An alternative method to quantify solids phase separation in a narrow rectangular CFB riser

In this study, a newly developed image calibration method is applied to "visualize" gas-solid flow under different operating conditions in a 19 mm × 114 mm narrow rectangular riser through a high-speed video camera. For the first time, a one-to-one correspondence between image grayscale and solids holdup of the bed material is achieved in a specially designed calibration column. By transforming the original images into Hue, Saturation and Value (HSV) images, the solids holdup distribution under a certain operating condition can be clearly visualized. With the calibration equation between the image grayscale and the solids holdup, grayscale images can be transformed into binary images with different solids holdup thresholds, through which solids phase with different solids holdup can be "peeled off". The change of the dense phase is expressed by the term "relative dense phase area". By carefully examining the variation profiles of the relative dense phase area with solids holdup thresholds, a critical solids holdup value of ε_s = 0.04 is chosen to demarcate the dilute and dense cluster phases. With images divided into three regions along the lateral direction, it is found that cluster fraction at the wall region is higher than those of the core and the middle regions. The cluster fraction is also found to increase with the mean solids holdup value.