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FIGURE 12.15 Influence of riser diameter (from Ranade, 1999).

profiles for the small diameter riser (0.06 m) and for larger diameter risers (0.3 and 1.0 m). For the small diameter riser, pronounced wall peaking was predicted even in the absence of cluster corrections. For the large diameter risers, the model predicts a qualitatively different profile with minima in solids hold-up near the wall. As the riser diameter increases, the location of the maximum in the predicted solids flux profile shifts towards the riser wall. In view of the significant influence of riser diameter on the characteristics of gas-solid flows in risers (especially on solids granular temperature), it is inappropriate to use empirical cluster corrections derived by fitting the experimental data obtained in a smaller diameter riser. The formation of clusters and the role of riser diameter in cluster formation need to be studied in detail to develop industrially useful models (Sunderesan, 2000). Simulations using DPMs may be useful in this regard.

Additional numerical experiments were carried out for three values of particle diameters and two values of solids density. These results are shown in Fig. 12.16a

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