On the two-phase theory of fluidisation

Theory is given to predict the value of k in the equation for a bubbling fluidised bed U=kU0+QB/A; here U is the superficial fludising velocity, U=UO at incipient fluidisation, QB is the observed bubble flow, and A the cross-sectional area of the bed. In the simple two-phase theory, k=1; but the theory herein, assuming constant voidage fraction between the bubbles, shows that for a regular array of bubbles, k=1+eb for a two-dimensional system, provided eb=volume of all bubbles/bed volume, is small. It is inferred that for a three-dimensional system, k=1+2eb. Thus the simple two-phase theory, though not strictly correct, is not much in error; the error is likely to be largest when U is not much greater than UO, and particularly just above the distributor of a fluidised bed.