| The instability of a bed of particles fluidized by a source of fluid entirely within the bed has been observed previously, but there have been no quantitative studies of the phenomenon to date. Therefore, an experimental study of a bed of drying silica gel was performed in order to determine the nature of the instability and to measure its characteristics. A linear stability analysis of the two-fluid equations of motion written for self-fluidization was also undertaken.; The experiments showed that the self-fluidized system was unstable for all flow rates greater than the minimum necessary to fluidize the bed. Two stages of the instability were observed. A primary instability, characterized by a spatially periodic array of small spouts of particles erupting at the surface of the bed, was found for all flow rates above the minimum fluidizing flow rate. For flow rates above 2-3 times the minimum fluidizing flow rate, this primary instability was superseded by a temporally periodic secondary instability in which a single large spout repeatedly grew from the initial array and then died out. The spatial frequency of the primary instability and the temporal frequency of the secondary instability were measured as functions of particle diameter, strength of the source and slumped height of the bed.; A mechanism, by which infinitesimal perturbations to the local void fraction are rendered unstable, is proposed and further investigated with the linear stability analysis. Predictions of the wavelength of the primary instability as a function of the source strength, particle diameter and slumped height were found to be in qualitative agreement with the experimental observations. Quantitative comparison suggested values of the solid phase rheological parameters that are consistent with previous estimates. |
