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CFX Reliable for Development of Hydraulic Machines
Posted Thu June 27, 2002 @04:54PM
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News by Andrej Lipej, Turboinstitut, Republic of Slovenia

For the past 50 years, Turboinstitut has been involved in the research and development of hydraulic machines. CFD has become an indispensable tool in our activities, and this is especially true for our turbine refurbishment projects, which are usually realized entirely with CFD. Using CFX-TASCflow, we can analyze the flow throughout the whole turbine, taking into account the stator-rotor-draft tube interaction and avoiding potentially inaccurate boundary conditions between components. However, compared to individual simulations for each part, such calculations require large models and long run times. Separate analysis is much faster, but it is essential to know when coupled analysis must be performed and what are the differences between separate and coupled analyses. At the Institute, by comparing results for coupled and separated calculations, we have found that the separate analysis usually overpredicts flow energy losses in all turbine parts and only coupled analysis is suitable for accurate prediction of turbine efficiency.

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Another area where CFD is essential is in our testing procedure, where we use CFX to optimize the positioning of the instruments which are used to measure power, cavitation and dynamic performance. Site testing of flow rate, usually performed for guarantee purposes after commissioning, is necessary for efficiency calculations. These values are easily obtained for Pelton and Francis turbines, but in Kaplan turbines the short intakes and large cross sections make this difficult. Our experience has shown that the best technique for these measurements is the current-meter method with up to 320 current meters. Positioning these correctly is the hardest task in the preparation of the measurement, as each current meter must be perpendicular to the measuring plane and flow streamlines. We use CFX to predetermine the flow direction inside the spiral casing, permitting correct positioning of the instruments.

Development of a new pump also starts with CFD analysis. By calculating the pressure and velocity distributions along the blades, we can optimize the design before commissioning a prototype and testing it in our pump test rig. Because of CFXs high reliability, which has been confirmed by numerous experimental projects, we can now design machines with better characteristics, more quickly and with less experimental testing than before.

Turbine test rigs
Test rigs for Pelton, Francis, Kaplan and bulb turbine types are available in our laboratory.

Francis turbine simulation
Flow simulation in a Francis turbine.

Kaplan turbine casing simulation
Streaklines in the semi-spiral casing of a Kaplan turbine.

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