Analysis Of Pressure And Voidage Fluctuations In Gas-solid Fluidized Beds

The gas-solid flow state determines the gas-solid mixing,heat and mass transfer behavior in the fluidized beds.It is vital for the design,operation and monitoring of the fluidized beds.In this work,the flow state in gas-solid bubbling fluidized beds was quantitatively characterized by analyzing the pressure and voidage fluctuation signals in the bed.In a self-built cold gas-solid fluidized bed,pressure and voidage fluctuation signals were measured with differential pressure sensors and optical fiber probes,respectively.Using time-series analysis methods such as time domain analysis,frequency domain analysis,and chaos analysis,a number of indicators characterizing gas-solid flow states were extracted from the fluctuation signals.The effects of superficial gas velocity(0-0.8 m/s),particle size(176,246,and 354 ?m),static bed height(5-27 cm),and bed size(four types of cross-sections: three squares of 5,10 and 15 cm in side length and one pseudo-2D bed of 2×10 cm)on each characteristic indicator were studied.The relationship between the physical fluctuation signals and the gas-solid flow states was established.For the pressure signal,the bubbling bed can be divided into a partial fluidization stage and a full fluidization stage according to changes in bed pressure drop with the gas velocity.Based on the change in the Kolmogorov entropy and bubble behavior with the gas velocity,the bubbling bed can be further divided into four zones.The Kolmogorov entropy has a good ability to discern the change of bubble behavior.In the full fluidization stage,the pressure fluctuation amplitude increases monotonically with the gas velocity,and is sensitive to the change of the gas velocity.The relative order of the pressure fluctuation amplitudes at different axial heights varies with the gas velocity,which is related to the size of the bubble passing by the probe and the amplitude of bed oscillation.At higher gas velocities,pressure fluctuations in the entire bed are dominated by the macro-scale bed oscillation.As the gas velocity increases,the skewness increases from negative to positive and the kurtosis first decreases rapidly and then increases slowly.The probability density function(PDF)of the pressure near the bed surface changes from a left-skewed unimodal PDF to a bimodal PDF and then to a right-skewed unimodal PDF.The bimodal distribution corresponds to the skewness close to zero and the kurtosis close to its minimum.The Kolmogorov entropy in the lower part of the bed is higher than that in the upper part of the bed,indicating that the flow state in the lower part of the bed is more complex.The Kolmogorov entropy decreases with increasing static bed height and particle size.Both the Kolmogorov entropy and the dominant frequency increase with increasing bed size,and the Kolmogorov entropy and the dominant frequency of the pseudo-2D bed are between the values of the 5-cm and 10-cm beds.The dominant frequency does not change with gas velocity,axial height,and particle size over a wide gas velocity range,but decreases as the static bed height increases.For the voidage signal,as the gas velocity increases,the bubble velocity increases.The larger the bed size,the greater the bubble velocity.At the same excess gas velocity,the larger the particle size,the smaller the bubble velocity.Overall,the voidage increases with increasing the gas velocity,but the voidage at the bottom of the two large-size beds shows a first-increase-and-then-reduction peak at low gas velocities.This peak is also reflected in the standard deviation,skewness,kurtosis,and Kolmogorov entropy correspondingly.With the increase in gas velocity,the standard deviation of the voidage increases first and then stabilizes;the Kolmogorov entropy tends to increase gradually in the full fluidization stage.The changing trends of the skewness and kurtosis of the voidage signal with various factors are the same.Static bed height and particle size have little effect on the mean,standard deviation,skewness,and kurtosis of the voidage signal.The characteristic indicators of the pressure signal are more sensitive to changes in various factors as a whole than that of the voidage signal.