G.B. Tatterson

Image processing of tracer particle motions as applied to mixing and turbulent flow—I. The technique

Abstract Techniques of digital image processing of stereoscopic motion pictures are presented which were used to automate the tracking of neutrally buoyant tracer particles in turbulent flow. A software package is described which performed the following tasks: (1) particle image identification, (2) particle image tracking and (3) stereo pair matching of particle trajectories. Various criteria and objective functions are given which were used to accomplish the tasks.

LIQUID DISPERSION MECHANISMS IN AGITATED TANKS: PART I. PITCHED BLADE TURBINE

An oil into water dispersion, created by a pitched blade turbine, was observed using high speed, stereoscopic motion pictures. Two different dispersion mechanisms were responsible for the break-up of the oil drops, even though both mechanisms occurred in the vortex system trailing from the impeller blade tips. The first mechanism could be described as ligament stretching, since large oil drops were stretched by fluid shear to form elongated ligaments, which subsequently ruptured into small drops. The second mechanism was turbulent fragmentation, where large oil drops were shattered into large droplet clouds the instant they entered the trailing vortex system. Observations of the oil drops undergoing ligament stretching also indicated that velocities in the trailing vortex system were proportional to impeller tip speed.

A THREE-DIMENSIONAL MEASUREMENT TECHNIQUE FOR TURBULENT FLOWS

An experimental technique is presented which will help resolve the nature of bulk motions occurring in turbulent flow. The technique is based upon photogrammetric analysis of stereoscopic motion pictures of the flow Held containing small neutrally buoyant tracer particles. The resulting data essentially consist of discrete sets of three-dimensional particle paths which characterize the bulk motions. The data obtained from the particle paths can also provide such information as point velocity measurements, velocity profiles and estimates of fluid accelerations. Furthermore the technique provides this information at a number of locations simultaneously and with time. In this work, the technique has been specifically applied to the study of flow phenomena occurring in an agitated tank.

Sound spectra of gas dispersion in an agitated tank

Abstract Sound spectra of gas dispersion phenomena were obtained using a hydrophone and a spectrum analyzer at three locations (the sparge ring, the impeller and a position in the bulk of the tank) in an agitated tank as a function of gassing rate and impeller rotational speed. For the sparge ring, the spectra made it possible to distinguish between gas sparging controlled by the sparge ring itself and the natural volume pulsation frequency of the bubbles and gas sparging controlled by the impeller flow. Sound pressure level spectra, obtained for an impeller under various flooding and non-flooding conditions, showed harmonies in the sound pressure levels only for non-flooding conditions. The number and amplitude of the harmonics diminished as the impeller approached flooding. The sound pressure level spectra for a position in the bulk of the tank resonated with the impeller rotational speed in some cases in which the impeller was not flooded. In other cases in which the impeller was flooded, the sound pressure level spectra of the bulk of the tank resembled the impeller spectra at frequencies below 500 Hz.

Image processing of tracer particle motions as applied to mixing and turbulent flow. II: Results and discussion

Abstract The results of the application of digital image processing of tracer particle motions recorded on stereoscopic motion picture films are discussed. Effects of threshold level on particle identification, data on location and velocity error analyses of the particles and results of the particle tracking and matching are investigated.

LIQUID DISPERSION MECHANISMS IN AGITATED TANKS: PART II. STRAIGHT BLADE AND DISC STYLE TURBINES

The dispersion of oil in water in an agitated vessel was studied for two types of radial discharge impellers, straight blade and disc style turbines. Two different dispersion mechanisms, ligament stretching and turbulent fragmentation, were observed to occur in the vortex systems of the impeller discharge. Although these two dispersion mechanisms were similar to pitched blade turbine performance, differences in the velocity magnitudes and vortex interactions were observed with the radial flow impellers. The ligament stretching mechanism was observed between the vortex formation regime and the transition to the fragmentation regime. The turbulent fragmentation mechanism was observed only in highly turbulent flow. Blade thickness was found to influence the ligament stretching mechanism. A thin blade on the straight blade turbine created higher vortex velocities and smaller drop sizes than a thick blade for the same tip speed and processing time. The consequences of this blade thickness effect could be signi...

THE EFFECT OF DRAFT TUBES ON CIRCULATION AND MIXING TIMES

Low circulation times do not necessarily imply good mixing. Data on circulation and mixing times plus their various standard deviations should be obtained and reported in any mixing study. The standard deviation in circulation times inversely measures the uniformity of momentum transfer in an agitated tank but should not be used as a measurement of mixing. The standard deviations of the mixing times obtained were substantially different from those obtained from circulation time data. The use of draft tubes and cone bottoms will improve the mixing in a processing vessel.

CHARACTERIZATION OF MIXING IN FLAT-BOTTOMED SPHERICAL FLASKS AGITATED WITH MAGNETIC STIRRERS

Mixing in a flat-bottomed spherical flask with magnetic stirrers is studied using dye addition and visual studies to measure mixing time. Different flow regimes that occurred in the flask and two regimes, i.e., a well-mixed regime and a Taylor wall regime, were identified. The effects of stirrer diameter, volume of material to be mixed, and rotational speed on mixing time were studied.

ELBOW FLOWMETER CALIBRATIONS FOR SLURRIES

The use of an elbow flowmeter for the volumetric flowrate measurements of water, a glass bead/water slurry, and a sand/water slurry was experimentally investigated. It was found that as long as the slurry behaved as a homogeneous single phase fluid, the calibration of the elbow flowmeter was the same as that for a single phase fluid and the pressure difference across the elbow was that expected from single phase correlations. The effects of concentration were accounted for by using the density of the slurry for these calculations. The coefficient of discharge was calculated and analytically predicted to within 5%. If the conditions were such that a considerable slip velocity existed between the fluid and the slurry particles, the flowmeter behaved differently from that expected for a homogeneous fluid. The flow of a sand/water slurry through the elbow flowmeter which consisted of large particles resulted in stratification of the sand in the elbow. This caused a substantial change in the value of the elbow...

SUSPENSION CHARACTERISTICS OF LARGE CYLINDERS IN AGITATED TANKS

Considerable research has been done concerning solid suspension in agitated tanks. Typically, such studies concern slurries or granular material, such as sand, and involve minimum suspension rotational speeds or suspension homogeneity. The objectives of this work are similar, however we are concerned with rather large cylinders of varying length to diameter ratios. The work involves: 1) the determination of minimum impeller rotational speed for suspension similar to criterion used by Zwietering, 2) the measurement of suspension heights and times, 3) an investigation into whether the correlations by Zwietering, Kolar and Rieger et al., can be used for predicting minimum suspension rotational speed and 4) weight fraction effects on minimum suspension rotational speed.