Studies of the Effect of Vortex-Control Grooves on Pressure Oscillations in a Francis Turbine Draft Tube

2015 ◽  
Author(s):  
Yu An ◽  
Luo Xianwu ◽  
Ji Bin
Keyword(s):  
Swirling Flow ◽  
Draft Tube ◽  
Francis Turbine ◽  
Pressure Oscillations ◽  
Hydro Turbine ◽  
Vortex Control ◽  
Operation Conditions ◽  
Calculation Results ◽  
Sst Turbulence Model ◽  
Load Conditions

In case of the hydro turbine operated deviated from the designed condition, vortex ropes usually occur in the draft tube, and consequently generate large pressure oscillations. This kind of unsteady flow phenomenon is believed to be harmful for hydropower stations. In this paper, the authors designed a runner with vortex-control grooves and numerically simulate the flow in a Francis hydro turbine using the SAS-SST turbulence model. The pressure oscillations induced by the vortex rope under several operation conditions were analyzed based on the calculation results. It is indicated that the runner with vortex-control grooves can alleviate the pressure fluctuation at part load conditions. However, vortex-control grooves may enhance the swirling flow, and cause a small hydraulic performance drop at full load conditions. Thus, the design optimization of vortex-control grooves is necessary and will be conducted in the future.

Simulations and Measurements of Pressure Oscillations Caused by Vortex Ropes

2006 ◽  
Vol 128 (4) ◽  
pp. 649-655 ◽  
Author(s):  
Zhengwei Wang ◽  
Lingjiu Zhou
Keyword(s):  
Stokes Equations ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Suction Side ◽  
Rotational Frequency ◽  
Francis Turbine ◽  
Pressure Oscillations ◽  
Hydraulic Unit ◽  
Load Conditions

Pressure oscillations caused by vortex rope were measured in the draft tube of a prototype Francis turbine. The three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations with the RNG κ−ϵ turbulence model were solved to model the flow within the entire flow path of the prototype hydraulic unit including the guide vanes, the runner, and the draft tube. The model was able to predict the pressure fluctuations that occur when operating at 67–83% of the optimum opening. The calculated frequencies and amplitudes of the oscillation show reasonable agreement with the experiment data. However, the results at 50% opening were not satisfactory. Pressure oscillations on the runner blades were found to be related to the precession of vortex ropes which caused pressure on the blades to fluctuate with frequencies of −fn+fd (fn is the rotational frequency and fd is vortex procession frequency). The peak-to-peak amplitudes of the pressure oscillations on the blades at the lower load conditions (67% opening) were higher than at higher load conditions (83% opening). Fluctuations on the suction side tended to be stronger than on the pressure side.

scholarly journals Stereo-PIV investigation of the unsteady flow in the draft tube of a model hydro turbine

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Ivan Litvinov ◽  
Dmitry Sharaborin ◽  
Sergey Shtork ◽  
Vladimir Dulin ◽  
Sergey Alekseenko ◽  
...  
Keyword(s):  
Velocity Field ◽  
Unsteady Flow ◽  
Swirling Flow ◽  
Flow Instability ◽  
Draft Tube ◽  
Pressure Pulsations ◽  
Mean Velocity ◽  
Hydro Turbine ◽  
Operation Conditions ◽  
Wide Range

Varying the generator load of a hydro turbine results in short-term changes in the rotation frequency of the runner, leading inevitably to flow instability and strong flow swirling behind the turbine. This may lead to the formation of unsteady flow regimes featured by vortex instability of the swirling flow behind the runner, known as the precessing vortex core (PVC) Dorfler et al. (2012). This effect causes dangerous periodic pressure pulsations that propagate throughout the water column in the draft tube. The present study reports on stereo PIV measurements of the air flow field inside a transparent draft tube of a model hydro turbine for a wide range of operation conditions. The research is focused on the time-averaged flow properties (mean velocity field and the second-order moments of velocity fluctuations), pressure pulsations and coherent flow structures in the velocity field.

scholarly journals Manipulation of the swirling flow instability in hydraulic turbine diffuser by different methods of water injection

2018 ◽  
Vol 180 ◽  
pp. 02090 ◽  
Author(s):  
Pavel Rudolf ◽  
Jiří Litera ◽  
Germán Alejandro Ibarra Bolanos ◽  
David Štefan
Keyword(s):  
Swirling Flow ◽  
Flow Instability ◽  
Draft Tube ◽  
Water Injection ◽  
Hydraulic Turbine ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Necessary Condition ◽  
Vortex Rope ◽  
Wide Range

Vortex rope, which induces substantial pressure pulsations, arises in the draft tube (diffuser) of Francis turbine for off-design operating conditions. Present paper focuses on mitigation of those pulsations using active water jet injection control. Several modifications of the original Susan-Resiga’s idea were proposed. All modifications are driven by manipulation of the shear layer region, which is believed to play important role in swirling flow instability. While some of the methods provide results close to the original one, none of them works in such a wide range. Series of numerical experiments support the idea that the necessary condition for vortex rope pulsation mitigation is increasing the fluid momentum along the draft tube axis.

