Abstract
Any hydraulic reaction turbine is installed with a draft tube that impacts widely the entire turbine performance, on which its functions are as follows: drive the flux in appropriate manner after it releases its energy to the runner; recover the suction head by a suction effect; and improve the dynamic energy in the runner outlet. All these functions are strongly linked to the geometric definition of the draft tube. This article proposes a geometric parametrization and analysis of a Francis turbine draft tube. Based on the parametric definition, geometric changes in the draft tube are proposed and the turbine performance is modeled by computational fluid dynamics; the boundary conditions are set by measurements performed in a hydroelectric power plant. This modeling allows us to see the influence of the draft tube shape on the entire turbine performance. The numerical analysis is based on the steady-state solution of the turbine component flows for different guide vanes opening and multiple modified draft tubes. The computational fluid dynamics predictions are validated using hydroelectric plant measurements. The prediction of the turbine performance is successful and it is linked to the draft tube geometric features; therefore, it is possible to obtain a draft tube parameter value that results in a desired turbine performance.
Highlights
The draft tube is a necessary component installed in a reaction turbine, and it is the last component in the flux stream over the hydraulic turbine
The draft tube has an irregular geometry; the geometric definition is dictated by a large number of parameters that must be selected carefully according to their influence on the draft tube hydraulic behavior
The analysis is based on a computational fluid dynamics (CFD) modeling that includes the Francis turbine components: guide vane, runner, and draft tube
Summary
The draft tube is a necessary component installed in a reaction turbine, and it is the last component in the flux stream over the hydraulic turbine. The implemented CFD modeling solves the flow in the turbine guide vanes, runner, and draft tube.
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