Circulating fluidized beds (CFB) have been applied extensively in industry for catalytic cracking of oil, coal combustion and gasification, alumina calcination and others (for example, Reh, 1986; Contractor and Chaouki, 1990). A CFB can be operated at different flow regimes from turbulent and fast fluidization to dilute transport. This work examines the nature of the existence of the turbulent regime and its transition, and provides a unified interpretation of this transition based on existing definitions of the turbulent and transport regimes and available experimental data covering a wide range of operating conditions.
Transport Velocity, U,,
In a CFB, by varying the solids circulation rate at a given gas velocity, a critical solids circulating rate may exist where a sharp change in the solids holdup gradient takes place at the lower part of the bed. As the gas velocity increases beyond a certain point, such a sharp change in the solids holdup gradient disappears. The gas velocity at this critical point is known as the transport velocity U, , which earmarks the onset of fast fluidization regime (for example, Horio, 1990). In addition to pressure drop measurement, the transport velocity can be determined by other methods such as saturation carrying capacity (Chen et al.,