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Analysis of Cavitation Inside a Francis Turbine
A Francis turbine is a reaction-type turbine which can be designed to operate over a range of head conditions. For this reason they are the preferred class of turbines for generating power from kilo to several megawatts. During its operation, high pressure water radially enters the turbine, where its potential energy is converted into kinetic energy, driving the runner blade rotation before exiting vertically into the draft tube. The water recovers its pressure as it passes through the draft tube and exits into the tail race, from where it is discharged to the atmosphere.
Hydromachinery is generally subject to cavitation due to a localized drop in water pressure during its operation. This can be due to; a) rupture of the liquid if the local pressure drops below its vaporization pressure, and/or, b) due to the growth of small nuclei already present in the liquid. The cavities or voids subsequently collapse as they migrate or encounter high pressure in the surrounding liquid. Very high pressures are generated due to the collapse of these cavities causing noise, vibrations and eventually erosion of the turbine surfaces. In addition, the presence of cavitation leads to increased hydraulic losses in a turbine.
It is critical for turbine designers to identify regions of cavitation for optimal blade design under changing loads. Addressing these requirements through prototype development and experiments is prohibitively expensive when a new blade design is being considered. Efficient design process dictates quick turnaround and consideration of multiple scenarios. In a world where computational power is cheap and abundant, using simulation methods such as Computational Fluid Dynamics (CFD) analysis are cost effective alternatives for addressing these needs. Not only can several loading conditions and guide vane orientations be easily analyzed, their effect on turbine characteristics such as efficiency and cavitation can also be investigated.
CAE Associates’ CFD consultants performed a CFD analysis of flow through a Francis turbine, in order to identify potential cavitating regions within the turbine for different guide vane angles and operating pressures. We utilized the ANSYS® Fluent suite of CFD software for this application due to its multiphase and turbulence modeling capabilities. The software also supports parametric associativity with CAD files, which allows for easy geometric changes such as the guide vane orientation.
ANSYS® Fluent was used to model the interaction between the rotating and stationary components using a “frozen rotor” approach. Predicting cavitation requires modeling multiphase flow with two phases (water and water-vapor), with an appropriate mass transfer mechanism corresponding to the cavitation phenomenon. The cavitation model in ANSYS® Fluent accounts for the vaporization of liquid to vapor when the local pressure drops below a certain threshold value. This process is reversed when the pressure rises above the vapor pressure, leading to vapor condensation.
Analysis shows three dimensional cavitation regions near the trailing edge of the runner blade surface on its suction side, identified as regions of vapor volume fraction in Figure 2. For a given blade angle, cavitation was also observed along the axis just below the runner cone. Changing the guide vane angle significantly alters the cavitation dynamics, as far as completely suppressing it near the blade surface. However, doing so also dropped the efficiency by nearly 7%. Using the design point feature of ANSYS® Workbench, a parametric study was conducted to observe the effect of guide vane opening on efficiency and cavitation. This enabled a clear understanding of the design space and the trade-offs associated with changes in guide vane angle.
Since cavitation is closely tied to the operating conditions, any drop in the operating pressure, e.g. reduced back pressure, can lead to an increase in vapor volume created within the turbine. Identifying and isolating the problem areas was instrumental in creating an optimal blade design with minimal tradeoff.
For more information about CAE Associates’ CFD consulting services or to learn about turbulence and cavitation modeling from our team of CFD consultants, please contact our office at 203-758-2914.
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