Influence of Bed Diameter on Circulating Fluidized Beds

The thickness of the downflowing layers at the wall of the CFB is typically defined as the distance from the wall to the position of zero vertical solid flux. Measurements of the layer thickness were made on a 12 MW and 165 MW CFB boiler by Zhang, Johnsson and Leckner (1995). They found that the thickness increased for the larger bed. They related data from many different beds (Fig. 19), with the equivalent bed diameter, taken as the hydraulic diameter, using the following form 

The thickness, 8, was found to be insensitive to particle concentration, gas velocity and height within the furnace. That suggests that the thickness results from a balance of solids internal circulation which is generally much higher than netthroughflow. If the local solids flux profile, 

Patience et al. (1992) developed a dimensionless correlation for the mean slip factor between the gas and solid by using solid suspension data from various small laboratory beds. The proposed correlation relates the slip to the Froude number based on the bed diameter. It remains to be seen if the correlation will hold at Froude numbers typical of large beds and if other dimensionless factors are important. 

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