Simulation on Balde Duct Vortex in a Francis Turbine Runner Based on S-A Model

2013 ◽  
Vol 444-445 ◽  
pp. 476-478 ◽  
Author(s):  
Yong Zhong Zeng ◽  
Xiao Bing Liu
Keyword(s):  
Flow Field ◽  
Internal Flow ◽  
Optimal Operation ◽  
Francis Turbine ◽  
Operation Conditions ◽  
Low Load ◽  
Turbine Runner ◽  
Low Specific Speed ◽  
Specific Speed ◽  
Load Conditions

If deviating from the optimal operation conditions, flow separation will occur on the blade of the runner in a low specific speed turbine. At this time, the turbulent flow of flow field in the blade duct will be in a strong non-equilibrium state, and thus the blade duct vortexes will be generated. To further study the mechanism of blade duct vortexes and to control the generation of these vortexes, Spalart-Allmaras (S-A) model was used to numerically simulate and calculate the internal flow in the low specific speed turbine runner under low load conditions. The blade duct vortexes in the turbine runner were accurately predicted. The effect of short blade in eliminating and reducing the vortexes in the low specific speed turbine runner was analyzed and compared.

scholarly journals Design of Experiments Applied to Francis Turbine Draft Tube to Minimize Pressure Pulsations and Energy Losses in Off-Design Conditions

Energies ◽  
2021 ◽  
Vol 14 (13) ◽  
pp. 3894
Author(s):  
Arthur Favrel ◽  
Nak-Joong Lee ◽  
Tatsuya Irie ◽  
Kazuyoshi Miyagawa
Keyword(s):  
Draft Tube ◽  
Pressure Fluctuations ◽  
Internal Flow ◽  
Francis Turbine ◽  
Energy Losses ◽  
Geometrical Parameters ◽  
Cfd Simulations ◽  
Part Load ◽  
Full Load ◽  
Load Conditions

This paper proposes an original approach to investigate the influence of the geometry of Francis turbines draft tube on pressure fluctuations and energy losses in off-design conditions. It is based on Design of Experiments (DOE) of the draft tube geometry and steady/unsteady Computational Fluid Dynamics (CFD) simulations of the draft tube internal flow. The test case is a Francis turbine unit of specific speed Ns=120 m-kW which is required to operate continuously in off-design conditions, either with 45% (part-load) or 110% (full-load) of the design flow rate. Nine different draft tube geometries featuring a different set of geometrical parameters are first defined by an orthogonal array-based DOE approach. For each of them, unsteady and steady CFD simulations of the internal flow from guide vane to draft tube outlet are performed at part-load and full-load conditions, respectively. The influence of each geometrical parameter on both the flow instability and resulting pressure pulsations, as well as on energy losses in the draft tube, are investigated by applying an Analysis of Means (ANOM) to the numerical results. The whole methodology enables the identification of a set of geometrical parameters minimizing the pressure fluctuations occurring in part-load conditions as well as the energy losses in both full-load and part-load conditions while maintaining the requested pressure recovery. Finally, the results of the CFD simulations with the final draft tube geometry are compared with the results estimated by the ANOM, which demonstrates that the proposed methodology also enables a rough preliminary estimation of the draft tube losses and pressure fluctuations amplitude.

Characteristics and Control of the Draft-Tube Flow in Part-Load Francis Turbine

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Ri-kui Zhang ◽  
Feng Mao ◽  
Jie-Zhi Wu ◽  
Shi-Yi Chen ◽  
Yu-Lin Wu ◽  
...  
Keyword(s):  
Swirling Flow ◽  
Draft Tube ◽  
Flow Behavior ◽  
Load Condition ◽  
Francis Turbine ◽  
Tube Flow ◽  
Reversed Flow ◽  
Part Load ◽  
Axial Pressure Gradient ◽  
Flow Phenomena

Under part-load conditions, a Francis turbine often suffers from very severe low-frequency and large-amplitude pressure fluctuation, which is caused by the unsteady motion of vortices (known as “vortex ropes”) in the draft tube. This paper first reports our numerical investigation of relevant complex flow phenomena in the entire draft tube, based on the Reynolds-averaged Navier–Stokes (RANS) equations. We then focus on the physical mechanisms underlying these complex and somewhat chaotic flow phenomena of the draft-tube flow under a part-load condition. The flow stability and robustness are our special concern, since they determine what kind of control methodology will be effective for eliminating or alleviating those adverse phenomena. Our main findings about the flow behavior in the three segments of the draft tube, i.e., the cone inlet, the elbow segment, and the outlet segment with three exits, are as follows. (1) In the cone segment, we reconfirmed a previous finding of our research group based on the turbine’s whole-flow RANS computation that the harmful vortex rope is an inevitable consequence of the global instability of the swirling flow. We further identified that this instability is caused crucially by the reversed axial flow at the inlet of the draft tube. (2) In the elbow segment, we found a reversed flow continued from the inlet cone, which evolves to slow and chaotic motion. There is also a fast forward stream driven by a localized favorable axial pressure gradient, which carries the whole mass flux downstream. The forward stream and reversed flow coexist side-by-side in the elbow, with a complex and unstable shear layer in between. (3) In the outlet segment with three exits, the forward stream always goes through a fixed exit, leaving the other two exits with a chaotic and low-speed fluid motion. Based on these findings, we propose a few control principles to suppress the reversed flow and to eliminate the harmful helical vortex ropes. Of the methods we tested numerically, a simple jet injection in the inlet is proven successful.

