Bubbling/slugging flow behavior in a cylindrical fluidized bed: ECT measurements and two-fluid simulations

Abstract

While the Eulerian two-fluid models (TFM) are now reasonably well established to predict the “time-averaged” flow behavior of gas–solid flow in fluidized beds, quantitative predictions of inherently unsteady local solid-phase distribution and bubbling/slugging behavior using the TFM continues to be a challenge. In the present work, we have investigated time-evolution of solid volume fraction (αs) distribution and bubbling/slugging behavior in a laboratory-scale fluidized bed using the Electrical Capacitance Tomography (ECT) measurements and TFM simulations. The ECT was used to measure temporally- and spatially-resolved local αs distribution and these measurements were used further to characterize the bubble chord length fluctuations, bubbling frequency and bubble size distributions. The effects of different models used to calculate gas-solid momentum exchange on fluidization behavior were investigated. The results showed that the predicted time-averaged αs distribution differed marginally and were in a satisfactory agreement with the corresponding measurements. However, the predicted bubbling frequency and bubble size distribution, in particular, the formation of large bubbles/slugs predicted using different drag models were considerably different and were not even in a qualitative agreement with the measurements. The modification proposed to the drag model in the present work led to an improvement in the predicted bubbling behavior and showed a qualitative agreement with the measurements. Further, simulations performed to investigate the effect of solid-phase frictional viscosity (μs,fr) showed that an increase in the solid-phase frictional pressure leads to an increase in μs,fr. Such an increase in μs,fr leads to compaction of the solid phase and to the formation of large bubbles/slugs, in line with what is observed experimentally. The predicted bubbling frequency and bubble size distribution were in a quantitative agreement with those measured using the ECT for different gas velocities.