Bubble Velocity and Size

Bubble size and bubble velocity measurement techniques have been reviewed by Saxena et al. (1988) and Boyer et al. (2002) the interested reader is directed to these sources for detailed descriptions of the available hardware and data analysis procedures. 

It is common to report the average bubble diameter as the Sauter mean diameter which is defined as the diameter of a bubble equivalent to the volume-to-surface area ratio of the entire dispersion (Saxena et al 1988). Assuming all bubbles are spheres, 

If the time difference between successive frames of the same bubble is known and the bubble displacement can be measured, then bubble rise velocity can also be measured. However, visual methods are limited to systems with optical access, so observations are limited to regions near the wall even under moderate gassing rates in gas-liquid systems. The wall and liquid must also be transparent. 

In addition to optical methods, bubble size can be determined using optical probes and electrical conductivity (resistivity) probes (Saxena et al., 1988). For example, Magaud et al. (2001) used dual optical probes to determine the local instantaneous presence of the liquid or gas in a bubble column. With this information, local bubble chord length and bubble rise velocity can be determined. One advantage of optical probes is that its operation is independent of the electrical properties of the medium surrounding the probe (Saxena et al., 1988). Electrical conductivity or resistivity probes can be configured as needle probes, which have been used to determine mean bubble chord length, bubble size, and bubble rise velocity. 

Ultrasound Doppler velocimetry (UDV) has also been used to measure bubble velocity by measuring the frequency shift between the emitted ultrasound beam and the echo reflected from the gas-liquid interface (Vial et al., 2003). 

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We find that the numerical fluidized bed model predicts bubble size and velocity, at ambient pressure and temperature, which agrees with the data. Further in that figure we show the results of three calculations at high pressure (40 atm) and room temperature (293 K) which, when correlated in the same fashion, yield predictions of bubble size and velocity which also agree with the ambient pressure data. This agreement between high... 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

One continuous and one discrete fluid phase. Most often this will be a discrete (bubble) phase and a continuous liquid phase. The simplifying assumptions made above will be retained for this case as well. For the general model equations (8-191) and (8-192), with negligible dispersion and constant bubble size and velocity in the... 

Microorganisms have a complex cell envelope structure. Their surfaces charge and their hydrophobicity cannot be predicted, only experimentally determined [131]. Several microorganisms are not hydrophobic enough to be floated. They need collectors, similar to ore flotation. In cultivation media proteins which adsorb on the cell surface act as collectors. The interrelationship between cell envelope and proteins caimot be predicted, only experimentally evaluated. The accumulation of cells on the bubble surface depends not only on the properties of the interface, proteins and cells, but on the bubble size and velocity as well [132]. On account of this complex interrelationship between several parameters, prediction of flotation performance of microbial cells based on physicochemical fundamentals is not possible. Therefore, only empirical relationships are known which cannot be generalized. Based on the large amount of information collected in recent years, mathematical models have been developed for the calculation of the behavior of protein solutions and particular microbial cells. They hold true only for systems (e.g. BSA solutions and particular yeast strains) which are used for their evaluation. In spite of this, several recommendations for protein and microbial cell flotation can be made. 

At the same time, the vibration of the fluidized bed can be alleviated by eliminating large bubbles and realizing uniform distribution of bubble size. With the decrease of the bubble size and velocity, a bed with internals has a higher bed surface than a free bed at the same gas velocity. However, the baffles tend to impede solids movement so that surface/bed heat transfer coefficients decrease particle segregation can also occur, and then it is difficult to have a full fluidization in all compartments simultaneously. In addition, the total pressure drop across the bed will be slightly increased by the horizontal baffles. 

The model of Partridge and Rowe (1966) makes allowance for variable bubble sizes and velocities and for the presence of clouds. Unfortunately, for the conditions of their work, the overestimation of visible bubble flow by the two-phase theory of Toomey and Johnstone (1952) led to incompatibility between predicted cloud areas and the total bed cross section. This mechanical incompatibility prevented direct application of these models to the reaction data obtained in their work. 

N2 2000 Bubble size and velocity decrease bubble frequency increases with pressure more gas flows in bubble wakes and less in bubble voids. 

Bubble Size and Velocity in Freely Bubbling Beds... 

The first Five-Point Electro Conductivity Probe Technique (FPECPT) was developed by Burgess and Calderbank [30]. In this technique, the bubble size and velocity estimation were based on the assumption that the bubble is always moving up vertically. An advanced version of the Five-Point Electro Conductivity Probe Technique (FPCPT) probe was designed by Steinemann and Buchholz [188]. The bubble size and velocity estimation were not based on the assumption that the bubble is always moving up vertically but computed from the geometry of the probe design and the signals from the bubbles. 

The advantage of using Mo is that this number is a gas and fluid property independent of the actual bubble size and velocity. The Morton number is of the order of Mo = 10 for air bubbles in water. 

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