To understand the hydraulic vibration characteristics in a traditional hydropower system and identify possible exciting sources that may induce serious hydraulic vibrations in the flow passage, experimental tests and numerical calculations were conducted for different operating conditions. The experimental results show that the pressure fluctuations are mainly related to the vortex rope phenomena in the draft tube, and the dominant frequency of pressure fluctuation is 0.2~0.4 times the runner rotational frequency (fn). The numerical results show all the attenuating factors are negative, which indicates the system itself is stable on the condition that all the hydraulic elements have steady operating performance. The free vibration analyses confirm that the frequency range of the vortex rope in the draft tube partly overlaps the natural frequencies of the hydropower system. Apart from the vortex rope, the runner rotational frequency is another common frequency that is approximately equal to the frequency of the 10th vibration mode. From the vibration mode shapes, it is inferred that a small disturbance in its frequency close or equal to a specific natural frequency of the vibration mode could induce large pressure oscillations in the tail tunnel. In light of the system’s response to different forcing frequencies, the vortex rope formed under off-design conditions and runner rotational frequency is verified to be the potential exciting source of a traditional hydropower system, and the frequency 0.2 fn is much more dangerous than other disturbances to the system.

Hydraulic Vibration and Possible Exciting Sources Analysis in a Hydropower System

Vortex rope formation at part load (PL) with cavitation inception causes pressure fluctuations inside the draft tube (DT) of a Francis turbine which may fail the turbine due to resonance and erosion. The pressure surge can be minimized by using anti-swirl fins which ensure safe turbine operation. The present study examines the effect of fin sizes and locations on the internal flow characteristics of the Francis turbine and predicts its adverse effect on the pressure surge. Three cases are investigated in which internal flow physics are compared among DTs with longer fins, shorter fins and no fins. At the cavitation inception point under PL conditions, the characteristics are thoroughly studied numerically using ANSYS-CFX with structured and unstructured grids. Cavitation and PL conditions are designated by Thoma number 0.266 and guide vane angle . Numerical methodology is then verified by an experiment based on International Standard (IEC 60193). The vortex rope occurrence is suppressed using fins on the DT periphery and longer fins that are extended up to the elbow exhibit the lowest strength of the vortex rope. Maximum pressure recovery inside the DT is achieved using longer fins. The swirl intensity remarkably reduces by about 94% with longer fins. The pressure peak of low frequency is about 60% suppressed using longer fins. The PL and cavitation-induced instabilities and vibrations are significantly mitigated by longer fins while shorter fins also moderately minimize it. Hence energy production is preferable with longer fins because of the safe and stable turbine function.

Suppression of Vortex Rope Formation and Pressure Fluctuation using Anti-swirl Fins in a Francis Turbine Model at Part Load Condition with Cavitation Inception Point

Numerical investigation of inner structure and its formation mechanism of cloud cavitating flow

Effects of Vortex Structure on Hydraulic Loss in a Low Head Francis Turbine under Overall Operating Conditions Base on Entropy Production Method

Mode vortex and turbulence in ventilated cavitation over hydrofoils

The vortex rope usually occurs in the draft tube of the Francis turbine operated under part-load conditions, to induce strong low-frequency pressure vibrations, and therefore, is very harmful to the safety of the hydropower unit. In the present work, three kinds of strategies are extensively investigated, i.e., the installations of the ventilation and the fin, as well as the hybrid strategy of the air admission through a fin, so as to effectively suppress the vortex rope oscillation and the pressure vibration in the draft tube of a Francis turbine, whose specific speed is 125 m-kW. For the unsteady flow simulation, the Reynolds averaged Navier-Stokes (RANS) method is applied coupled with the k-ω SST turbulence model and a homogeneous cavitation model. The flow analysis confirms that the low-frequency pressure vibrations are originated from the periodical oscillation of the vortex rope, and the cavitation usually enhances the vortex rope oscillation in the draft tube. Under the part-load condition, the dominant component of the pressure vibration in the draft tube has a frequency, for example, f1, lower than the runner rotating frequency fn. It is shown that all three strategies can be adopted to alleviate the vortex rope oscillation and the pressure vibrations in the draft tube, but their suppression mechanisms are quite different. The ventilation of an adequate amount from the turbine runner cone can change the vortex rope geometry from the spiral type to the cylindrical type, suppress the vortex rope oscillation, and consequently create the homogeneous distributions of the pressure and the pressure gradient in the draft tube. On the other hand, a fin installed at the draft tube wall can induce a small extra rope, and the interaction between the main vortex rope and the extra rope changes the flow field and alleviates the pressure vibration in the draft tube. It should be noted that a fin is much more effective to suppress the pressure vibration in the draft tube under the cavitation condition than under the non-cavitation condition. A better effect of suppressing the vortex rope oscillation can be achieved by the air admission through a fin, which is studied numerically in this paper. The result indicates that the air admission can further improve the effect of a fin for suppressing the pressure vibration in the inlet cone of the draft tube. This improvement is due to the stronger interaction between the main vortex rope and the extra air rope. However, the air admission through a fin should be carefully treated because the strong interaction may induce a larger pressure vibration in the elbow of the draft tube. Finally, it is clear that any strategy for suppressing the pressure vibration hardly changes the dominant component frequency f1, which is in the range of 0.22 fn-0.23 fn due to the main vortex rope oscillation in this study. The current results may be used in various engineering applications, where the active control of the vortex oscillation and the pressure vibrations with or without the cavitation is necessary.

