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Flow analysis of draft tubes in hydraulic turbines

Improvement of efficiency/power output of hydropower plants
by better designing runner/turbine/draft tube combination to
reduce "flow blockage" and instabilities. A key area is
pressure recovery between turbine and tail water channel.

Background Hydropower represents only 16% of total European electricity generation (61% in SWITZERLAND). Nevertheless, it remains a key component in the management of energy demand and grid stability. However, most European hydroplants were designed many years ago and are not well adapted to current operation requirements. Thus, the rehabilitation of hydro potential is of great importance for the European power generation economy. For instance, an increase in the efficiency and net power output to match demand can be achieved simply by changing the runner of the hydraulic machines which leads to an excellent investment cost per kWh. Therefore, this gives rise to the technical problem of how to adapt the newly designed runner to an existing draft tube. The main role of this component consists of the pressure recovery between the turbine and tail water channel. However, most of the fluid dynamics problems are to be found in this element. In this case, recent experiences have shown undesirable phenomena such as "flow blockage" and/or dangerous instabilities, and these will have to be kept under control. It is therefore necessary to investigate the flow in the draft tubes to obtain a better understanding of these phenomena and an improvement of the runner-draft tube set. In addition, taking new environmental preoccupations into account, the draft tube can be a key component in the quality control of natural water resources. Description In order to achieve the objectives described in point 7, the partners decided to organise the project in two phases: * an experimental approach to provide the draft tube database * a theretical study to develop the analysing tools and techniques for draft tube flow predictions. Experimental studies Experimental studies will be carried out on a reduced scale model of an existing water turbine of high specific speed where two runners of different types can be adapted; The experimental database can be built up using the following procedure: * global measurements with both runners including: - hill chart; - inlet, outlet and boundary flow survey by velocity probe measurements and wall pressure measurements. These measurements will be performed with both runners. * local measurements only with one of the runners giving unsteady inforamtion: - unsteady boundary flow survey by unsteady pressure probe measurements and unsteady wall pressure measurements; - unsteady Laser Doppler Anemometry in several sections to be determined by the Technical Committee. Some chosen parts of the database will be specifically documented to be used in a numerical workshop with the partners in order to improve the know-how of each participant on draft tube behaviour prediction. Theoretical flow analysis The successive steps of theoretical flow analysis are: * steady flow computation with emphasis on the calibration of the outflow condition and on a choice of turbulence model; * unsteady flow computation with steady or unsteady inflow conditions and measured unsteady outflow boundary conditions. This analysis will detail the flow stability, the influence of a pier and finally the unstable behaviour of a draft tube. Analysis using experimental data and numerical results will be developed in order to characterise the draft tube's flow behaviour in terms of efficiency and stability according to operating conditions.
Acronym: 
FLINDT II
Project ID: 
1 625
Start date: 
01-01-1997
Project Duration: 
69months
Project costs: 
2 770 000.00€
Technological Area: 
Market Area: 

Raising the productivity and competitiveness of European businesses through technology. Boosting national economies on the international market, and strengthening the basis for sustainable prosperity and employment.