A novel method based on image processing to visualize clusters in a rectangular circulating fluidized bed riser

• An image calibration method is developed to visualize clusters clearly.
• The calibration equipment and experiment are well-designed.
• A calibration equation correlates the solids holdup and the image grayscale.
• Clusters can be “peeled off” into different “layers”.
• Clusters size and solids holdup varies differently with increasing thresholds.

A new image processing method based on image calibration has been developed to visualize clusters in a Plexiglas rectangular circulating fluidized bed (CFB) riser at the fully developed region. A high speed video camera is adopted with FCC particles of 67 μm used as the bed material. In order to make sure day-to-day shooting images reliable and comparable under the same light illumination during the whole shooting period, a reference plate is made first to verify the light illumination consistency. After the verification, the relationship between solids holdup and image grayscale is calibrated in the same riser with both calibration curve and equation obtained. With given solids holdup thresholds increasing from 0.01 to 0.1, cluster size clearly decreases while cluster solids holdup increases by transforming gray images into binary ones. The variation of cluster profiles with different thresholds also reveals that there is a solids holdup gradient inside the cluster with a lower value at the surface and a higher one in the core. The change in dense phase holdup or cluster population with operating conditions is indicated by the variation of the black area under given solids holdup thresholds, which compare well with the well-known fact reported by many previous studies: higher Gs and lower Ug are beneficial for clusters with higher solids holdup to form, while lower Gs and higher Ug induce clusters to breakup. This consistency proves that the image processing method developed in the present study is effective and very useful.

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