Magnetic Resonance Studies of Fluidization Regimes

Abstract

Measurements of the voidage and velocity distributions in a gas-fluidized bed operating in the bubbling, turbulent, and core-annular fluidization regimes were acquired using magnetic resonance imaging (MR). The bed studied was contained in a column 50 mm in diameter and was fluidized with air. The particles were silica-alumina catalyst support loaded with water doped with gadolinium (diameter 63 μm, density of the water loaded particles 1530 kg m -3 ). Both time-averaged and ultrafast measurements are presented and provide the first noninvasive measurements of the velocities of particles and local voidage in a fluidized bed in each of the flow regimes. Measurements of the pressure fluctuations were also recorded as a function of the superficial velocity of the fluidizing gas to compare with the MR measurements. Ultrafast measurements of the voidage were used to examine the void structures present in the different flow regimes and to provide a means of studying the dynamics. A novel MR technique was used to measure the velocities of the particles in a diametral region of 15 mm x 15 mm square cross-section through the center of the bed every 7.7 ms. These measurements confirmed that the highest particle velocities in the bubbling fluidization regime occurred in the wakes of bubbles. The distribution of particle velocities in a bubbling bed is highly skewed; however, it approaches a Gaussian distribution and appears to scale with the superficial gas velocity in the turbulent fluidization regime. Finally, a simple model to infer the slip velocity in core-annular fluidization indicates that the particles in the center of the column group together in clusters with a diameter of between 4 and 8 particles.