Innovative 3-D Numerical Simulation of Thermal Discharge From Browns Ferry Multiport Diffusers

The TVA Browns Ferry Nuclear Power Plant (BFNPP) withdraws its condenser cooling water from Wheeler Reservoir via an intake pumping station. Waste heat from the plant increases the condenser cooling water temperature. The heated water returns to the reservoir through three multiport diffusers containing a total of about 22,500 downstream facing ports. The ports are 2 inch diameter holes situated 6 inches apart both vertically and horizontally. The numerous small diffuser ports, the buoyancy of the heated effluent, and the large scale of the discharge area to be simulated present major challenges to developing a reliable three-dimensional computational fluid dynamics (CFD) method for predicting temperature distributions and flow patterns in Wheeler Reservoir. This paper presents an innovative CFD method and the results for numerically simulating under steady conditions the thermal discharge from the BFNPP. A finite-volume CFD package, FLUENT, was used to develop the required numerical models. The realizable k-ε turbulence model was selected for closure of the Reynolds-Averaged Navier-Stokes Equations. Second-order accurate schemes were used to discretize the governing equations. To simulate the discharge from thousands of diffuser ports, the CFD method features an innovative two-zone modeling approach, consisting of a multiple jet sectional model and an overall river model. The latter contains about 2 million computational cells for simulating the cooling water discharged from the thousands of multiport diffuser ports. The multiple jet sectional model, which simulates the flow from the individual discharge ports over a one foot slice of the diffuser pipe in great detail, provides the information for developing sectional boundary conditions for the diffuser effluent. These boundary conditions are then used in the overall river model for the whole study area. Validation and verification of the CFD models were performed using data from field measurements, hydraulic model tests, and other experiments. Comparisons between the CFD results and these data showed that the overall river model, based on the two-zone approach, can reproduce the major features of temperature and flow in the diffuser mixing zone. Model limitations exist, however, for low river flows, where requirements for steady behavior impede solution convergence.