Suppression of vortex rope oscillation and pressure vibrations in Francis turbine draft tube using various strategies

The waterjet propulsion is widely applied in the marine vessels over 30 knots, and the intake duct is considered as an essential component that strongly relates to the propulsion performance. This paper sheds light on the flow features inside an intake duct under mooring conditions by using the particle image velocimetry (PIV) technique with three-dimensional (3D) numerical simulations. The hydraulic loss gradually increase as the flow-rate increases. According to analyses via the Bernoulli equation, the hydraulic loss is composed of the frictional head loss (hf ~ V1.75) and the local head loss (hj ~ V2.0). A recirculation region is observed near the duct lower wall with a high-velocity flow near the upper wall, and subsequently a shear flow presents in the horizontal straight pipe with an obvious velocity gradient. Three-dimensional simulations demonstrate that the vortex pair is very strong in the recirculation region and then it gradually decreases as the fluid flows downstream. With the flow-rate increasing, the non-uniformity at the duct outlet firstly increases to a peak and then slightly decreases, while the perpendicularity at the duct outlet dramatically decreases to a minimum and then increases. This work not only reveals some physics of the waterjet propulsion under mooring conditions, but also promotes its efficient operation. The hydraulic loss gradually increase as the flow-rate increases. The hydraulic loss is composed of the frictional head loss (hf ~ V1.75) and the local head loss (hj ~ V2.0). A recirculation region is observed near the duct lower wall with a high-velocity flow near the upper wall, and a shear flow presents in the horizontal straight pipe with an obvious velocity gradient. Numerical simulations demonstrate that the vortex pair is very strong in the recirculation region and then it gradually decreases as the fluid flows downstream.

Experimental and numerical investigations into flow features in an intake duct for the waterjet propulsion under mooring conditions

The stabilizer installation and air admission have been proved to be two effective ways to alleviate the pressure fluctuation in the Francis turbine draft tube. In this research, a combined method of these two ways, i.e. air admission through a fin (one type of the stabilizer) is applied, and its effectiveness, especially air admission's effects, on pressure fluctuation suppression has been investigated through numerical calculation. Two cases of different air amount (Q*=0.01 and 0.02) are investigated. Results show that cavitation inside the draft tube is suppressed and the size of vortex rope is also slightly decreased after air is admitted. As a result, pressure fluctuation inside the draft tube is generally alleviated compared with no air admission case. The maximum amplitude of pressure fluctuation decreases by 9.2% with 0.01 air admission, and by 26.3% with 0.02 air admission. In addition, the position where the maximum amplitude of pressure fluctuation occurs also moves downstream, which should be due to the more violent interaction between the vortex rope in the main stream and a small vortex rope attached to the fin. Consequently, a slight increase of pressure fluctuation downstream the fin can be observed. Although air admission through the fin can suppress pressure fluctuation in the draft tube, it would also lead to turbine efficiency decrease. Therefore, the application of both fin and air admission should be carefully considered to achieve better performance for Francis turbines.

Ruizhi Zhang

Papers

Suppression of vortex rope oscillation and pressure vibrations in Francis turbine draft tube using various strategies

Experimental and numerical investigations into flow features in an intake duct for the waterjet propulsion under mooring conditions

Numerical Investigation of Pressure Fluctuation and Cavitation inside a Francis Turbine Draft Tube with Air Admission through a Fin