Pump-turbines are used for energy storage purposes. Flow-induced vibrations instigated by flow instabilities are a serious issue associated with pump-turbine operations. One of the manifestations of flow instabilities is the formation of the vortex rope inside the draft tube. The frequency of these pressure fluctuations can reach the natural frequency of the unit, causing damaging vibrations \[1\]. The magnitude of the pressure fluctuations can also cause severe structural damage \[2, 3\]. The injection of water and air from the draft tube are well-known mitigation strategies for flow-induced pressure fluctuations. Injecting fluid in the central region of the draft tube can alter the dynamics of the vortex rope, and thus aid in stabilizing the system.

      Large Eddy Simulations will be performed using ANSYS Fluent 17.1 to characterize the turbulent flow fields and the effectiveness of water injection in a pump-turbine designed by Schleicher et al. \[4\]. Simulations will be performed under partial load conditions, with an inlet flow rate of 0.13 m^3^/sec, 65% of the optimal flow rate. Water is injected at the center of the runner in the axial direction of the flow at 4% of the total inlet flow rate. The influence of water injection on the flow-induced pressure fluctuations inside the draft tube and its effect on pump-turbine performance are examined against the no injection case.

      A no-slip boundary condition is applied at the surfaces of the runner and the draft tube, an outflow condition is imposed at the outlet of the draft tube, and a mass flow inlet with fixed turbulence intensity is imposed at the runner. The runner rotates with a constant rotation speed of 1,200 rpm, or Reynolds number 1.06×10^7^ based on the runner reference diameter. Second-order discretization schemes are used to solve the filtered LES governing equations with a WALE subgrid scheme. The spatial resolution of the mesh inside the pump-turbine runner and the draft tube will be reported regarding yplus values and first layer thicknesses. Pressure signals are acquired using several probes along the length of the draft tube.

      There is a strong correlation between the amplitude and frequency of pressure fluctuations and the dynamics of the vortex rope. The dynamic behavior of the rope can lead to low-frequency, high amplitude, pressure fluctuations inside the draft tube. There could also be high-frequency pressure fluctuations typically caused by the interactions of small and large scale eddies from the runner blades. The vorticity, velocity, and iso-surfaces of Q-criterion will be acquired to characterize the flow field in the draft tube with and without injection. The pressure signals at various locations will be probed to assess the level of fluctuations.  The effectiveness of water injection into the draft tube on the mitigation of pressure fluctuations will be determined and presented.

 

References

\[1\] Kirschner, O., Ruprecht, A., Göde, E. and Riedelbauch, S., 2012. Experimental investigation of pressure fluctuations caused by a vortex rope in a draft tube. In IOP conference series: earth and environmental science (Vol. 15, No. 6, p. 062059). IOP Publishing.

\[2\] Qi, S., Shuhon, L., Yulin, W., Weizhang, W. and Xingming, T., 2004. Three-dimensional simulation of unsteady flow in a model Francis hydraulic turbine. Tsinghua Science and Technology, 9(6), pp.694-699.

\[3\] Qian, Z.D., Yang, J.D. and Huai, W.X., 2007. Numerical simulation and analysis of pressure pulsation in Francis hydraulic turbine with air admission. Journal of hydrodynamics, 19(4), pp.467-472.

\[4\] Schleicher, W.C. and Oztekin, A., 2015. Hydraulic design and optimization of a modular pump-turbine runner. Energy Conversion and Management, 93, pp.388-398.


Computational Study of Pump Turbine \u2013 Partial Load Operation

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
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Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

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