Membrane walls composed of vertical tubes connected by fins, as shown in Figure , are commonly employed as heattransfer surfaces to remove heat from circulating-fluidized bed (CFB) combustors. The riser reactors are rectangular or square in cross section. The membrane tubes influence the dynamics of gas and particle flow in CFB risers. reported that particles were stripped from a flat smooth wall by upflowing gas more readily than from a membrane surface where downflowing particles appear to be protected in the fin region. Local heat-transfer coefficients near a membrane wall have been shown to be nonuniform by An- derson and Leckner (1992) and . Andersson and Leckner (1994) observed highly concentrated particle downflow with a long residence time in the fin area.
Despite the importance of the membrane wall geometry to CFB combustors, little research has been carried out with respect to the influence of the membrane wall geometry on the flow and voidage in CFB risers. To understand the heattransfer mechanism and erosion near the membrane wall, a better picture of local flow structure is needed. In this study, simulated membrane walls were installed in an experimental cold model CFB riser to investigate their influence on local hydrodynamics. Both voidage and particle velocity near the tubes were measured using separate fiber optic probes. Experimental results are compared with corresponding results for the same riser with smooth flat walls.