Francis Hydro Turbine Research Articles

Numerical investigation of the behaviour of the cavitation rope in a Francis turbine with an optimized runner cone

In case of the hydro turbine operated departing from the optimum condition, vortex ropes usually occur in the draft tube, and consequently generate pressure oscillations. In most conditions, cavitation occurred in the center of the rope and turned it to be a multiphase cavitating rope. This kind of unsteady flow phenomenon is believed to be harmful for hydropower stations. This paper designs a kind of vortex-control grooves and make a numerical simulation in a Francis hydro turbine using the partially averaged Navier-Stokes method. The behavior of the cavitation rope and the induced pressure oscillations under several operation conditions with and without the grooves will be analyzed based on the calculation results.

Effect of blade thickness on the hydraulic performance of a Francis hydro turbine model

Francis turbines are the most commonly used turbines for hydroelectric power generation. Preliminary studies to verify turbine designs are often performed with small-scale models; however, when the runner blade of a full-size turbine is geometrically scaled down to prepare a model for evaluating the design variables and performance characteristics, the blades become very thin and difficult to manufacture. Hence, the blockage effect of the runner blade should be considered to find a suitable blade thickness that satisfies the required hydraulic performance. Furthermore, a clear understanding of the blockage ratio at the highest efficiency point and off-design condition is required to investigate different blade thicknesses and performance characteristics. Here, the blockage effect of the runner blade on the hydraulic performance and internal flow characteristics of a 300-class Francis hydro turbine was investigated. Three-dimensional Reynolds-averaged Navier–Stokes calculations were performed with a shear stress transport turbulence model to analyze the internal flow characteristics near the runner blade and compare the blockage effect with various blade thicknesses on major performance parameters such as the hydraulic efficiency. Flow analyses for the off-design conditions were also performed with various blade thicknesses. The obtained results indicated that the power and efficiency gradually decreased with increasing blockage ratio. The runner head loss increased due to the mismatches between the flow angle and blade angle with changing the inlet velocity triangle components according to blockage ratio. Especially the efficiency of approximate 3.4% decreased as the blockage ratio increased with 12.5%, compared to the reference model. It was verified that the blockage effect significantly affects the design of Francis turbine models.

프란시스 수차 축소 모델의 수력학적 성능 특성에 대한 형상 매개변수 연구

This paper presents a geometrical parametric study on the hydraulic performance characteristics of a small-scale Francis hydro turbine model. The upper and lower clearances on the variable guide vane and the surface roughness on each component of the turbine model were considered as geometric parameters. Three-dimensional steady-state Reynolds-averaged Navier-Stokes (RANS) calculations with a shear stress transport (SST) turbulence model were performed to evaluate the hydraulic performance characteristics by varying these geometric parameters. Variable guide vane clearances and surface roughness were applied, respectively and simultaneously, to conduct flow analysis of the small-scale Francis hydro turbine model. Additionally, the loss distribution for each component of the turbine model was analyzed to obtain a better understanding of the effects of hydraulic performance on surface roughness. The results indicated it could be applied to the design and manufacture of a small-scale Francis hydro turbine model as the major geometric parameters.

Performance analysis of micro-class Francis hydro turbine by CFD

Hydropower is the largest source of renewable energy. Francis hydro turbines are the world’s most widely used hydro turbine. In this paper, CFD software used to simulate for the performance analysis of micro-class Francis hydro turbine. A draft tube is one of the most important parts of Francis turbine which connects the runner exit to the tailrace where the water is being finally discharged at atmospheric pressure from the reaction turbine. There are several issues in the draft tube like a pressure pulsation, effect of cavitation of draft tube performance, vortex rope study etc. At part-load condition vortex form, it hits structure constantly which affect the performance of the turbine. This study focused on prediction of vortex behavior at the draft tube and numerical results obtained the hydraulic performance of 3KW micro Francis turbine with the inlet pipe, a spiral casing with 12 guide vanes, 6 stay vanes and the runner 13 blades and a draft tube. Additionally, results from the CFD simulation and experimental results will be compared to 3KW micro-Francis turbine with misaligned guide vane.

Study on the effect of the runner design parameters on 50 MW Francis hydro turbine model performance

Francis hydro turbine is the dominant turbine in the hydropower generation. Francis turbine has been installed at most 60% of the hydropower in the world at present. Although the basic design for the Francis turbine has various method regarding the specific speed. The runner meridional shape varies with different specific speed. Despite having, the basic design but there is still some room for the optimization. In this study 50 MW, Francis hydro turbine with specific speed 323 m-kW was designed and considered for the optimization. The various parameter as runner meridional shape (curve profile of hub, shroud, leading edge and trailing edge), blade angle and its distribution, blade thickness, runner inlet width that has been considered for the optimization of the runner for enhancement of the performance.

