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The article discusses experimental measurements of the eruption of single bubbles and streams of bubbles in fluidized-bed reactors. It states that this was the first time that stereoscopic particle image velocimetry (PIV) was applied simultaneously with acetone planer laser induced fluorescence in the freeboard of a fluidized bed. A non-negligible, out-of-plane component of velocity was measured. The article notes agreement among these results and other research. It says there is no qualitative difference in the eruption of single bubbles or streams of bubbles and it characterizes the decay of bubble-induced turbulence.

Abstract: This work presents an experimental study of the ejection velocity for different mechanisms of solid ejection in fluidized beds. The experiments were carried out in a 2D fluidized bed, where the bubble eruptions were recorded with a frequency of 250 frames per second using a high speed video camera with a resolution of 1.3 Megapixels. The results show that in isolated bubble eruption, the dome velocity is significantly reduced by the effect of a group of raining particles in the form of stalactites within the bubble. Higher velocities are observed when bubble coalescence takes place. If bubbles coalesce before the leading bubble breaks, the momentum of the trailing bubble together with the increase in the throughflow accelerate the dome of the leading bubble. In contrast, when coalescence occurs after the breakage of the leading bubble, the wake of the trailing bubble is projected into the freeboard with a very high velocity (wake spike mechanism). The last observed mechanism, the jet spike mechanism, occurs when a stream of bubbles reaches the bed surface following the path opened by the previous bubbles. A cloud of particles moving upward is observed, although their velocities are not as high as in the wake spike mechanism due to the interchange of momentum during the collisions with other particles. Finally, an explanation for some of the patterns of gas release from erupting bubbles recently observed by Hartung et al. [G. Hartung, C.R. Müller, J. Hult, J.S. Dennis, C.F. Kaminski, Laser diagnostic investigation of the bubble eruption patterns in the freeboard of fluidized beds. 1. Optimization of acetone planer laser induced fluorescence measurements. Ind. Eng. Chem. Res. 47 (2008) 5686–5697] is proposed. [Copyright &y& Elsevier]

Abstract: Ultra-fast Magnetic Resonance Imaging (MRI) and pressure sensor measurements have been applied to study: (i) pressure fluctuations, (ii) the eruption of bubbles at the top of a bed and (iii) the formation of bubbles in a gas-fluidized bed. Ultra-fast MRI has been applied for the first time to study the formation and eruption of bubbles; the technique is non-intrusive and provides measurements with good temporal and spatial resolutions. The MRI measurements revealed that bubbles are formed periodically, rather than randomly at a distributor, which in this case was a perforated plate. The frequency at which bubbles erupted from the top of the bed matched the frequency of the pressure fluctuations measured just above the distributor, where the measured pressure is predicted very well for the case of slug flow by Kehoe and Davidson''s [P.W.K. Kehoe, J.F. Davidson, Pressure fluctuations in slugging fluidized beds, AIChE Symp. Ser. 128 (69) (1973) 34–40] correlation, originally developed for locations high up a bed. Both findings lead to the conclusion that the passage and eruption of bubbles at the top of a bed are the dominant cause of the pressure fluctuations, which are subsequently propagated downwards through the bed. Two new correlations are proposed for predicting the frequency of pressure fluctuations in a bubbling bed; both correlations agree well with experimental measurements. A modification of Baeyens and Geldart''s [J. Baeyens, D. Geldart, An investigation into slugging fluidized beds, Chem. Eng. Sci. 29 (1974) 255–265] correlation predicts the frequency of pressure fluctuations when slugs are formed, but are not fully developed. The frequency of bubble formation, as measured by MRI, is equal to or higher than both the frequency of bubble eruption at the top of the bed and the frequency of pressure fluctuations, depending on the depth of the bed. The frequency of bubble formation is significantly lower than predicted by Davidson and Schüler''s [J.F. Davidson, B.O.G. Schüler, Bubble formation at an orifice in an inviscid liquid, Trans. Inst. Chem. Eng. 38 (1960) 335–342] equation, originally developed for gas–liquid systems. [Copyright &y& Elsevier]