Characterization of fluidization regimes by time-series analysis of pressure fluctuations

Abstract

This work compares time, frequency and state-space analyses of pressure measurements from fluidized beds. The experiments were carried out in a circulating fluidized bed, operated under ambient conditions and under different fluidization regimes. Interpretation of results in time domain, such as standard deviation of the pressure fluctuations, may lead to erroneous conclusions about the flow regime. The results from the frequency domain (power spectra) and state-space analyses (correlation dimension, D ML, and Kolmogorov entropy, K ML, together with a non-linearity test) of the pressure fluctuations are generally in agreement and can be used complementary to each other. The power spectra can be divided into three regions, a region corresponding to the macro-structure (due to the bubble flow) and, at higher frequencies, two regions representing finer structures that are not predominantly governed by the macro structure of the flow. In all fluidization regimes, the measured pressure fluctuations exhibited an intermittent structure, which is not revealed by power spectral analysis of the original signals. Fluctuations with pronounced peaks in the power spectrum and in the auto-correlation function, corresponding to passage of single bubbles through the bed, are non-linear with a low dimension ( D ML<5.5). For D ML<5.5, the Kolmogorov entropy is proportional to the amount of energy in the spectral range of the intermittent structures observed, whereas for D ML>5.5 both K ML (bits/cycle) and D ML are insensitive to changes in the distribution of energy in power spectra. Thus, the state-space analysis reflects that non-linearity is mostly found in the macro-structure of the flow. Fluidized bed time series treated in this work are available at http://www.entek.chalmers.se/∼fijo