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The hydrodynamics of an agitated tank have been studied byphase-Doppler anemometry. The focus is on the impeller anddischarge region of a 45o pitched blade turbine (PBT). Thestudy includes agitation of pure water as well as of a dilutesuspension of process particles. A three-dimensionalphase-Doppler anemometer is used to measure local,instantaneous, three-dimensional velocities of the fluid and ofthe suspended particles. A shaft encoding technique is used toresolve the turbulent fluctuations from the periodic velocityfluctuation due to the impeller blades, and to provide moredetailed information about the variations relative to theimpeller blade. Velocity bias is corrected for by the total3-dimensional velocity. The mean flow field, the fluctuating velocities, and thecomplete Reynolds stress tensor, are reported for the liquidphase flow. The periodic fluctuations in the flow that aregenerated by the impeller blades are eliminated in theexamination of the turbulence. The anisotropy of the turbulenceis assessed by the invariants of the anisotropy tensor. Thetrailing vortex structure is demonstrated to be associated withhigh kinetic energy and strong anisotropy of the turbulence.The vortex is still observable 130-140 degrees behind theblade. It gradually moves down from the impeller blade but thelocation in radial direction remains essentially unchanged. Theinfluence of the periodic fluctuations is examined and it isshown that the turbulence appears more isotropic when theperiodic fluctuations are not eliminated. The solid particle concentration is low below the impellerand is high above the impeller tip. The particles diverge fromthe liquid flow mean direction, especially below the agitatorclose to the tip where the strongest turbulence is found.Periodic fluctuations in the particle concentration relate tothe variations found in the angle-resolved mean velocity andfluctuating velocity. The ratio of the maximum to the minimumconcentration is about 2.0 in the present study. The baffles influence on the conditions in the impellerregion, and this influence can be observed on the fluid meanvelocity field, the angle-resolved velocities, the kineticenergy, and on the behavior of larger process particles. In theimpeller region the highest kinetic energies are about 15%higher upstream of the baffle than at the middle plane betweenthe baffles. The highest energy level in the middle plane isactually the lowest value and is therefore not representativewhen rotation symmetry is assumed. Local energy dissipation rates have been investigated, andthe integration of the local energy dissipation rates overdifferent control volumes has been compared with macroscopicenergy balance calculations. The discrepancy is significant.Different reasons have been analyzed and recommendations forfurther investigation are given. I n the outflow region there is a significant variation alsoin the direction of the instantaneous velocity, which may leadto direction bias in the case of non-spherical measurementvolume. In order to account for this direction bias, amathematical model is developed to estimate the projected areaof the measurement volume in LDA or PDA. It is shown that theprojected area variation can lead to a significant directionbias in determination of time averaged values and localparticle concentration in a highly turbulent stirred tank flow.This bias is however negligible for an orthogonal optical setup, as is used in the present study. <b>Keywords:</b>Hydrodynamics, phase-Doppler anemometer,suspension, pitched-blade turbine, anisotropy, turbulence,Reynolds stresses, trailing vortex, kinetic energy, stirredtank

<p>The hydrodynamics of an agitated tank have been studied byphase-Doppler anemometry. The focus is on the impeller anddischarge region of a 45o pitched blade turbine (PBT). Thestudy includes agitation of pure water as well as of a dilutesuspension of process particles. A three-dimensionalphase-Doppler anemometer is used to measure local,instantaneous, three-dimensional velocities of the fluid and ofthe suspended particles. A shaft encoding technique is used toresolve the turbulent fluctuations from the periodic velocityfluctuation due to the impeller blades, and to provide moredetailed information about the variations relative to theimpeller blade. Velocity bias is corrected for by the total3-dimensional velocity.</p><p>The mean flow field, the fluctuating velocities, and thecomplete Reynolds stress tensor, are reported for the liquidphase flow. The periodic fluctuations in the flow that aregenerated by the impeller blades are eliminated in theexamination of the turbulence. The anisotropy of the turbulenceis assessed by the invariants of the anisotropy tensor. Thetrailing vortex structure is demonstrated to be associated withhigh kinetic energy and strong anisotropy of the turbulence.The vortex is still observable 130-140 degrees behind theblade. It gradually moves down from the impeller blade but thelocation in radial direction remains essentially unchanged. Theinfluence of the periodic fluctuations is examined and it isshown that the turbulence appears more isotropic when theperiodic fluctuations are not eliminated.</p><p>The solid particle concentration is low below the impellerand is high above the impeller tip. The particles diverge fromthe liquid flow mean direction, especially below the agitatorclose to the tip where the strongest turbulence is found.Periodic fluctuations in the particle concentration relate tothe variations found in the angle-resolved mean velocity andfluctuating velocity. The ratio of the maximum to the minimumconcentration is about 2.0 in the present study.</p><p>The baffles influence on the conditions in the impellerregion, and this influence can be observed on the fluid meanvelocity field, the angle-resolved velocities, the kineticenergy, and on the behavior of larger process particles. In theimpeller region the highest kinetic energies are about 15%higher upstream of the baffle than at the middle plane betweenthe baffles. The highest energy level in the middle plane isactually the lowest value and is therefore not representativewhen rotation symmetry is assumed.</p><p>Local energy dissipation rates have been investigated, andthe integration of the local energy dissipation rates overdifferent control volumes has been compared with macroscopicenergy balance calculations. The discrepancy is significant.Different reasons have been analyzed and recommendations forfurther investigation are given. I</p><p>n the outflow region there is a significant variation alsoin the direction of the instantaneous velocity, which may leadto direction bias in the case of non-spherical measurementvolume. In order to account for this direction bias, amathematical model is developed to estimate the projected areaof the measurement volume in LDA or PDA. It is shown that theprojected area variation can lead to a significant directionbias in determination of time averaged values and localparticle concentration in a highly turbulent stirred tank flow.This bias is however negligible for an orthogonal optical setup, as is used in the present study.</p><p><b>Keywords:</b>Hydrodynamics, phase-Doppler anemometer,suspension, pitched-blade turbine, anisotropy, turbulence,Reynolds stresses, trailing vortex, kinetic energy, stirredtank</p>

