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Heat transfer near the transition to turbulent fluidization Stefanova, Ana

Abstract

The turbulent fluidization flow regime, a transitional flow regime situated between bubbling and fast fluidization, has received relatively little attention in the heat transfer literature, despite its advantages for operating commercial gas-solid fluidized bed reactors. This work investigates simultaneously bed–to–surface heat transfer coefficients and changes in the local hydrodynamics near the transition from bubbling to turbulent fluidization flow regimes. Experiments were conducted with fluid cracking catalyst (FCC) and alumina particles in columns of different diameters, a 0.29 m ID column at UBC in Vancouver, Canada, and a 1.56 m ID column at CSIRO Minerals in Clayton, Australia. Radial and axial locations of the heat transfer surface, static bed height and superficial gas velocity were varied. Two distributor configurations were examined in the smaller column: perforated plate and bubble cap. The transition to turbulent fluidization was determined based on changes in trends and features of pressure and optical probes signals. To measure the bed-to-immersed surface heat transfer coefficients, a modular electrically heated vertical tube was designed. The results showed maximum heat transfer coefficients near the onset of turbulent fluidization and increased uniformity of the radial and axial distributions of heat transfer coefficients when the turbulent fluidization regime became dominant. It was discovered that the transition to turbulent fluidization was more complex and occurred gradually across the bed in the larger column. However, the maximum heat transfer coefficient was not affected by the column size. The Froude number, based on the column diameter, provided a good scaling parameter in regions of similar flow structure. A novel transparent heat transfer probe was designed for transient local heat transfer measurement at the column wall and simultaneous evaluation of the hydrodynamics at the heat transfer surface. This study provided insight into the changing dominant mechanism of heat transfer at the onset of turbulent fluidization. A probabilistic heat transfer model was developed, based on the packet renewal theory and the probability of having particle packets corresponding to each hydrodynamic regime at the heat transfer surface. The novelty of the model is the contribution of packets of intermediate voidage, typical for turbulent fluidized beds, to the total heat transfer coefficient.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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