Investigations of Compressible Turbulent Flow in a High-Head Francis Turbine

2017 ◽  
Vol 140 (1) ◽  
Author(s):  
Chirag Trivedi
Keyword(s):  
Compressible Flow ◽  
Incompressible Flow ◽  
Draft Tube ◽  
Francis Turbine ◽  
Accurate Estimation ◽  
Compressibility Effect ◽  
Numerical Errors ◽  
Rotor Stator Interaction ◽  
High Head ◽  
Runner Design

Dynamic stability of the high-head Francis turbines is one of the challenging problems. Unsteady rotor–stator interaction (RSI) develops dynamic stresses and leads to crack in the blades. In a high-head turbine, vaneless space is small and the amplitudes of RSI frequencies are very high. Credible estimation of the amplitudes is vital for the runner design. The current study is aimed to investigate the amplitudes of RSI frequencies considering a compressible flow. The hydro-acoustic phenomenon is dominating the turbines, and the compressibility effect should be accounted for accurate estimation of the pressure amplitudes. Unsteady pressure measurements were performed in the turbine during the best efficiency point (BEP) and part load (PL) operations. The pressure data were used to validate the numerical model. The compressible flow simulations showed 0.5–3% improvement in the time-averaged pressure and the amplitudes over incompressible flow. The maximum numerical errors in the vaneless space and runner were 6% and 10%, respectively. Numerical errors in the instantaneous pressure amplitudes at the vaneless space, runner, and draft tube were ±1.6%, ±0.9%, and ±1.8%, respectively. In the draft tube, the incompressible flow study showed the pressure amplitudes up to eight times smaller than those of the compressible. Unexpectedly, the strong effect of RSI was seen in the upper and lower labyrinth seals, which was absent for the incompressible flow.

Long-Period Pressure Pulsation Estimated in Numerical Simulations for Excessive Flow Rate Condition of Francis Turbine

2014 ◽  
Vol 136 (7) ◽  
Author(s):  
Kenji Shingai ◽  
Nobuaki Okamoto ◽  
Yuta Tamura ◽  
Kiyohito Tani
Keyword(s):  
Numerical Simulations ◽  
Flow Rate ◽  
Draft Tube ◽  
Homogeneous Model ◽  
Francis Turbine ◽  
Computational Time ◽  
Condensation Process ◽  
Rate Condition ◽  
Long Period ◽  
Rotor Stator Interaction

A series of numerical simulations for a Francis turbine were carried out to estimate the unsteady motion of the cavity in the draft tube of the turbine under a much larger flow rate condition than the swirl-free flow rate. The evaporation and condensation process was described by using a simplified Rayleigh–Plesset equation. A two-phase homogeneous model was adopted to calculate the mixture of gas and liquid phases. Instantaneous pressure monitored at a point on the draft tube formed long-period pulsations. Detailed analysis of the simulation results clarified the occurrence of a uniquely shaped cavity and the corresponding flow pattern in every period of the pressure pulsations. The existence of a uniquely shaped cavity was verified with an experimental approach. A simulation without rotor-stator interaction also obtained long-period pulsations after an extremely long computational time. This result shows that the rotor-stator interaction does not contribute to the excitation of long-period pulsations.

scholarly journals Investigating the Performance of a Super High-head Francis Turbine under Variable Discharge Conditions Using Numerical and Experimental Approach

Energies ◽  
2020 ◽  
Vol 13 (15) ◽  
pp. 3868 ◽  
Author(s):  
Zheming Tong ◽  
Hao Liu ◽  
Jianfeng Ma ◽  
Shuiguang Tong ◽  
Ye Zhou ◽  
...  
Keyword(s):  
Draft Tube ◽  
Numerical Models ◽  
Guide Vane ◽  
Maximum Velocity ◽  
Francis Turbine ◽  
Inlet Pipe ◽  
Guide Vanes ◽  
Flow Acceleration ◽  
High Head ◽  
Reduced Scale

A super high-head Francis turbine with a gross head of nearly 700 m was designed with computational fluid dynamics (CFD) simulation and laboratory tests. Reduced-scale (1:3.7) physical and numerical models of the real-scale prototype were created to investigate the hydraulic performance. According to the CFD analysis, a strong rotor–stator interaction (RSI) between guide vanes and runner blades is observed as a result of the high-speed tangential flow towards runner created by the super high water head as well as the small gaps between the radial blades. At the designed best efficiency point (BEP), there is no significant flow recirculation inside the flow passage and minor loss occurs at the trailing edge of the stay vanes and guide vanes. Maximum velocity is observed at runner inlets due to flow acceleration through the narrow passages between the guide vanes. The elbow-shaped draft tube gradually decreases the flow velocity to keep the kinetic energy loss at a minimum. The laboratory test was conducted on a reduced-scale physical model to investigate the pressure pulsations and guide vane torque (GVT) under variable-discharge configurations, which are key concerns in the design of a high head turbine. Pressure sensor networks were installed at the inlet pipe, vaneless space and draft tube, respectively. The most intense pressure variation occurs at the inlet pipe and elbow at 0.04–0.2 GVOBEP and 1.5–1.8 GVOBEP with a low frequency about 0.3 times of the runner frequency, while the vibration in vaneless zone performs stable with the blade passing frequency caused by RSI. The GVT shows a declining trend and then keeps stable as GVOs increases at synchronized condition. For the misaligned conditions, the torque of adjacent guide vanes differs a lot except at the synchronous angle and maximum absolute value at least doubles than the synchronized condition.

