Visual Investigation On The Vapor-Liquid-Solid Three-phase Flow With CCD

The vapor-liquid-solid (V-L-S) circulating fluidized bed heat exchanger can effectively enhance heat transfer and prevent and remove the fouling, and has been widely applied in many industrial process. However, the mechanisms of flow and heat transfer are still not well understood due to the system complex and the limitation of the measuring methods due to the opaque characteristic of the system in practice. In this paper, a CCD measuring system and techniques of image dealing are used to carry out a visual investigation on a vapor-liquid-solid three-phase natural circulating flow boiling system.Based on summarizing the development and research situations of techniques of enhancing heat transfer and preventing and removing the foulings for multi-phase flow, measuring means and method of image dealing. The idea of this paper is given.A set of new type vapor-liquid-solid three-phase natural circulating flow boiling system, whose flow-up bed and flow-down bed are glasses plated with transparent electrically heating film, is established as experiment apparatus. Flow characteristics, distributions of suspensions and characteristics of heat transfer in the V-L-S system are investigated by using the CCD measuring system, software of image dealing developed by own-self, system of laser sheet and other hardware system. The results show that the axial distributions of solid holdup occupy the character of fluctuation. Solid holdup increases with the increase of the amount of solid particles and the increase of heat flux when it is relatively low, but decreases when heat flux is relatively high. The radial distribution rule in liquid-solid two-phase region is that radial solid holdup is greater near the wall than in the center of the heating tube, which is contrary to that in V-L-S three-phase region. Axial velocity of solid particles increases with the increase of heat flux, whose distributions also occupy the character of fluctuation. However, the amount of solid particles influences axial velocity of solid particles on many aspects. Axial and radial holdups of solid particles and radial velocity of solid particles are all fluctuating with time. Heat transfer coefficient of V-L-S three-phase flow increases with the increase of heat flux and the additive amount of solid particles, which is 1.5 or 3.5 times larger than that of vapor-liquid flow. In addition, the changes of holdups of liquid and vapor and velocity of bubble with the operation parameters are discussed in this paper. A flow boiling heat transfer model of the V-L-S three-phase flow incorporating the asymptotic model and the mechanism of surface renewal is proposed here. The values calculated by this model agree well with the experimental data.