube at air tube gas velocity of 31 m/s.

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