The state of uniform fluidization, where the uniform upflow of a fluid balances the weight of a bed of particles, it shown to be unstable to small disturbances when the effect of the lift force on the particles is included. The lift force is dominant during the initial growth of the disturbances. The lift force on the particles is assumed to be proportional to the slip and to the one‐half power of the shear rate. Perturbations from the state of uniform fluidization are studied. The resulting perturbation problem is such that the evolution of the smallest disturbances is governed by a nonlinear equation. Particular initial conditions corresponding to growing solutions are found.

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