Author(s): Guillermo Palau; Volker Weitbrecht; Thorsten Stosser; Tobias Bleninger; Bernd Hofmann; Matthias Maier; Karl Roth
Keywords: CFD; Numerical simulations; Water storage tank; Mixing 1 BACKGROUND AND
Abstract: To avoid microbiological pollution in drinking water storage tanks, the mixing efficiency and the related residence times need to be optimized especially in relation to flow and temperature variations. . Therefore, the design and operation of water storage tanks is of great importance and the prediction of hydrodynamics as well as turbulent mixing is a major task for designing engineers and operating utilities. Computational Fluid Dynamics (CFD) has supported engineers in manifold problems over the last 3 decades. However, an essential prestage before application of CFD in engineering is the proper validation of the CFD model, i. e. assessing how far the obtained CFD results are credible and whether the model describes reality with sufficient accuracy. In the present project the comercial software package FLUENT was used to perform 3-D RANS simulations to predict the hydrodynamics in a round water storage tank of 33. 5 m diameter. In comparison with field measurements it was shown that the standard k-ε approach produces misleading results with respect to the overall hydrodynamic behaviour of the mixing tank. Especially the centre region of the tank (where the outlet is located at the bottom), which is dominated by strong anisotropic turbulence conditions from a free vortex could not be resolved. This would lead to wrong results for the related mixing conditions. Therefore, different turbulence models (available in FLUENT) were evaluated using two different grid resolutions. Acceptable results regarding the hydrodynamic behaviour were only be achieved using a very fine grid resolution together with the Reynolds Stress Model as turbulence closure scheme.