Particle pressures generated around bubbles in gas-fluidized beds

Abstract

The particle pressure is the surface force in a particle/fluid mixture that is exerted solely by the particle phase. This paper presents measurements of the particle pressure on the faces of a two-dimensional gas-fluidized bed and gives insight into the mechanisms by which bubbles generate particle pressure. The particle pressure is measured by a specially designed ‘particle pressure transducer’. The results show that, around single bubbles, the most significant particle pressures are generated below and to the sides of the bubble and that these particle pressures steadily increase and reach a maximum value at bubble eruption. The dominant mechanism appears to be defluidization of material in the particle phase that results from the bubble attracting fluidizing gas away from the surrounding material; the surrounding material is no longer supported by the gas flow and can only be supported across interparticle contacts which results in the observed particle pressures. The contribution of particle motion to particle pressure generation is insignificant.

The magnitude of the particle pressure below a single bubble in a gas-fluidized bed depends on the bubble size and the density of the solid particles, as might be expected as the amount of gas attracted by the bubble should increase with bubble size and because the weight of defluidized material depends on the density of the solid material. A simple scaling of these quantities is suggested that is otherwise independent of the bed material.

In freely bubbling gas-fluidized beds the particle pressures generated behave differently. Overall they are smaller in magnitude and reach their maximum value soon after the bubble passes instead of at eruption. In this situation, it appears that the bubbles interact with one another in such a way that the de uidization effect below a leading bubble is largely counteracted by refluidizing gas exiting the roof of trailing bubbles.