Further measurements of flow dynamics in a high-density circulating fluidized bed riser

Local voidages were determined for conditions corresponding to dense suspen sion upflow and fast fluidization in a riser of diameter 76.1 mm and height 6.1 m using a reflective-type optical fiber probe at superficial air veloc ities between 4 and 8 m/s and solids circulation fluxes up to 425 kp/m(2) s . High-density flow of cross-sectional mean volumetric solids concentration of about 0.2 +/- 0.05 was achieved in the riser with fluid catalytic crack ing (FCC) particles of mean diameter 70 mu m and density 1600 kg/m(3). Loca l time-mean voidages were nearly as low as epsilon(mf) at the wall and as h igh as 0.9 at the axis. As in our other recent work, solids refluxing near the wall of dilute CFB risers no longer existed for high-density conditions . Statistical analysis of local voidage fluctuations was used to characteri ze the local flow dynamics. The core behaves as a relatively uniform dilute flow, interspersed with infrequent particle structures. The number and den sity of these structures increases with radius, Maximum heterogeneity for h igh-density conditions occurs at some distance from the wall, unlike dilute risers where the flow is least uniform at the wall. intermittency index vs . local voidage plots are "bell-shaped". with a maximum where the local tim e-mean voidage is approximately 0.75. (C) 2000 Elsevier Science S.A. All ri ghts reserved.