Improvement of Intake strucures with numerical simulations

INTRODUCTION
The design of inlet and outlet areas of hydro power plants significantly influences the overall cost of a plant not only in the design and construction phases but also during operation. Due to an incorrectly shaped inlet geometry, vortices and flow separation occurred at several river power plants. These phenomena can cause flow problems and losses at the turbines and must not be neglected, especially for low head turbines with relatively high specific speed turbines. Sources of such inhomogeneous inlet flow distribution could be a wrong design of dividing piers, gravel steps and other points of discontinuity in the flow guiding walls and river bed. In this paper, samples of waterway designs and their optimisations are presented aiming to avoid wrong inflow and outflow designs together with cost-optimised design.
POWER PLANT EXAMPLES
The first example is an Austrian river power plant where a gap between the powers of the two identical horizontal axis Kaplan bulb turbines was assessed. It is assumed that the difference is caused by flow problems at the inlet area. The hydro power plant consists of two units with a maximum discharge of Q = 100 m³/s per turbine, whereas the units were brought into operation in 2012. Multiphase calculations were carried out to verify the air-sucking turbulences and swirls. Based on the results of these simulations, different kinds of structural measures were developed. The various types of measures range from simple accessory elements to large-scale structural interventions at the inlet area of the power plant. The effects on the inflow of all these different structural adjustments were further investigated by applying computational fluid dynamics.
The second example is located on the Swiss-Austrian border and refers to the GKI power plant on the river Inn. The weir system is a multiple function system including the weir flap, the diversion channel to the main power plant, which is located 23 km downstream, and the connection of a residual water turbine as well as a fish pass. The main turbines for Austria’s largest diversion power plant are two vertical Francis turbines with a net head of H=160.7 m and a design flow rate of Q=75 m³/s for the power plant. The Kaplan S-turbine with its maximum flowrate of Q=20m³/s increases the feeding to Q=95m³/s for this installation. As the fine screen in the front has a clear diameter of only 20 mm, a special focus must be given to a correct inflow situation at the screener as well as at the residual water turbine.
The last example describes the intake situation of the MKF hydro power plant at the river Mur. The design flow rate for this residual water turbine is Q=40 m³/s. The model of the river bed starts 350m upstream and includes the diversion channel to the main power plant. A significant cost reduction could be realised by changing the rotational direction of the turbine, and with an optimised design of a diversion pier directly in the turbine intake a smooth and homologous flow situation could be realised.
CONCLUSIONS
Based on three case studies that have been worked on in recent years at the Institute of Hydraulic Turbomachinery at the Graz University of Technology, it can be shown that the numerical flow simulation is very well suited for the detailed analysis of the inflow of power plants as well as for the development of geometric improvements. Especially in the area low head power plants, where axial turbines are often used (such as Kaplan tube or S-turbines), such studies should be carried out already in the planning phase in order to avert potential problems in advance.