The mechanics of submerged multiport diffusers for bouyant discharges in shallow water
Author(s)Jirka, Gerhard H.; Harleman, Donald R. F.
A submerged multiport diffuser is an effective device for disposal of water containing heat or other degradable wastes into a natural body of water. A high degree of dilution can be obtained and the environmental impact of concentrated waste can be constrained to a small area. An analytical and experimental investigation is conducted for the purpose of developing predictive methods for buoyant discharges from submerged multiport diffusers. The following physical situation is considered: A multiport diffuser with given length, nozzle spacing and vertical angle of nozzles is located on the bottom of a large body of water of uniform depth. The ambient water is unstratified and may be stagnant or have a uniform current which runs at an arbitrary angle to the axis of the diffuser. The general case of a diffuser in arbitrary depth of water and arbitrary buoyancy is treated. However, emphasis is put on the diffuser in shallow receiving water with low buoyancy, the type used for discharge of condenser cooling water from thermal power plants. A multiport diffuser will produce a general three-dimensional flow field. Yet the predominantly two-dimensional flow which is postulated to exist in the center portion of the three-dimensional diffuser cart be analyzed as a two-dimensional "channel model", that is a diffuser section bounded by walls of finite length and openings at both ends into a large reservoir. Matching of the solutions for the four distinct flow regions which can be discerned in the channel model, namely, a buoyant jet region, a surface impingement region, an internal hydraulic jump region and a stratified counterflow region, yields these results: The near-field zone is stable only for a limited range of jet densimetric Froude numbers and relative depths. The stability is also dependent on the jet discharge angle. It is only in this limited range that previous buoyant jet models assuming an unbounded receiving water are applicable to predict dilutions. Outside of the parameter range which yields stable near-field conditions, the diffuser-induced dilutions are essentially determined by the interplay of two factors: frictional effects in the far-field and the horizontal momentum input of the jet discharge. Three far-field flow configurations are possible, a counter flow system, a stagnant wedge system and a vertically fully mixed flow, which is the extreme case of surface and bottom interaction. A three-dimensional model for the diffuser-induced flow field is developed. Based on equivalency of far-field effects, the predictions of the two-dimensional channel model can be linked to the three-dimensional diffuser characteristics. Diffusers with an unstable near-field produce three-dimensional circulations which lead to recirculation at the diffuser line: effective control of these circulations is possible through horizontal nozzle orientation. The diffuser in an ambient cross-current is studied experimentally. Different extreme regimes of diffuser behaviour can be described. Performance is dependent on the arrangement of the diffuser axis with respect to the crossflow direction. Experiments are performed in two set-ups, investigating both two- dimensional slots and three-dimensional diffusers. Good agreement between theoretical predictions and experimental results is found. The results of this study are presented in form of dilution graphs which can be used for three-dimensional diffuser design or preliminary design if proper schematization of the ambient geometry is possible. Design considerations are discussed and examples are given. For more complicated ambient conditions, hydraulic scale models are necessary. The results of this study indicate that only undistorted scale models simulate the correct areal extent of the temperature field and the interaction with currents, but are always somewhat conservative in dilution prediction. The degree of conservatism can be estimated. Distorted models are less conservative in predicting near-field dilutions, but exaggerate the extent of the near-field mixing zone.
Prepared by the Ralph M. Parsons Laboratory for Water Resources and Hydrodynamics in association with the Energy Laboratory
MIT Energy Lab
Waste heat, Thermal pollution of rivers, lakes, Diffusers, Two-phase flow, Buoyant ascent (Hydrodynamics)
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