During mesh generation mesh-regions were defined for evaluation of local fluxes. This definition of mesh region which can be surfaces or volumes allows the analysis of local time variations of fluxes and balances, e.g. in each guide vane or rotor cannel.
[Some details are omitted]
Results
The process of energy dissipation for operating points near runaway involves in- and outflows from the runner. The high energy flow is entering the runner from the guide vanes and drives the runner up to speed where parts of the channel start to pump flow outwards. The equilibrium of energy input and dissipation by pumping results to zero torque at the shaft.
The discharge being pumped out of the runner has to reenter the runner. This increases the inflow into the runner above the flow rate given at the inlet to the turbine scroll. This process of pumping seems to be an unsteady process for the investigated model turbine for an operating point slightly above runaway.
[Some details are omitted]
The question arises now how these flows lead to energy transfer to the vaneless space and how the in- and outflows look like in detail. Figure 9 clearly demonstrates the existence of enhanced vortices transporting fluid outwards. These vortices exit the runner channels in front of the leading edges of the runner vanes into the vaneless space. The vortex strength varies in time and space. For the chosen operating point, which is slightly above the runaway point, the variation in time is dominant, which results in the global flow rate fluctuation through the surface A. It can be assumed that with decreasing flow rate Q at the inlet to the turbine the effect of the spatial variation of the vortex formation will more and more dominate and that rotating stall will be observed for operating points below runaway, as it was experimentally observed for a pump turbine e.g. by Staubli [8].
The difference between the in- and out-energy fluxes through the surface A indicates that a large amount of the energy dissipation occurs in the vaneless space between guide vanes and runner for operating points near runaway.
Conclusions
The characteristics of the pump turbine close to runaway could be well predicted with transient flow simulations. Unstable flow fields were predicted for the simulations in the so called S-shaped portion of the characteristic.
This simulated instability shows time-varying in- and outflow from the runner into the vaneless space. For the investigated operating point, slightly above runaway, the band of the fluctuations corresponded to about 50 percent of the main inflow to the turbine. The existence of unstable operation is confirmed by the model test where also instability was observed in this range of operation.
With detailed information available in the simulated flow field local flow effects could be analyzed. It could be concluded that local vortices forming in the runner channels close to the leading edge is the source for the unsteady in- and outflow from the runner into the vaneless space between guide vanes and runner. Therefore, the vortices and the induced outflow can be considered as the origin of the instability. Most of the energy dissipation for operating points near runaway occurs in the vaneless space between guide vanes and runner.
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Acknowledgement
This study was made possible by a grant of the Swiss Commission for Technology and Innovation (CTI) and swiss electric research. Industrial funding was provided by VA TECH HYDRO.
References
[1] Yamabe, M., Hysteresis Characteristics of Francis Pump-Turbines When Operated as Turbine, Trans. ASME, J. Basic Engineering, Vol. 93, pp.80-84, March 1971
[2] Yamabe, M., Improvement of hysteresis characteristics of Francis pump-turbines when operated as turbine, Trans. ASME, J. Basic Engineering, pp. 581-585, September 1972
[3] Klemm, D., Stabilizing the characteristics of a pump-turbine in the range between turbine part-load and reverse pumping operation, Voith Forschung und Konstruktion, Vol. 28, 1982
[4] Martin, C. S., Stability of pump turbines during transient operation, 5th Intl. Conf. On Pressure Surges, BHRA, Hannover, September 1986, pp. 61-71
[5] Martin C. S., Instability of pump-turbines with S-shaped characteristics, Proc. 20th IAHR Symp. Hydraulic Machinery and Systems, Charlotte, NC, 2000
[6] Doerfler, P., Stable operation achieved on a single-stage reversible pump-turbine showing instability at noload, XIX Symposium of IAHR Section on Hydr. Machinery and Cavitation, Singapore, 1998
[7] Billdal, J.T., Wedmark, A., Recent experiences with single stage reversible pump turbines in GE Energys hydro business, Paper 10.3, Hydro 2007, Granada
[8] Staubli, T., Some Results of force measurements on the impeller of a model pump-turbine, IAHR Work Group on the Behavior of Hydraulic Machinery under Steady Oscillatory Condition, 3rd Meeting, Lille, 1987, P. 8, pg. 1-11
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