스파이럴 케이싱 설부의 누설벽이 프란시스 수차의 수력학적 성능에 미치는 영향

스파이럴 케이싱 설부의 누설벽이 프란시스 수차의 수력학적 성능에 미치는 영향

The influence of runner cone perforation on the draft tube vortex in Francis hydro-turbine

In Francis hydro-turbine, the pressure oscillation in the draft tube is the main source of flow-induced vibration and noise. The structural design of runner cone exerts significant influence on the tangential flow velocity within the draft tube and the pressure fluctuations downstream. In this work, we propose a simple and effective method by perforating through-holes on the runner cone to weaken or eliminate pressure fluctuations induced by the vortex ropes in draft tube. An 2 and 4 holes are perforated on the lower half of runner cone, and numerical simulations and experiments are conducted to understand the changing of phenomenology and features with regard to vortex ropes and pressure levels, comparing with the original unperforated case. The analyses are done for two different guide vane openings, ?, and the results were validated by experiments with high-speed photography. It is found that at the two openings, the runner cone with perforations can reduce the volume of vortex ropes, and decrease the pressure fluctuation level. The motion of vortex ropes tends to be less violent with more perforated holes. Therefore, the runner cone perforation has the potential of reducing the vortex rope strength.

Suppression of unsteady swirl flow in the draft tube of a Francis hydro turbine model using J-Groove

The pressure fluctuations of the turbines are caused by the swirl component in the draft tube, which is an undesired phenomenon that needs to be improved when the turbine operates in the off-design condition. In order to avoid these kinds of unsteady flow phenomena, a technology of J-Groove on the draft tube wall is investigated by numerical method. The validation test of performance is conducted to compare the result of numerical and experimental methods. The steady and unsteady state analyses are conducted to investigate the internal flow of a Francis hydro turbine model draft tube with and without J-Groove. The swirl flow in the draft tube is significantly suppressed by the J-Groove installation. Moreover, according to the unsteady state analysis, the amplitude of pressure fluctuation in the draft tube is reduced effectively by the J-Groove.

Fractional-Order Modeling and Dynamical Analysis of a Francis Hydro-Turbine Governing System with Complex Penstocks

This paper investigates the stability of the Francis hydro-turbine governing system with complex penstocks in the grid-connected mode. Firstly, a novel fractional-order nonlinear mathematical model of a Francis hydro-turbine governing system with complex penstocks is built from an engineering application perspective. This model is described by state-space equations and is composed of the Francis hydro-turbine model, the fractional-order complex penstocks model, the third-order generator model, and the hydraulic speed governing system model. Based on stability theory for a fractional-order nonlinear system, this study discovers a basic law of the bifurcation points of the above system with a change in the fractional-order α. Secondly, the stable region of the governing system is investigated in detail, and nonlinear dynamical behaviors of the system are identified and studied exhaustively via bifurcation diagrams, time waveforms, phase orbits, Poincare maps, power spectrums and spectrograms. Results of these numerical experiments provide a theoretical reference for further studies of the stability of hydropower stations.

Numerical investigation of the air injection effect on the cavitating flow in Francis hydro turbine

At full and over load operating points, some Francis turbines experience strong self-excited pressure and power oscillations. These oscillations are occuring due to the hydrodynamic instability of the cavitating fluid flow. In many cases, the amplitude of such pulsations may be reduced substantially during the turbine operation by the air injection/ admission below the runner. Such an effect is investigated numerically in the present work. To this end, the hybrid one-three-dimensional model of the flow of the mixture “liquid−vapor” in the duct of a hydroelectric power station, which was proposed previously by the present authors, is augmented by the second gaseous component — the noncondensable air. The boundary conditions and the numerical method for solving the equations of the model are described. To check the accuracy of computing the interface “liquid−gas”, the numerical method was applied at first for solving the dam break problem. The algorithm was then used for modeling the flow in a hydraulic turbine with air injection below the runner. It is shown that with increasing flow rate of the injected air, the amplitude of pressure pulsations decreases. The mechanism of the flow structure alteration in the draft tube cone has been elucidated, which leads to flow stabilization at air injection.

