Characterization of various structures in gas-solid fluidized beds by recurrence quantification analysis

Gas-solid fluidized beds are widely considered as nonlinear and chaotic dynamic systems. Pressure fluctuations were measured in a fluidized bed of 0.15 m in diameter and were analyzed using multiple approaches: discrete Fourier transform (DFT), discrete wavelet transform (DWT), and nonlinear recurrence quantification analysis (RQA). Three different methods proposed that the complex dynamics of a fluidized bed system can be presented as macro, meso and micro structures. It was found from DFT and DWT that a minimum in wide band energy with an increase in the velocity corresponds to the transition between macro structures and finer structures of the fluidization system. Corresponding transition velocity occurs at gas velocities of 0.3, 0.5 and 0.6 m/s for sands with mean diameters of 150, 280 and 490 μm, respectively. DFT, DWT, and RQA could determine frequency range of 0?3.125 Hz for macro, 3.125?50 Hz for meso, and 50?200 Hz for micro structures. The RQA showed that the micro structures have the least periodicity and consequently their determinism and laminarity are the lowest. The results show that a combination of DFT, DWT, and RQA can be used as an effective approach to characterize multi-scale flow behavior in gas-solid fluidized beds.