Experimental Evidence of Hydroacoustic Pressure Waves in a Francis Turbine Elbow Draft Tube for Low Discharge Conditions

2009 ◽  
Vol 131 (8) ◽  
Author(s):  
Jorge Arpe ◽  
Christophe Nicolet ◽  
François Avellan
Keyword(s):  
Pressure Fluctuation ◽  
Swirling Flow ◽  
Draft Tube ◽  
Flow Direction ◽  
Pressure Fluctuations ◽  
Operating Conditions ◽  
Hydropower Plant ◽  
Francis Turbine ◽  
Scale Model ◽  
Vortex Rope

The complex three-dimensional unsteady flow developing in the draft tube of a Francis turbine is responsible for pressure fluctuations, which could prevent the whole hydropower plant from operating safely. Indeed, the Francis draft tube is subjected to inlet swirling flow, divergent cross section, and the change of flow direction. As a result, in low discharge off-design operating conditions, a cavitation helical vortex, so-called the vortex rope develops in the draft tube and induces pressure fluctuations in the range of 0.2–0.4 times the runner frequency. This paper presents the extensive unsteady wall pressure measurements performed in the elbow draft tube of a high specific speed Francis turbine scale model at low discharge and at usual plant value of the Thoma cavitation number. The investigation is undertaken for operating conditions corresponding to low discharge, i.e., 0.65–0.85 times the design discharge, which exhibits pressure fluctuations at surprisingly high frequency value, between 2 and 4 times the runner rotation frequency. The pressure fluctuation measurements performed with 104 pressure transducers distributed on the draft tube wall, make apparent in the whole draft tube a fundamental frequency value at 2.5 times the runner frequency. Moreover, the modulations between this frequency with the vortex rope precession frequency are pointed out. The phase shift analysis performed for 2.5 times the runner frequency enables the identification of a pressure wave propagation phenomenon and indicates the location of the corresponding pressure fluctuation excitation source in the elbow; hydroacoustic waves propagate from this source both upstream and downstream the draft tube.

Unsteady vortical flow simulation in a Francis turbine with special emphasis on vortex rope behavior and pressure fluctuation alleviation

2017 ◽  
Vol 231 (3) ◽  
pp. 215-226 ◽  
Author(s):  
Xianwu Luo ◽  
An Yu ◽  
Bin Ji ◽  
Yulin Wu ◽  
Yoshinobu Tsujimoto
Keyword(s):  
Pressure Fluctuation ◽  
Swirling Flow ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Flow Simulation ◽  
Vortical Flow ◽  
Francis Turbine ◽  
Vortex Rope ◽  
Spiral Vortex ◽  
Baroclinic Torque

Hydro turbines operating at partial flow conditions usually have vortex ropes in the draft tube that generate large pressure fluctuations. This unsteady flow phenomenon is harmful to the safe operation of hydropower stations. This paper presents numerical simulations of the internal flow in the draft tube of a Francis turbine with particular emphasis on understanding the unsteady characteristics of the vortex rope structure and the underlying mechanisms for the interactions between the air and the vortices. The pressure fluctuations induced by the vortex rope are alleviated by air admission from the main shaft center, with the water-air two phase flow in the entire flow passage of a model turbine simulated based on the homogeneous flow assumption. The results show that aeration with suitable air flow rate can alleviate the pressure fluctuations in the draft tube, and the mechanism improving the flow stability in the draft tube is due to the change of vortex rope structure and distribution by aeration, i.e. a helical vortex rope at a small aeration volume while a cylindrical vortex rope with a large amount of aeration. The preferable vortex rope distribution can suppress the swirl at the smaller flow rates, and is helpful to alleviate the pressure fluctuation in the draft tube. The analysis based on the vorticity transport equation indicates that the vortex has strong stretching and dilation in the vortex rope evolution. The baroclinic torque term does not play a major role in the vortex evolution most of the time, but will much increase for some specific aeration volumes. The present study also depicts that vortex rope is mainly associated with a pair of spiral vortex stretching and dilation sources, and its swirling flow is alleviated little by the baroclinic torque term, whose effect region is only near the draft tube inlet.

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