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Bubbly flow through fixed beds: Microscale experiments in the dilute regime and modeling

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Abstract

In order to predict the pressure drop and the mean void fraction for bubbly flows in packed beds, a new one-dimensional (1-D) model is proposed. The balance equations for both phases are derived from local Eulerian two-fluid equations which are spatially averaged at a mesoscale, that is, at a length-scale large compared with the microscale that characterizes the fixed bed. This model, that differs from previous mechanistic models, has been supplemented by closure laws for the liquid-solid and the gas-liquid interactions, those structures account for the flow dynamics at the pore scale. It is first experimentally demonstrated that, in dilute conditions, the bubble-size distribution only depends on the pore size, when the later is smaller than the capillary length scale. It is also shown that the mean-bubble dynamics is similar to that of a slug, with a relative velocity at mesoscale linearly increasing with the liquid superficial velocity. Besides, that relative velocity monotonically increases with the gas flow rate ratio, a behavior that can be attributed to the formation of preferential paths for the gas phase. Concerning the liquid-solid interactions, the two-phase flow pressure drop scaled by its single-phase flow counterpart at the same superficial liquid velocity is predicted to linearly increase with the void fraction, with a prefactor evolving with the Capillary number. These closures prove consistent with available experiments, both in upward and in downward situations. Although these proposals deserve to be further tested over an extended range of flow parameters, this model paves the way to reasonably accurate predictions of bubbly flows in packed beds able to account for refined parameters related with the flow dynamics. © 2006 American Institute of Chemical Engineers AIChE J, 2006

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