Results

Procedures to analyze fluxes

Validation

Numerical flow simulation

The flow near the no load operation of turbines becomes very complex in a sense that the flow is dominated by backflow regions and vortex formations in all parts of the turbine. Furthermore, partial pumping flows start to build up in some or all runner channels. Additionally, the flow becomes vigorously unsteady. Recirculation zones build up and disappear, vortical flows are swept away. To predict such flows - at least qualitatively correct - grid generation must be carried out carefully. The grids used in this study were generated using only hexaedra elements. Grid generation was done with the commercial software ICEMCFD v11.0.

For validation the numerical flow simulations for operation near the runaway point experimental data from model test were used. The validation was carried out in two steps. In a first step stationary simulations were performed. The demand with respect to computational power is much lower for stationary simulations compared to unsteady, transient simulations. However, the expectations in the accuracy of the results of the stationary simulations are low, since the flow is certainly not stationary near runaway.

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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.

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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.

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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.

Дата добавления: 2014-10-31