Magnetic resonance imaging (MRI) study of jet formation in packed beds

• The formation of gas jets in packed beds is studied non-intrusively using MRI.
• The bed filling and fluidization history critically affects the measured jet height.
• The jet height becomes independent of bed geometry for L/dp>55L/dp>55.
• An expression to correlate the jet height with operating parameters is proposed.

In this study magnetic resonance imaging (MRI) was applied to image non-intrusively the formation of jets in packed beds of square cross section (L=46.8 mm, L=50.6 mm, L=67.4 mm and L=72 mm) containing a central square orifice (Lo=3.6 mm). Poppy and mustard seeds were used as MRI detectable particles. We critically assessed the influence of two different start up procedures on the measured jet height, viz.   filling the bed with particles at a low orifice velocity followed by a stepwise increase of the orifice velocity, or filling the bed at a high orifice velocity followed by a stepwise reduction of the orifice velocity. It was found that the different start up protocols can lead to a difference in the observed jet height of a factor of two, with beds filled at the higher orifice velocity generally giving larger jets. Further measurements were performed to determine the effect of (i) fill level, (ii) particle diameter, (iii) bed dimensions and (iv) orifice velocity on jet height. Interestingly, the fill level did not influence the jet height. However, with increasing fill level the transition to a spouting bed shifted to higher orifice velocities. Additionally, it was established that, for a given orifice velocity, the cross-sections of the bed typically affected the jet height, in particular for larger particles. Based on the set of measurements obtained here we postulate that the jet height is independent of the bed dimensions at flow rates of Q<0.7QmfQ<0.7Qmf and bed dimensions of L/dp>55L/dp>55, where Q is the volumetric flow rate of the gas, Qmf is the volumetric flow rate of the gas at minimum fluidization and dp is the particle diameter. Finally an expression was developed to correlate the jet length against the bed and particle properties.

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