Mixing of Thermal Discharges at Low Froude Numbers

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by C. Liang Lee, (M.ASCE), Bechtel Corp, San Francisco, United States,
Patrick J. Ryan, (M.ASCE), Bechtel Corp, San Francisco, United States,

Document Type: Proceeding Paper

Part of: Hydraulic Engineering:

Abstract: A study was performed to develop an optimum configuration for the discharge canal of a cooling lake. The objective was to minimize the initial mixing at the end of the discharge canal, and, thus, maximize the heat transfer efficiency of the pond. Preliminary analysis had found that the appropriate Froude number for the discharge canal should be in the range of 0.5. Little data were available in the literature for Froude numbers less than 1. Therefore, a hydraulic model study was conducted. Results of the model study show that, contrary to common assumptions, the bulk dilution increased with decreasing Froude number at very low channel aspect ratios. One possible reason may be that the vertical entrainment, which is often neglected at low Froude numbers, becomes significant when the area available for vertical mixing becomes 1 or 2 orders of magnitude larger than that available for horizontal mixing. For the same flow rate and channel depth, a wider channel might induce more mixing than a narrower channel, when the densimetric Froude numbers for both channels (based on channel depth) are less than about 0.7.

Subject Headings: Froude number | Water discharge | Hydraulic models | Thermal pollution | Model analysis | Channel flow | Canals | Lakes |

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