Impact of Different Operating Conditions on the Dynamic Excitation of a High Head Francis Turbine

2016 ◽  
Author(s):  
Markus Eichhorn ◽  
Eduard Doujak
Keyword(s):  
Fatigue Analysis ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Fluid Structure Interactions ◽  
Cfd Simulations ◽  
Dynamic Excitation ◽  
Hydropower Plants ◽  
Rotor Stator Interaction ◽  
High Head ◽  
Strain Gauge Measurements

Fatigue analysis becomes more important in the design phase of Francis turbine runners due to the changing requirements on hydropower plants, affected by the increasing amount of volatile energy sources. Francis turbines are operated more often and over longer periods of time at off-design conditions to provide regulating power to the electric grid. The lifetime of a Francis runner depends mainly on the dynamic excitation induced by unsteady pressure pulsations like the rotor-stator interaction or draft tube vortex ropes. An approach using one-way coupled fluid-structure interactions has been developed and is now extended using unsteady CFD simulations as well as harmonic and transient FEM computations. The results are compared to strain gauge measurements on the according high head Francis turbine to validate the overall procedure. The investigations should be further used to perform a fatigue analysis and to examine the applicability for lifetime investigations on Francis machines with different specific speeds.

Numerical Simulation of Cavitating Turbulent Flow in a Francis Turbine with Draft Tube Natural Air Admission

2013 ◽  
Vol 291-294 ◽  
pp. 1963-1968
Author(s):  
Hong Ming Zhang ◽  
Li Xiang Zhang
Keyword(s):  
Turbulent Flow ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Francis Turbine ◽  
Transfer Model ◽  
Mass Transfer Model ◽  
Governing Equations ◽  
Operation Analysis ◽  
High Head ◽  
Volume Method

The paper presents numerical analysis of cavitating turbulent flow in a high head Francis turbine with draft tube natural air admission at part load operation. Analysis was performed by OpenFOAM code. A mixture assumption and a finite rate mass transfer model were introduced. The finite volume method is used to solve the governing equations of the mixture model and the pressure-velocity coupling is handled via a Pressure Implicit with Splitting of Operators (PISO) procedure. The pressure distribution and the flow of air in the draft tube are analyzed in detail. Simulation results show that the pressure fluctuations on the draft tube wall can reduce with natural air admission.

Vortex Rope Formation in a High Head Model Francis Turbine

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Rahul Goyal ◽  
Michel J. Cervantes ◽  
B. K. Gandhi
Keyword(s):  
Physical Mechanism ◽  
Swirling Flow ◽  
Draft Tube ◽  
Dynamic Pressure ◽  
Angular Position ◽  
Francis Turbine ◽  
Head Model ◽  
Stagnation Region ◽  
Vortex Rope ◽  
High Head

Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the two-dimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4 ± 0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor–stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope.

scholarly journals Quasi-Three-Dimensional and Three-Dimensional Flow Calculation in a Francis Turbine

1996 ◽  
Author(s):  
Manfred J. Sallaberger
Keyword(s):  
Incompressible Flow ◽  
Draft Tube ◽  
Three Dimensional ◽  
Computational Method ◽  
Francis Turbine ◽  
Governing Equations ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Specific Speed ◽  
Double Grid

The complex three-dimensional flow in the wicket gate and the runner of a Francis turbine is investigated by applying both a quasi-three-dimensional and a three-dimensional computational method. The computations were conducted on a double grid containing the stationary wicket gate and the rotating runner. The equations for inviscid and incompressible flow are solved, assuming that the relative flow field in the runner is stationary. In the quasi-three-dimensional method the governing equations are solved on stream surfaces using a Finite-Element-Method. In the three-dimensional method, the equations of continuity and motion are solved by a Finite-Volume technique using Denton’s code for incompressible flow. Both methods are used in order to compute the flow in a Francis-runner of high specific speed at the operating point of optimum efficiency. The results of the calculations are compared with measurements taken at the draft-tube inlet. Differences between results of computations and measurements are presented.

Sign in / Sign up

Export Citation Format

Share Document