Aquator B.V.

Development and application of 3D numerical model THREETOX to the prediction of cooling water transport and mixing in the inland and coastal waters


A modelling system—THREETOX—has been developed to simulate the transport and mixing of cooling water in both freshwater and marine environment. A 3D hydrostatic free-surface model describes the heat dispersion in the far-field, whereas an integral buoyant jet model coupled with a far-field model is applied to the near-field. The equations of hydrodynamics of the far-field model are completed by equations for heat and salt transport, and by the k _ ε turbulence model. Special attention is paid to the parameterization of heat fluxes between water and atmosphere and between water and bottom sediments. Wetting and drying (WAD) processes were built into the model to describe areas where tide and floods play a dominant role. The model was enhanced by processes describing the effects of ship traffic on the dispersion of the discharged heat in stagnant canals. The sigma coordinate in the upper layer can be combined in the lower layer with a second sigma coordinate system or with a z coordinate system. An orthogonal curvilinear horizontal grid with two-way nesting capabilities has been used to describe the area of interest accurately. A high order advection scheme has been applied in the model. Several examples of the application and validation of the THREETOX model are presented. Studies were performed on the dispersion of cooling water, discharged by various power plants in the Netherlands located at different types of aquatic systems, varying from rivers, canals to tidal river reaches. Copyright 2008 John Wiley & Sons, Ltd.

Power plants (PP) and other industrial facilities often apply once-through cooling water systems. Generally, the cooling water discharges in the large rivers were not critical in terms of environmental impact. However, the recent coincidence of low volumetric flow rates and relatively high water temperatures in the warm summers of 2003 and 2004 in Europe demonstrated the limitations of once-through cooling. The problem of discharge of cooling water in closed and semi-closed stagnant water bodies is worse than it is for rivers. The climate change resulted in increasing background water temperature and made it necessary to implement new regulations that force industries to assess the thermal impact on aquatic environment. The special model tools are required to investigate the available heat capacity of the receiving surface water and to evaluate the dispersion of discharged water in aquatic systems.

The complexity of processes of the transport and mixing of the cooling water is high due to the buoyancy, which determines the hydrodynamics of both the discharged water and the ambient water, and due to the heat exchange with the atmosphere. Commonly, three zones around the cooling water outfall are defined: (1) the near-field, where the transport is dominated by turbulent entrainment of the incoming buoyant jet, (2) the intermediate field, where the buoyancy forces in the plume are dominated, and (3) the far-field, where the cooling water is transported passively by the ambient currents. In turn, the trajectory of the jet in the near-field is governed by the ambient flow and by its stratification. The spreading buoyant plume interacts with the currents in the intermediate field, while the stratification in the plume suppresses the ambient turbulence. An accurate description of the jet near the outfall every so often is complicated due to the non-hydrostatic character of the flows and due to the complex geometry of outfall. The heat exchange with the atmosphere plays only a role in closed and semi-closed water bodies such as lakes, harbours, and canals but to a lesser extent in river systems, estuaries, and coastal waters, where advective transport dominates.

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