A method for analysis of head cover deformation and vibration amplitude in Francis hydro-turbine system by combination of CFD and FEA

It is of great importance to predict the deformation and vibration characteristics of the hydro-turbine structure accurately for the stable operation when the turbine is running under the alternating load of the flow field. In this paper, we proposed a methodology for analysis of the head cover deformation and vibration amplitude in the high-head Francis hydro-turbine system, in which Computational fluid dynamics (CFD) is employed to simulate the complex flow field in the head cover flow passage and the CFD-captured pressure distribution is provided for Finite element analysis (FEA) as a new load input to obtain more accurate head cover deformation and vibration calculation results. The results obtained by this method are compared with the results of the conventional algorithm and laboratory experimental results, demonstrated a better consistency. The great advantage of this method is that it allows vibration amplitude analysis and predictions under varied working conditions by adjusting the flow field parameters according to the specific working conditions. Therefore, it has supportive significance in optimization of the hydro-turbine structural turbine design as well as stabilizing operations at hydropower stations.

Structural Safety Evaluation of a Francis Hydro Turbine Model at Normal Operation Condition

Structural Safety Evaluation of a Francis Hydro Turbine Model at Normal Operation Condition

Laboratory modeling of flow regimes in a draft tube of Francis hydro-turbine

The paper reports on some results of the experimental study of flow and pressure pulsations in a laboratory model of the draft tube (DT) of Francis-99 hydro-turbine over a broad range of operating regimes. Velocity distributions at the model inlet varied within 866 modes of the turbine load conditions, including those with maximum coherent pressure pulsations on the model walls. The contact and non-contact methods were used to measure pressure pulsations on the model walls using acoustic sensors and to measure the averaged velocity distribution with a laser Doppler anemometer “LAD-06i”. Analysis of the results have showed that in the model cone there are flow modes with forming of precessing vortex cores, accompanied by a sharp increase in the amplitude of coherent pressure pulsations on the wall and the velocity field rearrangement. It is shown that the vortex core starts forming with an increase in the integral parameter of swirl up to S>0.5. A novelty of the work is the combination of the rapid prototyping (3-d printing) of the inflow swirl generators and computerized measurement techniques that makes it possible to acquire rapidly a large amount of experimental data for a variety of designs and operating conditions. The results can provide insight into the effect of various design and operating parameters on the flow physics, as well as serve for verification of the numerical simulations.

Numerical investigation of flow structure and pressure pulsation in the Francis-99 turbine during startup

We performed numerical simulation of flow in a laboratory model of a Francis hydroturbine at startup regimes. Numerical technique for calculating of low frequency pressure pulsations in a water turbine is based on the use of DES (k-ω Shear Stress Transport) turbulence model and the approach of “frozen rotor”. The structure of the flow behind the runner of turbine was analysed. Shows the effect of flow structure on the frequency and intensity of non-stationary processes in the flow path. Two version of the inlet boundary conditions were considered. The first one corresponded measured time dependence of the discharge. Comparison of the calculation results with the experimental data shows the considerable delay of the discharge in this calculation. Second version corresponded linear approximation of time dependence of the discharge. This calculation shows good agreement with experimental results.

Francis-99 turbine numerical flow simulation of steady state operation using RANS and RANS/LES turbulence model

We performed numerical simulation of flow in a laboratory model of a Francis hydroturbine at three regimes, using two eddy-viscosity- (EVM) and a Reynolds stress (RSM) RANS models (realizable k-ϵ, k-ω SST, LRR) and detached-eddy-simulations (DES), as well as large-eddy simulations (LES). Comparison of calculation results with the experimental data was carried out. Unlike the linear EVMs, the RSM, DES, and LES reproduced well the mean velocity components, and pressure pulsations in the diffusor draft tube. Despite relatively coarse meshes and insufficient resolution of the near-wall region, LES, DES also reproduced well the intrinsic flow unsteadiness and the dominant flow structures and the associated pressure pulsations in the draft tube.

Experience of an assessment of the vertical Francis hydroturbines vibration state at heads from 40 to 300 m

The article covers a choice of main vibration parameter at an assessment of a vibration state of vertical Francis hydroturbines. At present time vibration velocity and vibration displacement are adopted as main parameters of non-rotating parts vibration in the international standard ISO 10816-5:2000 «Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts — Part 5: Machine sets in hydraulic power generating and pumping plants» (further ISO 10816-5:2000).The hydraulic turbines refer to the slow-speed machines with rotation speed from 60 to 600 rpm (∼ 1 - 10 Hz).So maximum vibration displacements and dynamic stresses in hydraulic turbines supporting parts are in low-frequency region of vibration spectrum.In this report comparative data of hydro units supporting parts vibration velocity and vibration displacement measurements are presented. Using these data assessment of hydro units vibration state has been done.It is shown that the assessment of a hydro unit vibration state using parameter "vibration displacement" corresponds to the fundamental principles of operational reliability and fatigue strength of hydro units supporting parts.It is noted that when hydro units operate at small and partial loads with high low-frequency unsteady flow (f < frot) we have the smallest vibration velocity and the greatest vibration displacement of hydro units supporting parts.Specialists of LMZ (Saint-Petersburg) have developed Russian standard RD 24.023.117-88 «Vibration measurement and evaluation vibration state of vertical hydraulic turbines» which was published in 1989. In this document vibration displacement was considered as a main parameter. Evaluation of turbine vibration was performed according to the effecrive value of turbine supporting parts vibration displacement.

