Bubble Velocity Distribution Research Articles

Gas‐phase properties in stirred tank bioreactors

AbstractBubble properties in stirred tank bioreactors equipped with standard radial flow Rushton turbines have been investigated. Bubble velocity patterns within a stirred tank reactor were derived from measured local bubble velocity distributions. Local information on specific interfacial areas, bubble number densities and bubble diameters together with local gas hold‐ups, measured in a model medium at gas flow rates which are employed in practice, is presented. All measurements were performed with two new ultrasound pulse techniques which can be used in model as well as during real cultivation processes.

Flow structure of the dispersed gasphase in real multiphase chemical reactors investigated by a new ultrasound–doppler technique

AbstractThe flow structure of the dispersed gas phase within an aerated stirred tank reactor of pilot scale (600L) has been investigated by means of a new ultrasound reflection technique. A compact implementation of the new measuring technique is described in this article. It allows the investigation the flow structure of the dispersed gas phase in real multiphase flows within arbitrary large reaction vessels. Measurements have successfuly been performed even in complex biotechnological and chemical production systems. The measuring device exploits the Doppler–shift of ultrasound pulses upon reflections at the surfaces of moving bubbles. Only a single measuring probe needs to be introduced into the dispersion. Bubble velocity distributions can be obtained within a few minutes, even during production processes in highly viscous media. The device is simple to use, since it is fully automized by means of integrated microprocessors.

Numerical Analysis of a Single Bubble with VOF Method.

In order to examine the feasibility of direct simulation of bubbly flows, the applicability of the VOF (volume of fluid) method to the analyses of a single rising bubble was examined in this study. Calculated bubble shapes and terminal velocities under wide ranges of Eotvos number and Morton number were compared with the experimental data summarized by Grace et al. Except for the cases when bubble shapes are spherical-cap and skirted, the VOF method could predict them well by assigning only eight cells to the bubble diameter. Hence, it was confirmed that some modification of this method will enable us to simulate bubbly flows directly under a wide-range of flow conditions. Moreover, the relation between a bubble shape and velocity distribution was examined within the ranges in which the VOF method is valid. It was found that the secondary vortex appearing in wobbling bubbles induces the velocity component normal to the bubble interface, and this velocity is one of the causes of the wobbling shape of the bubble.

AN ASSESSMENT OF EXPERIMENTAL TECHNIQUES FOR THE MEASUREMENT OF BUBBLE SIZE IN A BUBBLE SLURRY REACTOR AS APPLIED TO INDIRECT COAL LIQUEFACTION

The proper design and control of an indirect coal liquefaction process plant require an accurate knowledge of bubble sizes and size distribution. Of particular importance regarding proper design is the understanding of the complicated dependence of bubble dynamics on bubble column geometry, and of the nature of the solid-liquid phases. The nature of the products and their relative proportions, on the other hand, are seriously influenced by the bubbling characteristics of the reactor because the mass transfer behavior and mixing are dependent on the bubble size and velocity distributions. This report reviews experimental techniques that have been employed to measure bubble sizes in multiphase reactors. The methods, which fall in three different categories, namely, the photographic method, the optical probe, and the electrical conductivity (resistivity) probe, are described in a historical sequence; and in each case, their unique features and design details are discussed. A general methodology of data analysis in each category is given, together with a state-of-the-art technology assessment and, finally, relative merits and demerits of the methods.

Properties of bubble swarm in a slurry bubble column.

The effects of solid concentration and liquid viscosity on bubble properties such as bubble size and bubble rising velocity were measured with a dual-electroresistivity probe in a slurry bubble column of 0.15m diameter. The behavior of volume-surface mean bubble diameter, dvs, was analyzed. By addition of solid particles at small gas velocity, the bubble size distribution shifted to a large-size region and the bubble velocity distribution shifted to a large-velocity region. At the same time, the flow pattern changed from homogeneous flow to heterogeneous flow. The effect of solid particles on bubble size, however, became small as the particle diameter decreased or the liquid viscosity increased. The following equation was derived to estimate dvs. dvs=0.59(VD/eg)2/gwhere VD is the drift flux of gas, eg is the cross-sectionally averaged gas holdup and g is the gravitational acceleration.

A new similarity rule for fluidized bed scale‐up

AbstractA rule of hydrodynamic similarity for a scale change of fluidized beds has been developed based on the governing equations of bubble and interstitial gas dynamics. When geometrically similar scale‐up is to be carried out maintaining hydrodynamic similarity, the proposed similarity rule requires that two conditions be satisfied. The first condition assures a similarity in bubble coalescence. The second assures the similarities in bubble splitting and in the interstitial flow pattern. The present work proves theoretically that these two conditions are the necessary, and almost sufficient, conditions for hydrodynamic similarity. They consider not only bubble coalescence but also bubble splitting. The theory proposed was first tested by previous correlations for bubble diameter and grid zone structure, and second by experiment. It has been proved that as long as the present rule is satisfied, the longitudinal distribution of the average bubble diameter, stochastic variation around it, and the radial distribution of superficial bubble velocity can be maintained similar for a scale change.Application of the present rule to predict the bubbling and solid circulation characteristics in a large‐scale unit by a small‐scale experiment has thus proved promising, at least for Geldart group B particles. In order to further study the possibility of a similarity in mass transfer and chemical reactions, computation was carried out using the three‐phase bubble assemblage model. If the effect of molecular diffusion is negligible, as in the case of fluidized bed combustion, even a chemical similarity is found to be possible.

Experience with an on-line surveillance system in LWR's

A computer controlled Multichannel On-Line Noise Analysis System (MONAS) had been successfully applied in a BWR for surveillance of thermal hydraulic behaviour of the core and is now installed in a PWR power plant for in-core neutron noise analysis and evaluation. The system is designed for automatic processing, reduction and evaluation of 32 noise signals of any kind with a cycling period of a few minutes. The characteristic data and functions of the past 40 or 50 cycles are stored temporarily and can be monitored on a colored video display. The actual values are checked against reference data and certain alert levels, so that any anomalies in the pattern and data evolution are indicated on-line, and the reactor history of the past several hours remains assessible. Long time behaviour is being surveyed optionally by periodical prints and plots. In its first application MONAS was connected for more than one year to the LPRM-instrumentation in in the core of the BWR Würgassen (KWW) analysing bubble and flow-induced noise effects and monitoring particularly the steam bubble velocity distribution. After modifications of hard- and software the system is now in operation since the end of 1983 at the PWR power plant Obrigheim (KWO) processing the signals of 35 self-powered neutron detectors. The task of surveying certain thermal hydraulic, vibrational or nuclear parameters and identifying any causes of distortions from normal reactor operation is being approached by a learning process. In cases of exceeding preset alert levels the system preserves all digital data and results from temporary memory and from the continuing analysis on a digital tape and furthermore initializes the storage of the original in-core signals on an analog magnetic tape. Thus, a subsequent off-line analysis enables to investigate the history of a developing anomaly and the further evolution after the event. The experience gained with MONAS in a BWR and recently in a PWR is reported in detail.

Investigation of the structure of two-phase flows. Model media in bubble-column bioreactors

The following two-phase properties were evaluated in bubble column reactors with porous plate (5 μm pore diameter) or perforated plate (1 mm and/or 3 mm hole diameter) gas distributors using distilled water or a 1% methanol solution: transverse profiles of the mean and Sauter bubble diameters, local gas holdups, true mean liquid and bubble velocities. Furthermore, swarm bubble velocity distributions were evaluted and compared with calculated values.