Sammendrag
The standard equations for hydraulic efficiency on turbines and pumps given by standards as IEC, ISO or ASME include pressure, kinetic, geodetic and thermal energy. The kinetic energy is calculated from the assumption of uniform velocity distribution without a swirl component, which is rather the exception than the rule for hydraulic machines. A non-uniform velocity profile will increase the kinetic energy, and if a swirl component exists even more kinetic energy is present. This is earlier presented by T. Staubli and S. Deniz (1994) in the article� Analyzing draft tube flow kinetics�. The article focus on the fact that the assumption of uniform velocity distribution underestimates the actual loss of kinetic energy at the exit of draft tubes. As advanced measuring techniques as Laser Doppler Velocimetry, LDV, and Particle Image Velocimetry, PIV, have been introduced to hydraulic laboratories researchers have become aware of the importance of the turbulent kinetic energy, TKE. This is another property that is not taken into consideration when the hydraulic efficiency is calculated. In a study of swirl flow in draft tubes at the Norwegian University of Science and Technology, NTNU, Ole G. Dahlhaug (1997) used LDV to measure the flux of TKE upstream and downstream of diffusers and bends. These measurements show that the flux of TKE varies as the swirl varies. For a hydraulic turbine, there is usually no swirl at the inlet but at the outlet it will vary as the load changes. At the inlet the flux of TKE only changes due to increased or decreased flowrate, but at the outlet it will also change due to varying swirl. In the measurements carried out at NTNU, the TKE represents a considerable amount of the kinetic energy stored in the mean velocity. Knowing that the TKE does not change much at the inlet of a turbine, the change of TKE at the outlet will represent another component that does not come into consideration when the hydraulic efficiency is calculated. This paper will discuss the influence of TKE on the hydraulic efficiency on low and high head turbines.
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