Numerical Investigation of the Internal Flow in a Banki Turbine

Abstract

The paper refers to the numerical analysis of the internal flow in a hydraulic cross-flow turbine type Banki. A 3D-CFD steady state flow simulation has been performed using ANSYS CFX codes. The simulation includes nozzle, runner, shaft, and casing. The turbine has a specific speed of 63 (metric units), an outside runner diameter of 294 mm. Simulations were carried out using a water-air free surface model and k-εturbulence model. The objectives of this study were to analyze the velocity and pressure fields of the cross-flow within the runner and to characterize its performance for different runner speeds. Absolute flow velocity angles are obtained at runner entrance for simulations with and without the runner. Flow recirculation in the runner interblade passages and shocks of the internal cross-flow cause considerable hydraulic losses by which the efficiency of the turbine decreases significantly. The CFD simulations results were compared with experimental data and were consistent with global performance parameters.

Banki-Michell Optimal Design by Computational Fluid Dynamics Testing and Hydrodynamic Analysis

“…Fiuzat and Akerkar [9] and De Andrade et al [11] have shown that the water flow within the impeller still possesses an appreciable energy content when it reaches the blade output (the energy at the blade output is 25%-35% of the energy at the inlet blades), where there is a second energy transfer from the water flow to the turbine.…”

“…We will show in the following section that Equation (3) is equivalent to Equation (2) when the velocity of the rotating system is half of the component of the particle velocity in the tangent direction. Many researchers agree that the optimum value of β 2 is π/2 [10][11][12][13]. This value provides a radial direction to the relative outlet velocity inside the impeller at the inner diameter (D 2 ), and this leaves to the fluid, in the inner part of the impeller, only the energy of the rotating system, which can be recovered during the next blade crossing.…”

“…This optimization allows one to limit the variation of the velocity directions close to the blades' surfaces. De Andrade et al have shown through experiments that for a fluid draft entry angle of α = 16°, the actual input angles vary between 7° and 23° [11]. Lastly, to ensure stable flow conditions within the impeller, it is important to provide the production chamber with one or more air inlets, in order to adjust the depression which arises in the chamber as a result of air entrainment by the jet.…”

The Flow Field of Undershot Cross-Flow Water Turbines Based on PIV Measurements and Numerical Analysis

“…The undershot cross-flow water turbine used by this study differs from cross-flow water turbines [6,7,10,11] for use with penstocks. Since there is no casing covering the runner, the interface between the water and air changes, particularly at the inlet and outlet regions of the runner.…”

“…Therefore, it is necessary to clarify the complicated flow fields involving free surfaces in order to design water turbines of high efficiency and high rotational speed for use in open channels. With the development of computational fluid dynamics technology, the flow fields involved with various water turbine designs, such as a cross-flow water turbine [6,7] for use with penstocks and a spiral water turbine [8,9] for use in open channels, have been investigated by numerical analysis. However, the treatment of a flow field involving a free surface must efficiently and correctly capture the interface between the fluids of water and air.…”