Characterization and Modeling of Concentration Profiles during Pulsed Jet Mixing of Slurries
Bamberger, Judith Ann
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The focus of this dissertation is investigating concentration profiles obtained during experiments to characterize pulse jet mixing to understand how the concentration profiles were affected by tank configuration, simulant properties and pulse jet mixer operating conditions for a range of solids loadings. This study provided an in depth investigation of solids suspension and settling during pulse jet mixing. The pulse jet array of four pulse tubes in a ring around the tank center formed a central plume that lifted solids during the drive portion of the mixing cycle. Measurements of concentration were taken at this central location. Measurements at a radial position halfway between the center and the wall characterized the suspension and settling produced by four pulse tubes located in two concentric rings. Measurements near the tank wall characterized the suspension and settling produced by the two pulse tubes in the outer ring nearest that location. Concentration data obtained during these experiments to characterize pulse jet mixing were evaluated to understand how the concentration profiles were affected by slurry properties, geometric configuration of the pulse tubes and test vessels, pulse tube operational parameters for a range of solids loadings. The concentration data were examined at increments during the jet pulse and at increments during settling. These data showed the presence of an "internal cloud height" within the confines of the visually observed cloud height. At the end of the pulse (end of the drive portion of the mixing cycle) the concentration profile of the "internal cloud height" was uniform. For data obtained at a duty cycle of 0.33, the "internal cloud" with uniform concentration profile contained 0.94 of the suspended solids at the end of drive. When compared over the mixing cycle, the measured value at the end of drive is 1.4 times greater than the cycle average concentration. Data were correlated to develop models of vol % solids per horizontal area and solids loading throughout the cycle at increments during the jet pulse and at increments during settling. The model fits the data with a standard deviation of 0.9 at the end of the pulse and 0.94 based on the cycle average. The concentration profiles during the pulse were evaluated at three elevations within the cloud: 0.25, 0.5, and 0.75 HC so that the profiles among tests could be visually compared. The main peak of the pulse is visible at 0.25 HC. It occurs soon after the start of the pulse. The main peak concentration then declines until the end of the pulse denoted by end of drive on each plot. During nondrive, the concentration continues to decrease and then reaches a constant value. At an elevation of 0.5 HC, the main peak of the pulse lags slightly that shown at 0.25 HC and is at a lower concentration. For most plots, the main concentration peak is also visible at 0.75 HC; however, the time for the concentration peak to reach that elevation is further delayed until after the end of drive and the concentration fluctuates cyclically at a frequency much greater than that of the pulse cycle. Correlations between the cycle average concentration at each elevation and solids loading for each of the data sets were developed. The exponents on the power law equations increase from 0.81 at 0.25 HC, to 0.94 at 0.5 HC, and 1.0 at 0.75 HC. The equations provide a relationship to determine average concentration at elevations within the tanks.Observations and evaluation of these tests including concentration profiles and video and photographs during operation showed many similarities between the performance at three scales with three head shapes and five simulants. The main observation is that for most tests the observed phenomena of cloud height and solids suspension during the operating cycle were quite similar in shape at 0.25 HC, 0.5 HC, and 0.75 HC. Differences were noted for smaller lighter particles and larger and heavier particles. These differences suggest that there is an optimum region of operation for the pulse jet system and the majority of the tests evaluated during this study fell within this optimal region of operation.