Multiport Diffusers for Heat Disposal: A Summary

Abstract

This paper reviews the fluid mechanical characteristics - as derived from theoretical and experimental studies during the past dozen years - of submerged multiport diffusers used for heat disposal from thermal power plants into the water environment. Foremost among these characteristics is the near field instability produced by such thermal diffusers in typical receiving water conditions. Rather than forming a distinct buoyant plume as is the case for the traditional sewage diffuser, the high discharge momentum of thermal diffusers leads to a flow breakdown with local recirculation zones and full vertical mixing. Stability diagrams for both stagnant and flowing ambient conditions are presented. The flow and temperature fields at larger distances, in the intermediate field, are, in turn, critically dependent upon how the discharge momentum is introduced into the ambient fluid layer. Out of a spectrum of possible diffuser designs three major types have evolved. The unidirectional and staged diffusers are designs which result in concentrated vertically mixed plume motions. The alternating diffuser with appropriate nozzle control generates a stratified flow field outside the unstable near field. Predictive techniques for these basic types are summarized. A typical case comparison illustrates their differences in engineering design and environmental impact.