Numerical Investigation Of Unsteady Flow And Air Admission For Francis Turbine With The Parallel Environment

Recently years, with hydra turbine's unit capacity and diameter increasing, the stability problem such as the vibration and blade's cracks of large-scale turbine is more and more prominent. The vibration and pressure fluctuation is the main factor of hydraulic units' stability running, and the pressure fluctuation is produced by the unsteady flow fields during the units' running. It is also one of the main sources of the hydraulic vibration which causes the hydraulic units' vibration and instability operation.In order to deal with the problem that the numerical simulation of the Francis turbine's whole channel demands the calculation resources, we build a parallel calculation platform with eight nodes based on Windows and got a higher efficiency through basic performance testing. And with the number of grids increasing, the parallel efficiency is increasing. From the testing results we found that the parallel efficiency in this small scale distributed parallel computer cluster is satisfied.This article used the Reynolds averaged N-S equations and the RNGk-εturbulence model to simulate the three-dimensional steady flow in Francis turbine, and got the pressure and velocity distribution of the main flowing parts in the whole flow passage. And also comparative analyzed the flow characteristic and energy property which under the same opening but different specific speeds. It lay the foundation for further carry on the three dimensional unsteady flow computation.Based on the steady flow foundation , we studied the unsteady flow including runner and guide apparatus,runner and draft tube these two-stage interaction in the Francis turbine , and got the amplitude and frequency characteristic of the pressure fluctuation . There is a frequency which is about 0.235 times the rotating frequency before the runner, and the amplitude of this pressure fluctuation is the maximum before the stay vane. This frequency is transmitted by the low frequency pressure fluctuation which in the draft from upstream. The pressure fluctuation which is about 2 times of rotating frequency and exiting in blade outlet is mainly due to the draft tube inlet's pressure fluctuation transmits to the upstream. The pressure fluctuation which is about one times of the product of rotating frequency and blade number is mainly caused by the effect which the flow field suffer from the rotating blade. But the main frequency of the pressure fluctuation which in the draft tube mainly fasten on elbow segment, and the pressure fluctuation in conical segment is small.Based on the study mentioned above, we further researched the influence which the air supply from Francis turbine head cover on the pressure fluctuation. The steady flow calculation results shows that the influence which the air on the internal flow of runner mainly focus on 0 to 0.4R, and the influence in the range 0.4R-1R is small. When the guide vane opening is 12 mm, 0.15% air supply was satisfactory; and when the opening is 10 mm, 0.1% air supply was satisfactory. The unsteady flow calculation results shows that air supply has relatively big influence on the pressure fluctuation in runner. The pressure fluctuation frequency in runner is unchanged before and after air supply, but the amplitude has relatively big change. It made the amplitude of pressure fluctuation which in runner outlet decreased about 50 percent. The main frequency of the pressure fluctuation in draft tube is increased from 1.66Hz to 8.28Hz, and the amplitude of frequency also reduced.