Preliminary evaluation of PSCM and BIPP melter design and operating conditions using physical modeling [report]

R.J. Skarda, S.G. Hauser, J.A. Fort
1985 unpublished
The Glass Melter Physical Modeling investigation was initiated to support Pacific Northwest Laboratory (PNL) Hanford Waste Vitrification Program. Specifically, results discussed herein are those of the modeled B-Plant Immobilization Pilot Plant (BIPP) and Pilot Scale Ceramic Melter (PSCM) designs. The purpose of this study was to evaluate various melter design features using laboratory scale models. Hydrodynamic, thermal, and electrical similarity between the modeling fluid and the molten glass
more » ... nd the molten glass were primary objectives. Stroboscopic velocity measurements (flow visualization), temperature measurements, and electrical potential measurements were used to investigate the molten glass behavior. Results from this effort are to provide input to melter design and proposed operation in addition to providing a data base for verifying numerical models. The approach used in the current glass melter modeling investigation was to obtain similarity parameters by nondimensionalizing the governing differential equations (i.e., mass, momentum, and energy equations) and the corresponding boundary conditions. Similarity of the four dimensionless groups: Peclet number, Pe, Raleigh number, Ra, Nusselt number, Nu, and the Power number, ¢s was required. For the convenience of determining scaling factors, Ra was regrouped into two dimensionless groups, ~6T and GaPr. Model to prototype scaling factors were then derived from the relevant dimensionless groups. Determination of scaling factors was required because they provide a systematic method of determining modeling fluid properties and model operating conditions that correspond to the desired prototype operating conditions. A modeling fluid was desired that could simultaneously be scaled to the glass kinematic viscosity and electrical resistivity for a given melter operating configuration. Model fluid criteria was satisfied by Glycerin-Lithium Chloride mixtures which were prepared using an iterative procedure. The 1/3 scale PSCM model was fabricated from plexiglass, while a 1/4 scale BIPP model was fabricated from glass. Plexiglass® cooling jackets for both models were ® Rohm & Hass Co. v constructed on all sides of the models to maintain the proper wall temperatures, however, heat loss from the bottom occurred by natural convection. \~ater cooled plate electrodes were fabricated from copper to simulate the PSCf·l prototype Inconel® electrodes. The BIPP water cooled electrode configuration, an upper and lower copper plate electrode pair simulated the BIPP Inconel® electrode configuration. Power was supplied to the electrodes, in both configurations by phase angle forced SCR-power controllers. Lastly, a molten crust or "cold cap" which exists at the surface of the molten glass was simulated with a water cooled "cold cap." An Argon ion laser was the source of a 1/8" thick vertical light sheet which was required for the stroboscopic velocity measurements. Neutrally buoyant glassy carbon particles which were developed at PNL were used as flow tracers/light scatters for flow visualization. Data was collected using the Integrated System for Automated Acquisition and Control, ISAAC, which provided data with 12 bit binary resolution to an Apple IIE PC. Temperature r:1easure ments were taken of the model walls, bottom & too and modeling fluid. Cooling jacket temperatures and flows as well as electrode, power current, voltage, and temperature were also monitored and recorded. f1easurement and result accuracies were determined using the Kline and fkClintock uncertainty 1Jethod. The objectives of both the BIPP and PSCf>' l studies were to minimize glass temperature gradients and increase fluid velocities by promoting stable convective cells in the fluid which increase fluid mixing. Fluid regions with low velocities promote particle settling. Large temperature gradients within the glass are undesirable because large variations of both viscosity and electrical conductivity occur. Large electrical conductivity variations produce nonuniform current densities which could result in excess electrode corrosion due to high local current densities. Nonuniform electrical conductivity in the BIPP melter could also lead to crossfiring of the electrodes. Cooler regions in the melter can also provide regions of crystallization due to the tendency of the glass to devitrify at lower temperatures [11]. High viscosity in cooler regions also promotes settling of nonsoluble particles which creates nucleation ®Carborundum Co. vi • • • • • • sites for crystallization formation. The sludge formation could potentially lead to blockage of the throat or riser over longer periods of operation [11]. In addition, the formation of a conductive sludge could lead to shorting of electrodes. Specific objectives of the BIPP model investigation were to measure the effects of electrode spacing and upper/lower Electrode Power Ratio (U/L EPR) on model operating performance. Preliminary BIPP tests at U/L EPR's of 1.0:0.0, 0.5:0.5, and 0.26:0.74 were completed, although no effects of electrode spacing were examined. Additional BIPP model testing is scheduled to address botn objectives in greater detail. Preliminary testing revealed that fluid is divided into two flow regions for operation at a U/l EPR of 1.0:0.0. Fluid velocities are faster and fluid temperatures warmer in the upper half of the model and between the powered upper electrodes, relative to the fluid in the lower portion of the melter. A more uniform temperature was obtained for a U/l EPR of 0.5:0.5 as compared to 1.0:0.0 U/l EPR. Well defined convective cells were observed to penetrate through roughly the full fluid depth for a U/L EPR of 0.26:0.74. During U/l EPR 1.0:0.0 testing, unstable convective cells were observed in the upper portion of the model while no convective cells appeared in the lower fluid regions. During PSCf~ testing, large temperature gradients existed in the vertical direction and across the horizontal length of the model when bubbler induced mixing was not provided. The temperature differential in the model was reduced from l2°C to 2°C (55°C to 9°C in the melter) by implementation of bubblers. Less mixing occurred near the electrodes where the temperature profiles were also increasingly stratified. During bubbler mixing, two counter rotating eddies. symmetric about the melter center were observed. Bubblers increased the speed at which these eddies rotated and also induced secondary eddies near the electrodes. Fluid velocities in the laminar flow regions near boundaries were also increased by bubbler induced mixing • In all BIPP and PSCM test runs, large temperature gradients existed in the immediate region of the cold cap. The flow field was symmetric about the melter center with the top completely cooled. However, cooling only an offset portion (60%} of the top resulted in a distorted or nonsymmetrical flow-field vii about the melter center. The flow field was skewed in the direction of the cold cap. Cold cap orientation, bubbler flow rates, and number of bubblers produced no observable change in potential field. BIPP testing suggests that varying the power density of fluid layers i? a viable means of enhancing fluid mixing, however, these preliminary results are not sufficient to recommend an electrode operating configuration. As expected, a U/L EPR of 1.0:0.0 was a poor operating configuration. The U/L EPR of 0.26:0.74 provided better overall mixing and may be a possible operating configuration. The effects of upper to lower electrode spacing need to be considered in addition to the possibility of electrode crossfiring. Use of bubblers in the PSCM model enhances mixing which results in a more uniform temperature. Bubblers not only provide greater mixing in the center of the melter but also increase fluid velocities in laminar flow regions near the model floor. The smaller temperature variations and higher fluid velocities are believed to minimize the opportunity for crystallization and particle settling in the melter. Scaling criteria were not sufficiently developed to provide relative bubbler sizes and bubbler gas flow rates for the operating mel ter. These sealing criteria should be de vel oped for present PSC~l modeling results and future modeling efforts. Installation of bubblers in the PSOl should be considered to improve fluid mixing. viii • • • • • • • CONTENTS
doi:10.2172/5514078 fatcat:maioxvg5xzaeldaqno76bq67dm