Keyvani, Bahram; (1995) An experimental study and modelling of the response of a vibrofluidization technique for particle sizing. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the results of an experimental study and modelling of the response of a vibrofluidization technique for particle sizing. The principle of operation of the device is simple and relies on the fact that when a container, partially filled with a powder is vibrated in the vertical direction with a maximum acceleration greater than that due to gravity, the particles inside the container become effectively "fluidised". The degree of damping experienced by the vibrating system is a direct function of the average particle size; larger particles give rise to a higher degree of damping. In the case where the container is attached at the free end of a cantilever, the average particle size is directly related to the resonant amplitude of vibration of the cantilever. This forms the basis for the application of this type of device for particle sizing. A convenient and deceptively logical explanation would be to attribute the observed damping of oscillation to particle/wall friction. Indeed, the response of the system was modelled accordingly by Mahgerefteh and Al-Khoory (1991) using visco-elastic vibration phenomena. The results of this work show, using direct experimental evidence that the primary mechanism responsible for the observed damping is a consequence of the phase lag between the vibrating cavity and the test powder. The previously proposed particle/wall interaction mechanism although applicable, is simply a direct manifestation of the phase lag phenomenon. In addition preliminary results obtained proving the applicability of the device for particle size distribution analysis are also presented. The phase lag between the vibrating cavity and the powder is monitored using a sonic technique. This is achieved by measuring the intensity of the noise generated as a result of the impact of the powder with the containing cavity during oscillations. The results show a direct correlation between phase lag and the average particle size. A semi-empirical model is also developed which satisfactorily predicts the system's response in terms of a large number of design and operating parameters. This model is expected to serve as a powerful tool for design optimization. Particle size distribution data are obtained by vibration segregation of the test powder to various size fractions and measuring the average size and mass of each sub-sample using the vibrofluidization technique. The thesis also describes the optimal segregation conditions leading to the generation of particle distribution histograms for typical 2 g samples in ca 3 minutes. The corresponding resolution in terms of the average size for each size fraction is better than [plus-minus] 5 µm for a test sample in the 20 - 1000 µm size range.

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