Study of channeling phenomena in fluidized foundry sands

摘要

In aggregative (fluidization with gas) fluidization, there are several abnormalities primarily related to the characteristics of the solid phase, the fluid phase and/or the design of the equipment. Channeling or cohesive flow is an abnormality in which large amounts of gas flows preferentially through paths in a sand bed. All previous investigations concerning channeling phenomena neglected the complexity of the flow structure. However the existence of the eddy flow considerably reduces the uniformity and efficiency of fluidized beds. In this paper, we report theoretical predictions and experimental results concerning the through channeling flow considering the eddy flow. Our analytical solution concentrates on Lavrentyev's model of separated flow. The model assumes that the flow of fluid is sheared into separated and potential regions. It postulates the existence of constant vorticity in the separated region. Between these regions there is an intermediate turbulent-eddy flow zone. Expressions for the pressure drop, the friction factor and the resistance coefficient as functions of air velocity and the Reynolds number have been developed for channeling flow in fluidized beds based on the above mentioned model. Experiments were run in a fluidization chamber of 70 mm internal diameter and 1829 mm length with foundry sand of 2.593 g/cc density and 30-270 mesh size. A pressure drop-flow rate diagram is presented for the channeling flow regime. Experimental data obtained for the friction factor and the resistance coefficient as functions of the Reynolds number compare well with theoretical predictions.