Measuring the Velocity of Gas and Particles in and Around a Single Bubble in a 3D Fluidised Bed

Understanding the flow of gas and particles is critical to predicting the behaviour of gas-solid fluidised beds, and is not yet fully resolved. The bubbles in a fluidised bed drive the mixing behaviour of the particulate phase, while the flow of gas through the bubbles and the particulate phase determines the gas-solid contact. In this paper, we investigate some of the classical models of fluidised beds using Magnetic Resonance Imaging (MRI) measurements of the velocity of the gas and solid phases in a three dimensional gas-solid fluidised bed. In order to obtain sufficient signal from the gas phase, sulphur hexafluoride gas was used at a pressure of 7.5 barg; poppy seeds were used for the particulate phase. Even with this gas at pressure, MRI measurements of the velocity are typically slow, especially when measuring the gas phase owing to the low signal intensity of the gas. Therefore to enable images of the gas and solid velocity field, a system was developed to inject single, isolated bubbles reproducibly into an incipiently fluidised bed. Images of the flow of gas and solid are then obtained by averaging measurements from a large number of these isolated bubbles. The MRI measurements were triggered such that data were acquired at specific times after the injection to ensure that all of the bubbles that were measured were in the same position for each time. Here we first quantify the reproducibility of the gas injection system and then demonstrate that quantitative measurements of the gas and particle velocities can be obtained. The resulting images clearly show a recirculation pattern in the gas phase that is consistent with the potential flow solution of Davidson and Harrison, for these group D particles. From the gas velocity field it is also possible to estimate the through-flow of gas through the bubble and show that this is consistent with the estimate of three times the minimum fluidisation velocity, U_mf. The velocity of the gas in the particulate phase is found to be less than expected in the classical theory owing to the net downward velocity of the particles relative to the rise velocity of the bubble. If the particle velocity is used to correct the expected gas velocity, it is in good agreement with the estimated interstitial velocity U_mf/e_mf, where e_mf is the voidage at minimum fluidisation.