ADVANCED MIXING MODELS

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The process of recovering the waste in storage tanks at the Savannah River Site (SRS) typically requires mixing the contents of the tank with one to four dual-nozzle jet mixers located within the tank. The typical criteria to establish a mixed condition in a tank are based on the number of pumps in operation and the time duration of operation. To ensure that a mixed condition is achieved, operating times are set conservatively long. This approach results in high operational costs because of the long mixing times and high maintenance and repair costs for the same reason. A significant reduction ... continued below

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Lee, S; Richard Dimenna, R & David Tamburello, D November 13, 2008.

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Description

The process of recovering the waste in storage tanks at the Savannah River Site (SRS) typically requires mixing the contents of the tank with one to four dual-nozzle jet mixers located within the tank. The typical criteria to establish a mixed condition in a tank are based on the number of pumps in operation and the time duration of operation. To ensure that a mixed condition is achieved, operating times are set conservatively long. This approach results in high operational costs because of the long mixing times and high maintenance and repair costs for the same reason. A significant reduction in both of these costs might be realized by reducing the required mixing time based on calculating a reliable indicator of mixing with a suitably validated computer code. The work described in this report establishes the basis for further development of the theory leading to the identified mixing indicators, the benchmark analyses demonstrating their consistency with widely accepted correlations, and the application of those indicators to SRS waste tanks to provide a better, physically based estimate of the required mixing time. Waste storage tanks at SRS contain settled sludge which varies in height from zero to 10 ft. The sludge has been characterized and modeled as micron-sized solids, typically 1 to 5 microns, at weight fractions as high as 20 to 30 wt%, specific gravities to 1.4, and viscosities up to 64 cp during motion. The sludge is suspended and mixed through the use of submersible slurry jet pumps. To suspend settled sludge, water is added to the tank as a slurry medium and stirred with the jet pump. Although there is considerable technical literature on mixing and solid suspension in agitated tanks, very little literature has been published on jet mixing in a large-scale tank. If shorter mixing times can be shown to support Defense Waste Processing Facility (DWPF) or other feed requirements, longer pump lifetimes can be achieved with associated operational cost and schedule savings. The focus of the present work is to establish mixing criteria associated with the waste processing at SRS and to quantify the mixing time required to suspend sludge particles with the submersible jet pump. Literature results for a turbulent jet flow are reviewed briefly, since the decay of the axial jet velocity and the evolution of the jet flow patterns are important phenomena affecting sludge suspension and mixing operations. One of the main objectives in the waste processing is to provide the DWPF a uniform slurry composition at a certain weight percentage (typically {approx}13 wt%) over an extended period of time. In preparation of the sludge for slurrying to DWPF, several important questions have been raised with regard to sludge suspension and mixing of the solid suspension in the bulk of the tank: (1) How much time is required to prepare a slurry with a uniform solid composition for DWPF? (2) How long will it take to suspend and mix the sludge for uniform composition in any particular waste tank? (3) What are good mixing indicators to answer the questions concerning sludge mixing stated above in a general fashion applicable to any waste tank/slurry pump geometry and fluid/sludge combination? Grenville and Tilton (1996) investigated the mixing process by giving a pulse of tracer (electrolyte) through the submersible jet nozzle and by monitoring the conductivity at three locations within the cylindrical tank. They proposed that the mixing process was controlled by the turbulent kinetic energy dissipation rate in the region far away from the jet entrance. They took the energy dissipation rates in the regions remote from the nozzle to be proportional to jet velocity and jet diameter at that location. The reduction in the jet velocity was taken to be proportional to the nozzle velocity and distance from the nozzle. Based on their analysis, a correlation was proposed. The proposed correlation was shown to be valid over a wide range of Reynolds numbers (50,000 to 300,000) with a relative standard deviation of {+-} 11.83%. An improved correlation including the effect of circulation time was proposed by Grenville and Tilton [11] via a better fit of mixing time data for turbulent jet mixing under a wider range of jet Reynolds numbers (50,000 to 300,000). The circulation time was defined as the liquid volume divided by the entrained flow rate. They assumed that the mixing rate at the end of the jet length controls the mixing time for the entire tank by estimating the kinetic energy dissipation rate as discussed earlier. They predicted that for a given volume, an optimum geometry exists for a mixing vessel, allowing a desired mixing time to be achieved for a minimum power input. The current work will compare their correlation of the jet mixing time with CFD modeling results for their experimental tanks in an attempt to achieve a fundamental understanding of the turbulent jet mixing and to establish mixing indicators.

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  • Report No.: SRNL-STI-2008-00417
  • Grant Number: DE-AC09-08SR22470
  • DOI: 10.2172/941114 | External Link
  • Office of Scientific & Technical Information Report Number: 941114
  • Archival Resource Key: ark:/67531/metadc899543

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  • November 13, 2008

Added to The UNT Digital Library

  • Sept. 27, 2016, 1:39 a.m.

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  • Nov. 2, 2016, 5:13 p.m.

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Lee, S; Richard Dimenna, R & David Tamburello, D. ADVANCED MIXING MODELS, report, November 13, 2008; [Aiken, South Carolina]. (digital.library.unt.edu/ark:/67531/metadc899543/: accessed September 22, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.