Hydrodynamics of a bubbling fluidized bed: influence of pressure and fluidization velocity in terms of drag force

Journal article, 1992

Measurements of the visible bubble flow rate and the through-flow velocity of gas inside bubbles have been carried out in a pressurized fluidized bed. Based on the results, it is demonstrated that the representation of in-bed hydrodynamics in terms of particle drag force facilitates a comparison between the influence of pressure and fluidization velocity. By calculating a "potentially available drag force" corresponding to the different operating conditions, it is shown that most of the in-bed parameters, such as bed expansion, bubble volume fraction, bubble rise velocity and local visible bubble flow rate, fall on single curves when plotted versus this force. Deviations occur due to bubble instability, which is largely a pressure-dependent effect. Some mechanisms that together govern bubble instability and splitting are established: (a) The through-flow velocity of gas through the bubbles decreases considerably as the pressure increases. (b) The fluctuations of the through-flow velocity of gas through the bubbles are of the same order of magnitude at all the operating conditions investigated. (c) A given fluctuation in gas velocity has a higher relative influence on the fluctuation in particle drag force at high pressures than at low pressures. In addition, the overall bed behaviour becomes less stable at high Reynolds numbers, i.e. at high pressures and fluidization velocities. The bubble size at a given location in the bed is determined by a complex balance between bubble splitting and coalescence. Both splitting and coalescence are governed by fluctuations in the particle drag force caused by fluctuations in gas velocity. These velocity fluctuations are largely caused by the gas short-cutting between adjacent bubbles. Due to increased coalescence, the bubble flow is redistributed towards the centre of the bed cross-section with both increasing pressure and fluidization velocity.