Chaotic dynamic characteristics of pressure fluctuation signals in hydro-turbine

The pressure fluctuation characteristics in a Francis hydro-turbine running at partial flow conditions were studied based on the chaotic dynamic methods. Firstly, the experimental data of pressure fluctuations in the draft tube at various flow conditions was de-noised using lifting wavelet transformation, then, for the de-noised signals, their spectrum distribution on the frequency domain, the energy variation and the energy partition accounting for the total energy was calculated. Hereby, for the flow conditions ranging from no cavitation to severe cavitation, the chaos dynamic features of fluctuation signals were analyzed, including the temporal-frequency distribution, phase trajectory, Lyapunov exponent and Poincare map etc. It is revealed that, the main energy of pressure fluctuations in the draft tube locates at low-frequency region. As the cavitation grows, the amplitude of power spectrum at frequency domain becomes larger. For all the flow conditions, all the maximal Lyapunov exponents are larger than zero, and they increase with the cavitation level. Therefore, it is believed that there indeed exist the chaotic attractors in the pressure fluctuation signals for a hydro-turbine.

Runner Design and Internal Flow Characteristics Analysis for an Ns=200 Francis Hydro Turbine Model

Francis hydro turbines have been most widely used throughout the world because of their wide range of head and flow rate applications. In most applications, they are used for high heads and flow rates. Currently, Korea is developing technology for Francis hydro turbine design and manufacture. In order to understand the internal details of Francis hydro turbines further, a new Francis turbine model runner is designed and model internal flow characteristics are investigated. The specific speed of the Francis hydro turbine model runner is Ns = 200 m-kW-min-1. The runner blade is designed successfully according to the port area and one-dimensional loss analysis. The best efficiency point of the Francis hydro turbine model achieves 90% at the design condition. CFD analysis yields a hill chart of the Francis hydro turbine model for use in predicting performance.

Nonlinear dynamics of a novel fractional-order Francis hydro-turbine governing system with time delay

This paper focuses on the stability of a hydropower station. First, we established a novel nonlinear mathematical model of a Francis hydro-turbine governing system considering both fractional-order derivative and time delay. The fractional-order α, which is introduced into the penstock system, in the range from 0.82 to 1.00 is on the left side of the model in a incommensurate manner in increment of 0.03 to provide an adjustable degree of system memory. The time delay τ, which exists between the signal and response in the hydraulic servo system, in the range from 0s to 0.26s is inserted on the right side of the model in increment of 0.04s. Utilizing the principle of statistical physics, we respectively explored the effects of the fractional-order α and the time delay τ on the stable region of the system. Furthermore, we exhaustively investigated the nonlinear dynamic behaviors of the system with different governor parameters by using bifurcation diagrams, time waveforms and power spectrums, finding that only under the condition of reasonable collocation of governor parameters the system can maintain stable operation. Finally, all of the above numerical experiments supply new methods for studying the stability of a hydropower station.

Vortical structures and pressure pulsations in draft tube of a Francis-99 turbine at part load: RANS and hybrid RANS/LES analysis

Recognizing the limitations of the conventional linear-eddy-viscosity (LEVM) Reynolds-averaged Navier–Stokes (RANS) models to reproduce complex three-dimensional unsteady flows in hydraulic machinery, we performed a comparative assessment of a second-moment (Re-stress model, RSM) RANS closure and a hybrid RANS/LES method in capturing the flow and vortical structures in the draft tube of a Francis hydroturbine at off-design conditions. Considered is a case of part load (PL) at a flow rate of only 35% of the best efficiency point (BEP) characterised by multiple unsteady vortex systems. Despite some remaining uncertainties in generating the inflow conditions, both approaches reproduced reasonably well the measured mean velocity and the rms of its fluctuations, as well as the pressure spectrum with peaks detecting the precessing vortex core. In contrast to the common LEVMs, the Re-stress closure showed sufficient receptivity to intrinsic unsteadiness and reproduced well the overall flow and vortical patterns as well as the associated pressure pulsations in accord with the experiments. The hybrid RANS/LES method gave similar predictions as the RSM, but resolving a wider range of scales, which however, showed no significant effect on the dynamics of the dominant processing vortex core and the pressure pulsations.