Wpływ pozaoptymalnej pracy turbiny Francisa na zjawiska przepływowe w rurze ssącej
[The influence of partial load operation of the Francis turbine on the flow structure 
in the draft tube]

Aneta Nycz (Doctoral School - Environmental Engineering, Mining, and Power Engineering, 
PWr, Wrocław)


The position of the machine's operating point in the hydraulic system of thrust turbines 
determines the flow phenomena occurring in the system. The change in the operating range 
of the water motor is of great importance for the operation of the liquid discharge 
element, the draft tube. Its function is to enable the use of the geometric slope 
contained between the turbine rotor and the lower water table, as well as to recover the 
kinetic energy of the fluid flowing out of the rotor (the dynamic component of the 
pressure) owing to the diffuser shape and closed form of the pipe. The importance of 
draft tubes in the operation of water engines is well known. It increases as the speed 
distinction increases and in some cases can reach as much as 50-55% of the power 
generated by the water turbine. The process of designing draft tubes is very important 
and requires great care to ensure that they work correctly over as wide a range of water 
engine operations as possible. The draft tubes of dual-controlled water turbines are less 
affected by the non-uniformity of the fluid velocity profile at the inlet. This is 
beneficial energetically. In the case of single-stage turbines, using a guide vane, any 
reduction in the esophagus from the optimum point will be distinguished by the occurrence 
of flow with a curl in the inlet section to the draft tube.

This study aimed to identify the effect of non-optimal Francis turbine operation on flow 
phenomena in the draft tube. The presented problem concerning the stability of Francis 
turbine operation at part load is an important operational topic for the water engine. 
Emerging flow disturbances in the draft tube are a cause of the reduced efficiency of the 
machine and reduced service life. The presentation includes a discussion of several 
introductory topics: an introduction to hydropower, the construction of a hydropower 
plant using the example of the research facility analyzed, the classification of 
hydropower turbines, the theory of operation of the Francis turbine, the elements of 
fluid discharge with particular emphasis on the draft tube and cavitation in hydraulic 
machines.

The indicated flow structure in the draft tube, in the form of a cavitation vortex rope, 
was identified as the cause of excessive vibration and a decrease in machine efficiency. 
The final part of the research was to test one of the methods to partially or completely 
solve the flow structure disturbance. A research plan via computational fluid dynamics 
was drawn up to verify exemplary forms that structurally level or reduce the effect of 
cavitation string vortex - fins. With the data obtained from the analyses and 
calculations, a comprehensive analysis was made of the effect of the non-optimal 
operation of the Francis turbine on the flow phenomena in the draft tube.