Bubbles cloudless

In large particle beds H < Ue Slow bubble Cloudless bubble... 

Most bubbles in gas-solid fluidized beds are of spherical cap or ellipsoidal cap shape. Configurations of two basic types of bubbles, fast bubble (clouded bubble) and slow bubble (cloudless bubble), are schematically depicted in Fig. 9.7. The cloud is the region established... 

Figure 9.7. Bubble configurations and gas flow patterns around a bubble in gas-solid fluidized beds (a) Fast bubble (clouded bubble) Ub > /mf/ mf (b) Slow bubble (cloudless bubble)...

D 1,000 coarse e.g., wheat can be spouted mix poorly when fluidized appreciable particle attrition rapid elutriation of fines relatively sticky materials can be fluidized bubbles cloudless slow bubbles bubble velocity less than interstitial gas velocity... 

Data Since this is a large-particle fluidized bed containing cloudless bubbles, assume plug flow of gas through the unit. Also assume that the volume of gases released by the solids is small compared to the volume of carrier gas passing through the bed. 

The gas flow through the bubbles depends on whether the bubble rise velocity, 0.7l/g d (29) is slow or fast relative to the gas velocity in the emulsion phase (umf/emf). When the bubble rise velocity exceeds the emulsion velocity, the bubbles are said to be fast or clouded and the gas circulates through a cloud surrounding the bubble as shown schematically in Figure 5 (adapted from Ref. 32). When the emulsion velocity is fast relative to the bubble rise velocity the bubbles are said to be slow or cloudless and the emulsion gas uses the bubble as a by-pass (see Figure 5). The transition from slow to fast bubble depends upon the... 

It is seen that the Nusselt numbers for BFBs fall below those for convection from a single sphere, for Reynolds numbers less than 20. In fact, the magnitude of Nup for fluidized beds drops below the value of 2.0, which represents the lower limit of conduction heat transfer. The cause of this is the bubbling phenomenon. Low Reynolds numbers correspond to beds of fine particles (small flip and C/g), wherein bubbles tend to be clouded with entrained particles. This diminishes the efficiency of particle-gas contact below that represented by idealized plug flow, resulting in reduced values of Nup. As particle diameter increases (coarse particle beds), bubbles are relatively cloudless and gas particle contact improves. This is shown in Fig. 2 where the Nusselt numbers of fluidized beds are seen to increase with... 

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