Anomalous collapse of interacting bubbles in a fluidized bed: A magnetic resonance imaging study

The collapse, or reduction in size to zero volume, of bubbles injected into incipiently fluidized beds was studied using rapid magnetic resonance imaging. The collapse of a smaller lower bubble trailing a larger upper bubble and the collapse of one bubble when two bubbles rose side by side were found to occur. Under the same conditions with the use of finer particles or the injection of an isolated bubble, no collapse occurred. Thus, results indicate that gas leakage into the particulate phase of coarse particles and bubble interaction promote bubble collapse. Furthermore, injection of larger bubbles also resulted in bubbles rising to the bed surface instead of collapsing, indicating that bubbles must be below a critical size in order to collapse. For side-by-side bubbles, bubble collapse is attributed to gas flow channeling to the larger bubble; for consecutive bubbles, bubble collapse is attributed to increased gas leakage in a dilated bubble wake.

Figure 1

Schematic of the process for determining bubble volume: (a) raw image of particle concentration field; (b) binary image of gas and particulate pixels after applying a signal threshold to (a). The centroid of the bubble in (b) is shown with a red $\times$, and this is used to draw the vertical center line. The bubble volume is determined based on the radius of each gas pixel from the center line and assuming axis symmetry.

Figure 2

Time series of images of local particle concentration (upper row) and local particle velocity (lower row) for single bubbles injected into incipiently fluidized beds of 3 mm particles for an injection time of 25 ms. Green arrows indicate the direction of particle velocity.

Figure 3

Bubble volume normalized by volume injected vs normalized vertical position of the centroid of the bubble for an injection time of 25 ms into an incipiently fluidized bed of 3 mm particles. Error bars show the standard deviation from three measurements.

Figure 4

Time series of signal intensity images for two bubbles injected side by side into (a) 1 mm particles with $t_{\mathrm{inj}}=30$ ms, (b) 1 mm particles with $t_{\mathrm{inj}}=40$ ms, (c) 3 mm particles with $t_{\mathrm{inj}}=30$ ms, and (d) 3 mm particles with $t_{\mathrm{inj}}=40$ ms.

Figure 5

Normalized bubble volume vs vertical position of the centroid of the bubble for two bubbles injected side by side (L = left, R = right) into an incipiently fluidized bed of (a) 1 mm particles and (b) 3 mm particles. (c),(d) The total normalized bubble volume from both the right and left bubbles in (a) and (b), respectively.

Figure 6

Time series of images particle signal intensity (upper row) and particle velocity (lower row) for side-by-side bubbles in incipiently fluidized beds of (a) 1 mm particles and (b) 3 mm particles with injection times of 30 ms.

Figure 7

Schematic showing gas flow (red arrows) through side-by-side bubbles, leading to the larger bubble growing due to gas channeling through it, while the smaller bubble decreases in volume due to gas channeling away from it.

Figure 8

Time series of signal intensity images for two bubbles injected consecutively into incipiently fluidized beds with an injection time of $t_{\mathrm{inj},U}=75$ ms for the first injection and a separation time between injections of $t_{\mathrm{sep}}=50$ ms. Different values for particle diameter and injection time of the lower bubble were used: (a) $t_{\mathrm{inj},L}=50$ ms, 1 mm particles; (b) $t_{\mathrm{inj},L}=25$ ms, 1 mm particles; (c) $t_{\mathrm{inj},L}=50$ ms, 3 mm particles; and (d) $t_{\mathrm{inj},L}=25$ ms, 3 mm particles.

Figure 9

Normalized bubble volume vs normalized vertical position of the bubble for upper (U) and lower (L) bubbles for consecutively injected bubbles with bubble injection time of $t_{\mathrm{inj},U}=75$ ms, a separation time between injections of $t_{\mathrm{sep}}=50$ ms, and variable lower bubble injection times $t_{\mathrm{inj},L}$ in (a) 1 mm particles and (b) 3 mm particles.

Figure 10

Normalized bubble volume at the time of bubble breakoff from the injection port for two consecutive bubbles injected for the upper (U) and lower (L) bubble into 1 and 3 mm particles.

Figure 11

Time series of images particle signal intensity (upper row) and particle velocity (lower row) for consecutively injected bubbles into incipiently fluidized beds of 3 mm particles with an upper bubble injection time of 75 ms, a separation time of 50 ms between bubble injections, and a lower bubble injection time of (a) 25 and (b) 50 ms.

Figure 12

Time series of images particle signal intensity (upper row) and particle velocity (lower row) for consecutively injected bubbles into incipiently fluidized beds of 3 mm particles with an upper bubble injection time of 25 ms, a separation time of 100 ms between bubble injections, and a lower bubble injection time of 25 ms with a separation distance of 80 mm between injection ports.

Figure 13

Time series of images particle signal intensity (upper row) and particle velocity (lower row) for consecutively injected bubbles into incipiently fluidized beds of 3 mm particles with an upper bubble injection time of 25 ms, a separation time of 200 ms between bubble injections, and a lower bubble injection time of 25 ms with a separation distance of 80 mm between injection ports.

Figure 14

Time series of images particle signal intensity (upper row) and particle velocity (lower row) for consecutively injected bubbles into incipiently fluidized beds of 3 mm particles with an upper bubble injection time of 25 ms, a separation time of 200 ms between bubble injections, and a lower bubble injection time of 25 ms with a separation distance of 40 mm between injection ports.

Figure 15

Effect of threshold value of normalized bubble volume vs normalized vertical position of the bubble for a 25 ms injection time into an incipiently fluidized bed of 3 mm particles. Error bars show the standard deviation from three measurements.