Visualization of flow structures inside a circulating fluidized bed by means of laser sheet and image processing

In the present paper, an attempt has been made to visualize local flow stru ctures inside the upper dilute zone of a pilot-scale circulating fluidized bed (CFB) under realistic flow conditions, i.e. with cross-sectional averag e solids volume concentrations ranging from 0.1 to 1 vol.%. The high-speed video technique in combination with the laser sheet techniqu e has been used. Since it is not possible to penetrate from outside with a laser sheet into the flow, an endoscope has been used to guide the laser li ght into the fluidized bed. Particles or loosely packed agglomerates passin g the illuminated light sheet area reflect the laser light. This provides t he possibility for an undisturbed observation of the local flow pattern via the camera endoscope. To understand the governing optical effects, a model has been developed which is able to estimate the influences of the solids properties and the operating conditions on the ratio of the received light intensity to the emitted one. Digital image processing was successfully applied to pre-process the obtain ed image sequences and to gain a deeper insight into the observed flow stru ctures. Using specific image processing methods, shapes and velocities of t he flow structure were calculated. (C) 2001 Elsevier Science S.A. All right s reserved.