Local suspension density

The data of Fig. 20 also point out an interesting phenomenon—while the heat transfer coefficients at bed wall and bed centerline both correlate with suspension density, their correlations are quantitatively different. This strongly suggests that the cross-sectional solid concentration is an important, but not primary parameter. Dou et al. speculated that the difference may be attributed to variations in the local solid concentration across the diameter of the fast fluidized bed. They show that when the cross-sectional averaged density is modified by an empirical radial distribution to obtain local suspension densities, the heat transfer coefficient indeed than correlates as a single function with local suspension density. This is shown in Fig. 21 where the two sets of data for different radial positions now correlate as a single function with local mixture density. The conclusion is That the convective heat transfer coefficient for surfaces in a fast fluidized bed is determined primarily by the local two-phase mixture density (solid concentration) at the location of that surface, for any given type of particle. The early observed parametric effects of elevation, gas velocity, solid mass flux, and radial position are all secondary to this primary functional dependence. 

Figure 21. Significance of local suspension density for heat transfer in fast fluidized beds. (Data of Don, Herb, Tuzla and Chen, 1991.)...

Initial studies of circulating fluidised bed heat transfer show the importance of the solids in the heat transfer process. Kobro and Brereton (1986) presented results for CFBC sized solids at intermediate and combustion temperatures. These results suggest an approximately linear dependence of the heat transfer coefficient for a short annular calorimetric section upon the local suspension density. These results appear in Figure 16. Simple correlations for heat transfer coefficients may be based upon this type of data, with radiation accounting for most of the difference in heat transfer coefficient at elevated and ambient temperatures (Wu et al, 1989a). Assuming that the gas radiates as a grey cloud accounts for the radiation contribution adequately. 

The experiments of Dou et al. (1991) also indicate that the heat transfer coefficient varied with radial position across the bed, even for a given cross-sectional-averaged suspension density. Their data, as shown in Fig. 20, clearly indicate that the heat transfer coefficient at the bed wall is significantly higher than that for vertical surfaces at the centerline of the bed, over the entire range of suspension densities tested. Almost certainly, this parametric effect can be attributed to radial variations in local solid concentration which tends to be high at the bed wall and low at the bed centerline. 

Crystal size too small low suspension density/high circulation rate/solids in feed causing nucleation sites/feed flowrate > design/excessive turbulence/local cold spots/subsurface boiling/supersaturation too high or too dose to the metastable limit. 

Results from Yang et al. s study are in good agreement with other work gas velocity in the core region increases with solids circulation rate it decreases near the wall and, gas wall velocities are zero [101]. Other optical techniques, used to measure solids velocities, also may be applied to infer gas velocities. For example, in the core region suspension densities are low and particles may be reasonably well-dispersed therefore, the slip velocity is approximately equal to the particle terminal velocity. The measured particle velocity in a dispersed suspension of solids then can be readily translated into a local gas velocity. 

Bulk Suspension Temperature The heat transfer coefficient increases with bulk temperature (e.g., see Wu et al., 1989 Basu, 1990 Golriz and Sunden, 1994a,b), due to both the higher gas thermal conductivity and the increased radiation. At temperatures > 500° C, especially for relatively dilute beds (suspension densities <15 kg/m ), where radiation tends to be the dominant mode of transfer, the increase in heat transfer with temperature can be very significant. Figure 29 shows the local heat transfer coefficient along a 1.5 m long heat transfer surface for two bulk temperatures (Wu et al., 1989). The 30 0% in-... 

Figure 19. Variation of the instantaneous local heat transfer coefficient and the point voidage with time on the wall of the cold model circulating fluidised bed combustor. The signals are seen to be strongly cross-correlated. The cross sectionally area averaged suspension densities are from top to bottom, 46.7 kg/m3, 32.0 kg/m3,15.3 kg/m3, (Wu et al, 1991).

L ir in free suspension in moving water, no limit, local effects under high current density may increase wastage rate M May be used in the environment under special circumstances N High consumption rate in this environment... 

The subject of main interest in the present study is the layering phenomenon in films that are formed from like-charged particles confined between two uncharged surfaces. In the case when confining surfaces are parallel, the particle layering is characterized by a local density distribution p(z) across the slit. Two kinds of films in a plane-parallel slit can be distingushed. The first one is that formed from the macroion suspension adsorbed into a slit of the fixed thickness. The other kind of film can be formed in the case when slit surfaces are movable. 

In nature, free-living amoebae phagocytose bacteria and divide every 4h. In the laboratory, the amoebae can live on bacterial lawns, suspensions, in a simple axenic broth and even on a completely defined medium. It is quite easy to grow >10" cells per day for biochemical analysis or development. When cells deplete the local food supply and reach a critical density, they begin a complex developmental program [1]. 

Measurements of local instantaneous velocity, density, and mass flow of phases of a gas-solid suspension are needed in determining transport properties, validating theoretical predictions, and formulating design procedures. 

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