Fluctuation bubble density

Fig. 6 shows the FFT spectrum for calculated bed pressure drop fluctuations at various centrifugal accelerations. The excess gas velocity, defined by (Uo-U ,, was set at 0.5 m/s. Here, 1 G means numerical result of particle fluidization behavior in a conventional fluidized bed. In Fig. 6, the power spectrum density function has typical peak in each centrifugal acceleration. However, as centrifugal acceleration increased, typical peak shifted to high frequency region. Therefore, it is considered that periods of bubble generation and eruption are shorter, and bubble velocity is faster at hi er centrifugal acceleration. 

Baumgarten and Pigford (B2) have employed the y-ray method for their study of density fluctuations in fluidized beds. The method is laborious and time-consuming, and yields only approximate values based on a large number of bubbles. Because of this, the x-ray cinephotographic method is to be preferred for the study of the behavior of bubbles in fluidized beds. 

The onset of turbulent fluidisation appears to be almost independent of bed height, or height at the minimum fluidisation velocity, if this condition is sufficiently well defined. It is, however, strongly influenced by the bed diameter which clearly imposes a maximum on the size of the bubble which can form. The critical fluidising velocity tends to become smaller as the column diameter and gas density, and hence pressure, increase. Particle size distribution appears to assert a strong influence on the transition velocity. With particles of wide size distributions, pressure fluctuations in the bed are smaller and the transition velocity tends to be lower. 

Measurements of the current density revealed a linear increase as the applied potential increases up to a certain level, followed by fluctuations. This phenomenon is related to the presence of hydrogen bubbles, and specifically to the bubble growth, detachment, coalescence, and collapse. This agrees with the results of Kahanda and Tomkiewicz [76] who explained the current density fluctuations in terms of the competition between the metal deposition and the hydrogen evolution. 

The cross-correlation technique measures the time of flight of an inherent flow tag passing through two sensors separated by a known distance. The technique has been used successfully to monitor single-phase fluid flows in which turbulent eddies modulate the interrogating ultrasonic beams. This type of correlation flowmeter has also been developed for solid/liquid and gas/liquid flows, in which the density fluctuation, caused by clusters of solids and by gas bubbles, is the prime inherent flow tag. 

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