Riser region

The reactor vessel contains the core, twelve once through steam generators, six canned rotor pumps at a high level in the vessel, and a control rod assembly in each of the 65 fuel assemblies. The top part of the vessel forms the pressuriser with its electric heaters. There is a passive spray system in the pressuriser which takes water from the riser region and sprays it into the steam space in the event of pressure rise in the core. The containment has a novel form of pressure suppression where the water for pressure control is contained in a tank farm connected to the reactor cavity by large diameter pipes. 

Figure 8 relates the flowline and riser conditions anticipated in Troll subsea oil wells to the possible hydrate formation region, defined by the experimentally... 

The inside of the upper region of a 4-cm rotor can be seen in Figure 10.6. One can see the holes of the underflow as they open into the riser that leads to the upper weir. The upper weir, which has been removed from the rotor body, is face up so that the vanes in the riser area are hidden. These riser vanes can be clearly seen... 

Consider a riser where the voidage in the bottom dense region is uniform and the top dilute region behaves as the freeboard of a dense-phase fluidized bed. The axial profile of the voidage in the top dilute region can be expressed by [Kunii and Levenspiel, 1990] (see 10.4.1)... 

Figure 10.18 shows the conceptual configuration of Bolton and Davidson s model. In the model, the flow structure is idealized as a dilute core region surrounded by a dense particle film which falls adjacent to the wall. The mathematical model for a circular riser can be based on the following assumptions. 

Particles are transported by turbulent diffusion from the core region to the surface of the falling film, where they are trapped and are carried downward. The implication of this assumption is that the net upward particle flow will decrease along the riser height in the core. 

Considering an element of dz in the riser, the mass balance of the solids in the core region gives... 

Example 10.3 A riser is of 0.15 m in diameter and 8 m in height. Particles with a mean diameter of 200 pm and a density of 384 kg/m3 are used in the riser, which operates at U — 2.21m/sand/p = 3.45kg/m2 -s. The gas used is air. For this operating condition, Davidson (1991) reported a particle downward velocity, npw, of 0.5 m/s and a particle downward flow rate, Ww, of 0.2 kg/s in the annular region. Assume that the solids volume fraction in the central core region, apc, is 0.015. Calculate the cross-sectionally averaged solids holdup and the decay constant, Kd, defined in Eq. (10.33) in terms of the core-annular model. 

As the gas velocity increases, the solids holdup decreases and, thus, hgc begins to become as important as hpc. In the center region of the riser, hgc is dominant, and its influence decreases with an increase in the solids holdup along the radial direction toward the wall. In the near-wall region, hpc dominates the heat transfer. The contribution of hpc decreases with a decrease in the particle concentration toward the bed center. As a result, a minimum value of h appears at r/R of about 0.5-0.8, as indicated in Fig. 12.16(b). 

With further decrease in the particle concentration at a > 0.93, hgc becomes dominant except at a region very close to the wall. Thus, the heat transfer coefficient decreases with increasing r/R in most parts of the riser, as shown in Fig. 12.16(c), exemplifying the same trend as the radial profile of the gas velocity. In the region near the wall hpc increases sharply, apparently as the effect of relatively high solids concentration in that region. 

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