Fluid-dominated regime

The FD region at the top is characterized by the dominance of the fluid over the movement of particles, as already shown in Fig. 11. When the fluid-dominated FD regime is first formed, the clusters of fast fluidization are disintegrated to form an essentially one-phase structure in which the particles are, however, not completely discretely suspended, that is, at a much higher concentration as compared to the ef computed for the broth before Upt. This is shown by the fluctuating voidage considerably above zero, as can be seen in the upper right-hand side of Fig. 11. 

According to the degree of uniformity of the system, the fluid-dominated FD-regime can be divided into two subregions dilute-phase transport for real systems and idealized dilute-phase transport. The transition between the two subregimes occurring at the value of emM. 

Although this mechanism is consistent with available data, a full test of the theory is still lacking. Also, some ER fluids don t seem to show a minimum in viscosity over the range of frequencies tested. Frequency dependencies can also occur because of a crossover to a conduction-dominated regime at low frequencies, and this was not included in the See-Doi calculations. 

Measurement of the hotness or coldness of a body or fluid is commonplace in the process industries. Temperature-measuring devices utilize systems with properties that vaiy with temperature in a simple, reproducible manner and thus can be cahbrated against known references (sometimes called secondaiy thermometers). The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers and are applicable over different temperature regimes. 

Because of the closeness of the low-velocity regime to the 100% quality line, it is to be expected that latent heat will be the dominant fluid property controlling burn-out in this regime. In fact, it has been found (M3) that latent heat alone can adequately represent all the low-velocity regime data, which includes pressures ranging from 15 to 2000 psia. For uniformly heated round tubes, for example, the appropriate burn-out equation obtained, based on Eq. (18), is ... 

Rotational fluid velocities are calculated since horizontal (rotational) flow prevails in the hydrodynamic regime within the dissolution vessels. Thus, the overall hydrodynamics and hence dissolution is dominated by the substantially higher rotational (tangential) fluid velocities. 

The fu st term is a modified Archimedes number, while the second one is the Froude number based on particle size. Alternatively, the first term can be substituted by the Reynolds number. To attain complete similar behavior between a hot bed and a model at ambient conditions, the value of each nondimensional parameter must be the same for the two beds. When all the independent nondimensional parameters are set, the dependent parameters of the bed are fixed. The dependent parameters include the fluid and particle velocities throughout the bed, pressure distribution, voidage distribution of the bed, and the bubble size and distribution (Glicksman, 1984). In the region of low Reynolds number, where viscous forces dominate over inertial forces, the ratio of gas-to-solid density does not need to be matched, except for beds operating near the slugging regime. 

When external gradients correspond to substantial differences in concentration or temperature between the bulk of the fluid and the external surface of the catalyst particle, the rate of reaction at the surface is significantly different from that which would prevail if the concentration or temperature at the surface were equal to that in the bulk of the fluid. The catalytic reaction is then said to be influenced by external mass or heat transfer, respectively, and, when this influence is the dominant one, the rate corresponds to a regime of external mass or heat transfer. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

In cocurrent-up fluid-particle two-phase flow, the fluid tends to choose an upward path with minimal resistance, while the particles tend to array themselves with minimal potential energy. Stability of the two-phase system calls for mutual coordination, as much as possible, between the fluid and the particles in following their respective tendencies. This applies for all the three broad regimes of operation fixed bed, fluidization and transport. When neither the fluid nor the particles can dominate the system, either has to compromise and yield its intrinsic tendencies to those of the other to reach a stable state. However, if the system is fully dominated by either the fluid or the particles, the intrinsic tendency of the dominant one will be satisfied exclusively, with full suppression of that of the other. 

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