A Hydro-Québec power plant commissioned in the early 1980's was operating with less efficiency than was expected from the model tests conducted by the manufacturer. This was confirmed by an independent test stand. Fluid flow simulations using FIDAP were done for the entire turbine system, from the water intake through the spiral casing, the distributor, the runner and the draft tube. CFD analysis of fluid flow in the water intake and in the penstock correlated well with the manufacturer's model measurements, and did not show any anomalies.The analysis indicated that the flow at the runner inlet was uniform along the circumference. In addition, no problematic flow pattern was seen in the spiral casing. The problem was then suspected to reside in the turbine itself.

In less than a year of operation, this improved efficiency has already paid for the cost of the modifications.

The draft tube before and after modification. The draft tube after modification clearly shows reduced losses.

FIDAP's simulation of the turbine showed a large eddy in the elbow of the draft tube. Researchers modified the runner outlet and designed a new trailing edge. A parametric study was conducted on the draft tube flow to determine the influence of the inlet flow parameters on its performance. Several blade cuts and extensions were tried in order to modify the critical parameters. Once the draft tube flow was optimized, the flow in the runner was examined to reduce runner losses at the same time.

Runner modification taking place in the power plant.

The modifications determined by CFD were implemented at the plant, for a single 195 MW Francis turbine. This raised the power output at the maximum efficiency point by 7.8 MW. In less than a year of operation, this improved efficiency has already paid for the $200,000 (Canadian) cost of the modifications. Hydro-Québec management is currently considering implementing the modifications on the other 11 turbines in the plant. Revenue gains of up to $5 million per year are anticipated.

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