Thesis--M. Phil., University of Hong Kong, 1980.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

This work presents the design, fabrication, calibration and testing of a pressure-based three-component anemometer capable of measuring accurate wind speeds in extreme weather conditions. The groundwork, at the outset, covers the development of a 12-hole omni-directional flow-velocity measurement probe capable of measuring flows up to 155° from the probe axis. The new 12-hole design is optimal in the sense that the calculation of the four unknown flow quantities, i.e., two flow angles, flow speed and static pressure, is achieved with the minimum necessary number of holes/ports on the probe tip. The fact that this design has 33% lesser number of holes compared to an earlier design, has significant implications in the instrument’s spatial resolution, frequency response as well as cost of interfacing and usage. A prototype 12-hole probe with a spherical tip diameter of 3/8 inches was fabricated and tested. Good flow prediction accuracy was obtained. Further groundwork on multi-hole probe technology was carried out, developing new methods for correcting and refining the calibration and reduction procedures. When calibrating multi-hole velocity probes in a wind-tunnel, offset (or bias) errors often exist in the recorded flow angles due to errors in aligning the traverse system exactly with the flow direction and due to the angularity of the tunnel flow itself. These offset angles are hard to quantify from direct measurements with any degree of accuracy. Although usually small (less than 0.5° in most good calibrations), these errors still need to be corrected to increase the flow measurement accuracy of the probe. In this work, a method is developed that computes offset errors in all types of multi-hole probes – from the traditional 5- and 7-hole probes to the omni-directional 18-hole probe and the nextgeneration 12-hole probe – using simply the pressure data obtained during their calibration. The algorithm doubled the measurement accuracy for most probes. Other issues related to post-processing of the pressure data from flow studies, when the multihole probe encountered unsteady and reversed flow conditions, were also examined. The design of the anemometer (herein called a Weatherprobe) builds on that of the 12- hole probe and is capable of measuring wind velocities up to ±45° to the horizontal plane and 360° around the horizontal plane. Due to the non-conventional arrangement of its pressure ports, newly developed calibration and data-reduction algorithms were used. The probe was calibrated and its measurement accuracy assessed in a calibration facility. All associated instrumentation was assembled from the ground up and ruggedized for harsh-weather applications. Field tests performed over many days next to a 3-D sonic anemometer showed good agreement in measured flow properties, thus validating the entire Weatherprobe system. This probe has widespread applications in weather monitoring, wind energy potential estimations and structural wind load evaluations.

A turbulent diffusion flame that is typically used in a thermal spray coating system was analyzed in this study, as part of a diagnostic and development program undertaken by a research group at Queen’s University. Contributions made by this researcher were to numerically and experimentally investigate velocity and turbulent fields of the gaseous phase of the jet. Numerical and experimental analyses have been further developed upon previous research, with improved numerical methods and advanced experimental instrumentation. Numerous numerical simulations were performed in both two dimensional axisymmetric and three dimensional wedge geometries, while testing the dependence of the final solution on various physical models. Numerical analyses revealed the requirement for simulating this problem in three dimensions and improved turbulence modeling to account for relatively high levels of anisotropy. Velocity and turbulent measurements of non-reacting and combusting jets were made with a laser Doppler anemometer to validate numerical models. Excellent agreement was found between predicted and measured velocity and turbulent quantities for cold flow cases. However, numerical predictions did not agree quite as well with experiments of the flame due to limitations in modeling techniques and flow tracking abilities of tracer particles used in experimentation. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2007-09-28 13:05:54.365