Heat Transfer in a 3D Dynamically Structured Fluidized Bed

Inspired by the origin of ordered structures generated by periodically oscillating fluid flows in nature, such as the sandy ripples on beaches and desert floors, it has been proposed to apply an oscillating gas flow at a state around the minimum fluidization velocity to create a “dynamically structured fluidized bed”, which manipulates the gas-solid drag flow periodically in time. The pulsed flow introduces order, and the system reaches a specific fluidization state between fixed and fluidized bed. In quasi-2D flat or annular fluidized beds, bubbles rise and form a hexagonal array with bubbles spaced at certain distances, which is different from the chaotic bubble flows in conventional fluidization. This special fluidization state provides opportunities for process intensification. Previous research has investigated the hydrodynamics in dynamically structured fluidized beds and identified the operation of a structured bubbling flow with bubble size and pattern wavelength that could be tuned by changing the oscillation frequency. However, the influences of dynamical structuring on transfer coefficients are still poorly understood.

An essential step towards utilizing dynamically structured fluidized beds for, e.g., heterogeneously catalysed reactions or drying, is the development of a heat transfer model that considers the unusual characteristics of dynamically structured fluidized bed, lying in between traditional fluidized and fixed beds. In this work, the heat transfer characteristics within a 3D annular pulsed fluidized bed were first studied numerically, and subsequently compared with the unstructured conventional bubbling fluidized bed. By using a thermocouple, the instantaneous temperatures were measured at various locations, and the simulation results showed good agreement with experimental results. The relationship between bubble patterns and temperature distribution was explored, and the effects of pulsed flow on local and overall heat transfer coefficients were also investigated.