Digital image analysis of hydrodynamics two-dimensional bubbling fluidized beds

A new method of digital image analysis has been developed to study the hydrodynamics of two-dimensional bubbling fluidized beds with a digital video camera. The method comprises simultaneous of the size and velocity of gas bubbles, and the axial and radial distribution of bubble voidage. It provides a better estimation of the visible bubble flow than from local probe methods. Also a good estimation of the throughflow can be gotten, which is of great importance for combustor applications. Parallel to the approach of Darton et al. (Transactions of the Institution Chemical Engineering 55 (1977) 274) for three-dimensional fluidized beds, an equation for the bubble diameter of two-dimensional beds is developed, D-b = 0.89[(U-0 - U-mf)(h + 3.0A(0/t))](2/3) g(1/3), where h is the bed height above air distributor; t is the thickness of the two-dimensional bed. For Group B particle and a given gas velocity U-0, if the bed height is sufficient, bubble diameter D-b could be able to reach a maximum value at a certain height. The height is defined as the maximum bubble height h*, beyond which bubbles do not grow further and become unstable and break up. The height h* is a particle-size dependency, and can be expressed as h* = A(1 + 3 exp(-U-0/U-mf))D-t, where A = 0.45 and D-t is the bed diameter. The bubble rising velocity U-b can be described by U-b = PhirootgD(b)(Phi = 0.80 - 1.0) within the height h*. Also, the bubble velocity is kept constant beyond the height h*. The bubble density delta(b) is not uniform over the bed cross-section. It increases with particle size, and increases quite slowly for high fluidization velocities. The gas throughflow decreases much slowly along with the bed height. Beyond the height h*, the gas throughflow is almost kept constant. Also, the throughflow is a significant part of the total gas flow, especially for high fluidization velocity, and increases almost linearly with the fluidization velocity. (C) 2004 Elsevier Ltd. All rights reserved.