Investigation numérique de la perte de rendement à l'avant- distributeur de deux turbines Francis

Abstract

The objective of this work is to determine the efficiency augmentation due to stay vanes modification by full turbine flow simulation of two hydraulic turbines. This method of simulation with unsteady RANS equations aims to determine the effect of reducing the head losses at the stay vanes on the downstream components of the turbine. To achieve this objective, a methodology of simulation by increasing the size of the turbine simulation domains is developped. It assesses the impact of simulation parameters and the capacity of smaller simulation domain to evaluate head losses reduction by stay vanes modification. Complete turbine simulations show that the main source of head losses reduction is from the stay vanes modifications. These eliminate the boundary layer separation at their leading edge and the large recirculation zone in the turbine stator. Head-losse reduction in the runner is mitigated in comparison to that in stay vanes. It is shown that the runner uniformizes the flow at its entrance. Thus, the small head-losse variation in the runner originates from the mean flow modification by the boundary layer separation at the stay vanes. Then, very similar flow at runner outlet, except for the different point of operation between the original and modified geometry admits a very small variation of the head losses in the draft tube. The effect of the operating point difference of the flow is also verified with a more accurate simulation of the draft tube alone. These results indicate that the methodology used for the numerical simulations of a complete turbine cannot determine all the head-losse reduction measured experimentally. It appears that the numerical parameters implemented show the limits of RANS modeling of the flow in the entire hydraulic turbine for determining a precise reduction of the head losses. For this reason, it is wiser to prefer the use of simulations of fixed components upstream of the runner due to the computing resources needed for their implementation in comparison to the accuracy of their assessment of the head losses.

Date	9 Dec 2014
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Azzeddine Soulaïmani (Supervisor)