Regimes of jetting and bubbling in a fluidized bed studied using real-time magnetic resonance imaging

Abstract

Real-time magnetic resonance imaging was used to study the different flow regimes which occur in a fluidized bed containing a gas injection system. The gas flow rates through the main distributor and a central orifice were varied independently. We identified six different regimes of bubbling and jetting behavior: (1) freely bubbling, (2) permanent jet, (3) spouting, (4) pulsating jet, (5) pulsating jet with bubble collapse and (6) pulsating jet and freely bubbling. While regimes (1–4) have been described previously in the literature, regimes (5) and (6) are described here for the first time. To construct a regime map, the Froude number (Fr) and the ratio of the superficial gas velocity to the minimum fluidization velocity (U/Umf) were used to describe the system. We observed that bubbles formed predominantly, when U/Umf > 1. Further, we propose an empirical model that predicts the length of jets in the permanent jet regime as a function of Fr and background gas flow as ljetdor=0.12Fr0.381+UbgUmf0.81. The proposed model is in good agreement with tomographic measurements in smaller 3D systems reported in the literature, indicating that the non-dimensionalized description of jet length using a Fr number is valid throughout a large range of system diameters. Moreover, the bubble breakoff frequency of the pulsating jet regime was assessed by Fourier analysis, demonstrating that the frequency increases with increasing Fr before plateauing.