Image Based Measurement of Particle Phase Reynolds Stresses in a Laboratory Scale Circulating Fluidized Bed

To fully account for the complicated flow patterns of the dense gas-solid suspension in a circulating fluidized bed (CFB) fluid dynamic simulations of CFBs are typically conducted in the transient mode. The computational mesh in these simulations must be reasonably fine, which in the case of large industrial processes is not feasible. A better approach for large processes seems to be time-averaged modeling facilitating steady-state simulation of fluidization. Time-averaging of the transport equations creates fluctuation terms that require modeling. Of these terms the Reynolds stress terms usually are among the largest terms to be modeled. In the present paper, experiments were carried out at a laboratory scale pseudo-2D CFB at Åbo Akademi University, Finland. Image based measurement methods were used to simultaneously determine particle velocities and volume fraction. Backlight illuminated shadowgraphy was the imaging method of choice. From the images, particle velocities were measured with Particle Image Velocimetry (PIV). The sub-pixel accuracy of the PIV-algorithm with the large particle images was verified with synthetic images. The local instantaneous particle volume fraction was determined from the gray-scale value of the shadowgraphy images with a correlation suggested by Grasa and Abanades (2001). The simultaneous measurement of particle velocities and volume fraction allows calculation of particle phase Reynolds stresses and volume fraction weighted average velocities. The results are presented at different particle sizes and fluidization velocities in the present paper.