Pressure fluctuations and transition from bubbling to turbulent fluidization

Abstract

A simple mechanistic model of bubbly flow in nonslugging gas-solids fluidized beds with fine Group A particles has been developed for the prediction of pressure fluctuations induced by bubble motion. The model assumes that bubbles are aligned in vertical chains in the fluidized bed. The bubble growth is dominant at low velocities due to the coalescence of adjacent bubbles, while the ratio of the separation distance between the leading and the trailing bubbles and the bubble diameter remains constant. However, a maximum stable bubble size is reached when a balance between bubble splitting and coalescence is reached and the mean bubble size will remain at its maximum stable size when the gas velocity is further increased while the separation distance between adjacent bubbles starts to decrease. The model simulation shows both the amplitude and the standard deviation of pressure fluctuations reach a maximum with increasing superficial gas velocity. When the mean bubble size is allowed to decrease with further increasing gas velocity after the maximum stable size is reached, the amplitude and standard deviation also decrease but more gradually than with D B remaining constant. The transition velocity from bubbling to turbulent fluidization, U c, and bubble-phase volume fraction at U c predicted from this model are in good agreement with experimental data obtained in relatively large columns with fine particles where slugging does not occur since D t≫ D B.