Visual investigations on radial solid holdup in vapor-liquid-solid fluidized bed evaporator with a CCD measuring system

In previous visual investigations on the hydrodynamic and heat-transfer characteristics in an external natural circulating evaporator with vapor-liquid-solid (V-L-S) boiling flow, a charge-coupled device (CCD) measuring system with a white light source was developed (Liu et al., 2004, studies on the hydrodynamic and heat transfer in a vapor-liquid-solid flow boiling system with a CCD measuring technique. Chemical Engineering Science 59(4), 889). In this approach, a three-dimensional flow field was transformed into a twodimensional (2D) image and was recorded, and the holdup and the axial linear velocity distributions of solid particles in an axial location in a heated tube were estimated. However, the radial distribution of solid holdup has not been evaluated because the space position of each particle in a 2D image measured under an incandescent lamp light source is difficult to determine even though the particle overlapping does not appear. Hence, a CCD measuring system combined with a laser sheeting technique was chosen and developed to examine the solid holdup profile in the radial direction in a V-L-S fluidized bed evaporator by taking images of 2D flow fields in this work. The principles and steps are briefly elucidated and typical results are given. It is shown that the solid holdups in the radial direction in a heated tube are not uniform and the profiles vary with the flow regimes under the present experimental conditions. The solid holdups in the axis center are higher than those near the wall of the heated tube with the V-L-S boiling flow regime, while the opposite is true for the L-S flow regime. Asymmetric distributions of solid holdups in the radial direction are also obtained. Some likely qualitative explanations and demonstrations on experimental facts observed are given. This work may provide some valuable references for academic descriptions and industrial applications of such an installation.