Measurement of clustering flow by high resolution laser sheet technique and mathematical model development for large scale circulating fluidized beds.

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Measurement of clustering flow by high resolution laser sheet technique in a large scale circulating fluidized bed and numerical model developmentThe macroscopic flow structure of circulating fluidized beds including overall slip velocity and dense region voidage are closely related to meso-scale and microscale flow structures. From such a point of view, we have been studying the various aspect of cluster behavior in gas solid suspension structure. However, the three dimensional meso-scale structure of clustering suspensions has not been clarified yet.In this work the three dimensional flow structure of dilute gas solid suspensions in a circulating fluidized bed cold model (200mm riser diameter) was visualized by the scanning laser sheet technique. To compose the three dimensional image of gas solid suspensions the scanning laser sheet images were treated with a computer image processing technique, in particular, modification of weakened images of objects by reduced laser sheet illumin … More ation, two-level quantization, detection of boundary line and conneciton of images to obtain three dimensional surfaces. Shapes of clusters and gas channels were observed in detail. Clusters of roughly several centimeters still exist in the dilute suspension. In addition to the large gas channels, a complicated network of dilute and cluster phases was observed. From the images of horizontal cross sections at different height big gas drafts were seen around clusters and small channel exist in clusters.To quantify the shape change the fractal analysis was applied to the 3-D images of solid concentration.The clustering suspension has the fractal structure at any conditions tested. With the decrease in gas velocity and increase in solid circulation flux, fractal dimension increased indicating the formation of more complex 2 or 3 dimensional structures.A simple model is presented to predict the cluster size and the cluster voidage for a given set of operating conditions and suspension voidage. In this model a cluster is supposed to be suspended by both gas flows inside and outside the cluster, and the gas around the cluster is supposed to form bubble like gas pockets moving upward. Model predictions were successfully compared with the experimental data in the literature. Less