(121b) Instabilities in Liquid Fluidization Systems

Voidage instabilities appear when beds of particles are fluidized. In gas-fluidized beds, instabilities appear in form of bubbles rising up the beds, whereas in liquid fluidized beds, it is well established that it gives rise to more stable fluidization behaviour without the characteristic bubbling phenomenon often observed in the gas fluidized beds. Such systems have been found to be useful for studies of voidage-wave structures, especially in narrow tubes (Anderson and Jackson, 1969; Ham et al., 1990; Nicolas et al., 1996; Duru et al., 2002; Duru and Guazzelli, 2002). Duru et al. (2002) investigated experimentally primary wave instabilities in narrow beds, where a concentration plane wave propagated along the bed, with alternating dense and dilute layers. They also found that these instabilities could be adequately modeled using a two-fluid continuum model. Duru and Guazzelli (2002) further investigated the secondary instabilities of one-dimensional wavy structures and observed that the destabilization of the voidage waves leads to bubble formation. The focus of the present work is on the first type of wave instability observed in liquid-fluidized beds. The objective is to investigate the nature of these unstable flow structures through quantitative two-phase flow field measurements.

In the present work, a Particle Image Velocimetry system is used for the study of the microscopic behavior of wave instabilities in a liquid-solid fluidization system. The laser lightsheet is introduced to the liquid bed for illuminating the particles and images of the solids phase flow field are captured for further computational processing. From the PIV image of solids phase flow field, it can be seen that particles tend to move from the middle part of the bed towards the near wall regions. To obtain the PIV image of the liquid phase flow field, the liquid is seeded with fluorescent particles and an optical filter corresponding to the wavelength of the fluorescent light would be applied. A light scattering system is employed for the particle concentration measurement. The basic principle behind this technique is that under column backlighting conditions, the light intensity transmitted through the bed suspension varies strongly as a function of particle concentration. In this regards, a stabilized He-Ne light would be used as the column backlighting source. The transmitted laser light through the bed suspension would then be detected using a linear photodiode. The specific data obtained from this work would include liquid and particle velocities, granular temperatures, horizontal and vertical Reynolds normal stresses and shear stresses which can be derived from velocity fluctuations data. This provides better insights on liquid fluidization systems such as with regards to the amount of granular and turbulence energies which can develop in such systems.

The convective nature of these voidage instabilities was also investigated computationally using the Discrete Element Method (DEM) coupled with Computational Fluid Dynamics. The geometry of the fluidization system simulated consisted of a two-dimensional narrow tube of width 2 cm containing 2500 glass beads as the solid particulate phase and water as the interstitial fluid. Each glass bead had a diameter of 1.0 mm and specific gravity 2.5. The superficial velocities of the liquid used were 0.018 m/s and 0.030 m/s. The base of the fluidization column was allowed to undergo simple harmonic motion when desired in order to facilitate the study of the effects of harmonic perturbations on the stability of the bed. With a fixed base, the original packed bed of glass beads was observed to expand slightly upon introduction of liquid with a superficial velocity. The bed remained homogeneously fluidized and exhibited minimal tendency to develop any form of voidage instability. This shows that the system was intrinsically stable in the absence of any external perturbations while any internal noises were not sufficiently significant to cause instability. This was true for both liquid superficial velocities (0.018 m/s and 0.030 m/s) investigated in this study.

A second set of simulations was then performed with the base of the fluidization column vibrating sinusoidally with an amplitude of 1.5 times the diameter of a glass bead and frequency of 1 Hz. It was observed that at a liquid superficial velocity of 0.018 m/s, a small amount of voidage waves could be discerned in the system. These formed at the vibrating base but were propagated only a short distance up the bed. As the fluidized bed at this low superficial velocity was only expanded slightly and close to a packed condition, the likely reason for attenuation of the voidage waves could be the high effective solid viscosity (in the continuum sense). In contrast, when the liquid superficial velocity applied was 0.030 m/s such that the bed was expanded to a larger extent, voidage instabilities in the form of waves of high and low particle concentrations could be observed traveling up the expanded bed when the base was oscillating at 1 Hz. These clearly show the unstable nature of the system towards external perturbations and the convective characteristic of the resulting instability. One other interesting feature of such an instability was that particles in the high density regions were observed to be moving upwards in the direction of the wave while those in the low density regions were observed to be settling downwards. In other words, particles switched between upward and downward motions as different phases of the wave passed through them. When the oscillating frequency of the base was increased to 2 Hz, it was observed that the effective wavelength (distance between two adjacent regions of high or low particle concentrations) of the voidage wave increased. However, the same phenomenon of particles switching between upward and downward motions in different phases of the wave persisted.

Experiments as well as computer simulations are carried out in this study so as to construct a stability diagram which can potentially help predict the conditions under which such a liquid fluidization system would be stable (or correspondingly unstable) towards external perturbations. Experimental results are compared with those from the DEM simulations. The latter would also provide a means to achieve better understanding of the physical mechanisms which produce and eventually lead to either the amplification or attenuation of voidage instabilities in such liquid fluidized bed systems.

References:

Anderson, T. B. and R. Jackson. Fluid mechanical description of fluidized beds. Industrial and Engineering Chemistry Fundamemtals, 7, 12?21. 1968.

Duru, P. and E. Guazzelli. Experimental investigation on the secondary instability of liquid-fluidized beds and the formation of bubbles. Journal of Fluid Mechanics, 470, 359?382. 2002.

Duru, P., M. Nicolas, J. Hinch, E. Guazzelli. Constitutive laws in liquid-fluidized beds. Journal of Fluid Mechanics, 452, 371?404. 2002.

Ham, J. M., S. Thomas, E. Guazzelli, G. M. Homsy, M.-C. Anselmet. An experimental study of the instability of liquid-fluidized beds. International Journal of Multiphase Flow, 16, 171?185. 1990.

Nicolas, M., J.-M. Chomaz, D. Vallet, E. Guazzelli, E. Experimental investigations on the nature of the first wavy instability in liquid fluidized beds. Physics of Fluids, 8, 1987?1989. 1996.