The analysis and modeling of pressure fluctuations in a fluidized bed

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2003-01-01
Authors
Falkowski, David
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Mechanical Engineering
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The objective of this research was to evaluate different pressure probe techniques used to measure the pressure fluctuations in a fluidized bed, to determine the effect of bed parameters on the power spectrum, and to develop a second-order model that describes the spectrums. This work's motivation is to increase the knowledge of fluidized bed pressure phenomena and further the understanding of such fluidized bed research areas as similitude, chemical processes, and pressure diagnostic tools.;Pressure fluctuations in a fluidized bed were measured with four pressure probe techniques (static-absolute, dynamic-absolute, static-differential, and dynamic-differential) to determine if differences existed. Testing showed that the absolute and differential techniques produced drastically different results under certain conditions. Comparison of dynamic and static techniques showed that their results were very similar in most situations. The probe arm position was an important testing parameter for all techniques.;Pressure data at different bed heights, fluidization velocities, particle sizes/densities, and bed temperatures were taken to determine each parameter's effect on the spectrum. The relationship between bed height and dominant frequency agreed with the literature, but secondary peaks were a function of position in the bed and not affected by bed height. Through the Bode plot, the shifting of frequency peaks with fluidization velocity was documented as a continuous growth. Large-diameter, high-density particles (Group D) exhibited harmonic behavior, while small-diameter, low-density particles (Group A) yielded power spectrums with first-order characteristics. For temperatures up to 512°C, spectrums varied little when velocity ratios were kept nearly constant. Multiple-peak phenomena were associated with bubble coalescence, surface effects, and distributor jetting effects.;A second-order model that describes the dominant peak's location and magnitude was developed. An equation for the natural frequency as a function of bed height was developed from experimental data and agreed with the literature. An equation for the damping ratio as a function of bed height was developed from experimental data and agreed with a new theoretical damping ratio presented here. Using these equations, the model was developed such that its output was a function of bed height, and the model qualitatively described the experimental spectrums adequately.

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Mechanical engineering
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