3D-numerical Refinements to Simulate High-Head Power Plants
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The presented work deals with the coupling of 1D- and 3D-numerical approaches to simulate water hammer effects and surge tanks of high-head power plants. 3D-numerical simulations are used for an optimization process for nonstandard parts as well as to create more realistic local model parameters. Thus, the global 1D-numerical model can be refined and this leads to an overall improvement of the performance of the design. Three different examples are shown which illustrate the need for and the advantages of additional supporting 3D-numerical investigations in this field. As long as standard parts are used, the hydraulic specifications - especially the local head loss coefficient - are well known. Still, most hydro power plants require a unique design either due to specific local conditions or optimization needs. Hence, for some parts of a high-head power plant, a local investigation using 3D-numerical simulation is obligatory in order to increase the accuracy of the 1D-numerical simulation. The presented examples are all part of an actual modernization project of an existing high-head power plant operated by the TIWAG-Tiroler Wasserkraft AG, where a new penstock and surge tank had to be designed. For the 3D-numerical modeling, the two software packages (a) ANSYS-CFX (pipe flow) and (b) FLOW-3D (free surface) are used. The work concentrates on the quantification and optimization of hydraulic structures with a focus on the local head loss coefficient. As a first example, the simulation of the connection between the existing head race tunnel and the newly built structure (penstock and surge tank) is presented. For this case, in each flow direction the head loss should be minimized. Going up into the surge tank, the local head loss coefficient of the asymmetric orifice is an important design parameter for the mass oscillation in the surge tank. To minimize the construction costs of the chambers, the asymmetric orifice is optimized with the help of 3D-numerics. In this case the modeling results could be validated with a physical lab scale test (scale 1: 25). In addition, the filling process of the upper chamber of the surge tank is investigated. Based on the 1D-numerical simulation, the inflowing discharge is calculated with a peak flow of up to 140 m(3)/s. The existing chamber has been redesigned and extended so that a circular flow through the complete upper chamber is enabled. The main goal is to analyze the travelling of the surge front and to guarantee that no water flows out of the chamber. In addition, the free surface flow should be the main flow type in as many sections as possible over the complete filling time. Therefore, different built-in components are investigated.
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