Near field performance of staged diffusers in shallow water
Author(s)Adams, E. Eric; Trowbridge, John Howard
Submerged diffusers are commonly used to dilute condenser cooling water from coastal power plants. A staged diffuser, in which the diffuser centerline is perpendicular to shore and the nozzles are direc- ted essentially offshore, is often used at sites where there is a long- shore, reversing current. Because of the symmetry of this design, dilution is improved by a longshore current in either direction, and the diffuser's position perpendicular to shore allows it to intercept a crossflow effectively. The performance of a staged diffuser in shallow water of constant depth has been analysed previously by treating the diffuser as a continuously distributed line source of momentum (Almquist and Stolzen- bach, 1976). This theory has been reviewed and extended to consider the case of a sloping bottom and to compute the external (entrainment) flow field set up by the diffuser. In these analyses the important parameters are the gross diffuser dimensions, including total flow rate, discharge velocity, water depth and diffuser length. Length scales are on the order of one diffuser length, and the characteristics of the individual jets are assumed to be insignificant in describing diffuser performance at this level. A more detailed analysis of staged diffuser performance in the near field is useful if one wishes to describe the temperatures and shear stresses experienced by organisms that are entrained into the diffuser plume. Length scales in this problem are on the order of the port spacing, and characteristics of the individual jets are very important at this level. Relevant diffuser dimensions are discharge velocity, port diameter D , port spacing, port elevation h, water depth H, and discharge orientation. A description of the near field at this level has been obtained by solving for the trajectories, velocities, temperatures and flow rates of individual jets. Boundary layer approximations are made similar to those used in the classical analysis of free turbulent jets, and the analysis includes the effects of shallow water, the flowfield set up by adjacent jets, and an ambient current. Theoretical predictions are compared with the results of an experimental program. The analysis is then used to evaluate different diffuser designs from the stand- point of temperature and shear stress exposure of entrained organisms.
This work was performed by John H. Trowbridge as part of his masters thesis in the M.I.T. Dept. of Civil Engineering, 1979.
MIT Energy Laboratory
Diffusers, Pipe |x Hydrodynamics., Waste heat., Thermal pollution of rivers, lakes, etc.
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