Study On Simulations Of Gas-liquid-solid Flow In Microchannel And Experiments Of Liquid-solid Fluidization In Mini Fluidized Bed

The transport phenomena and mixing process can be intensified obviously inmultiphase flow at micro-and miniscale, for the large specific surface area betweenphases. Hence, microscale multiphase flow is an important part of micro chemicaltechnology. For the present study of microscale multiphase flow, it mainly focused ongas-liquid and liquid-liquid two immiscible phase flow. Oppositively, fewerresearches concerned other microscale multiphase processes. Based on the existinginvestigation results, author simulates flow and backmixing behaviors ofgas-liquid-solid flow at dilute particles in microchannels and experimental studiesliquid-solid expansion fluidization in mini fluidized beds, in order to provide basis forinvestigation of gas-liquid-solid fluidization in mini and micro fluidized beds.3D gas-liquid-solid flow and backmixing behaviors at dilute particles (particlehold-up ranges from0.3%~0.8%) in microchannels, with operation conditons(superficial gas and liquid-solid mixture velocities) which ranges from0.17~0.85m/s,were simulated by CFD-VOF-DPM method at Fluent software(6.3.26), in addition,simulations were validated against observations. Slug flow is the major flow patternof three-phase flow, the detachment time and length of gas slug are shortened inliquid-solid mixture compared with those in pure liquid due to higher superficialviscosity of liquid-solid mixture, which will move the bubble formation mechanismfrom squeezing regime towards dripping regime. Solid particles mainly distribute inliquid slug and particle flow shows obvious periodicity in both temporal and spatialscale. With the increase of contact angle of the inner wall, gas slug (0°~50°), stratified(77°~120°) and liquid drop (160°) flows are observed. Based on the analysis ofdimensionless variance of residence time, it is found that multi peaks appears on theresiendence timed distribution curves of slug and liqui-drop flows at typical operationconditions, i.e., the ratio of superficial gas velocity to superficial liquid-solid mixutreis5:1. Meanwhile, the backmixing of liquid-solid mixture in liquid drop flow is theweakest among the three flow patterns, and the backmixing of gas phase in slug flowis weaker than that in both stratified and liquid drop flows. In slug flows, if the ratiobetween gas and liquid-solid mixture decreases, the backmixing of both gas andliquid-solid mixture will be weakened.For liquid-solid expansion fluidization in mini fluidized beds, visible experimentswere conducted to study the effect of bed diameter(3.15×10-3m and1.1×10-2m),particle size ((80~200)×10-6m) and its size distribution. In a mini fluidized with hydraulic diameter of3.15×10-3m，when particle diameter is bigger than80×10-6m,the minimum fluidization velocity of particles is bigger than the theoretical valuewhich is deduced from Ergun equation and Wen-Yu drag coefficeient, i.e., wall effectexists. If wall effect exists, the particle size distribution will influence theRichardson-Zaki correlation obviously. A narrow particle size distribution will makethe R-Z exponent larger than the theoretical value, however, a wide particle sizedistribution will result in a smaller one.