Low Specific Speed Francis Turbine Research Articles

Numerical Investigation of the Influence of Cavitation on the Runner Speed at Speed-No-Load and Runaway for a Francis Turbine of Low Specific Speed

Abstract Studies have shown that the runner speed of hydraulic turbines at no-load conditions is affected by cavitation. However, those studies did not provide explanations relating the variation of the no-load runner speed to cavitation. Understanding why cavitation affects the runner speed is crucial because the maximum runner speed is reached in no-load condition, and this speed must remain below a limit to ensure the generator's safety. This paper uses numerical simulations to investigate the effect of cavitation on two no-load conditions, the runaway and the speed-no-load, for a low specific speed Francis turbine at model scale. The study is based on unsteady Reynolds-averaged Navier–Stokes simulations with and without cavitation and focuses on averaged quantities. At no-load, the regions over the blades producing a motor torque, i.e., oriented with the turbine rotating direction, must be balanced by regions producing a braking torque, opposed to the turbine rotation, to achieve a zero-torque condition. At runaway, cavitation mainly affects regions where a motor torque is produced. However, the zones affected by cavitation have a small contribution to the total motor torque. Therefore, for the runaway condition studied, the torque balance over the blade is hardly affected by cavitation, and the impact of cavitation on the runaway speed is negligible. At speed-no-load, comparisons between cavitating and noncavitating simulations indicated that cavitation affects mainly the braking torque regions. Those regions result from an interaction between the runner blades and a backflow extending from the draft tube cone to the runner outlet. In that case, cavitation strongly affects the torque balance over the blades, and consequently, the runner speed will adapt to find another zero torque condition.

Assessment of erosion wear in low specific speed Francis turbine due to particulate flow

The erosion wear of turbine components reduces the turbine life and consequently decreases the efficiency or power generation. In the present work, the effect and cause of erosion wear in a high head Francis turbine are investigated through the measurements and numerical simulations on a prototype turbine. Localised erosion patterns are observed over the surface of the guide vane, runner, faceplate, and labyrinth seal due to existing particulate flows. Further, the physical interpretation of erosion wear mechanisms is studied through numerical simulations, and the eroded zones of different components are qualitatively validated with the actual site in-situ measurements. The erosion rate prediction of the turbine components is performed by employing a recently developed erosion model for CA6NM turbine material. The results show that the continuous interblade vortex with significant velocity and crossflow between the blade passages is found accountable for the severe erosion of the runner shroud. The dominating effect of flow instability at the off-design operation depicts higher erosion of turbine components. Moreover, the exponent value for the particle size is found to vary in the range of 1.16–1.30 for the material removal rate of turbine components.

CFD analysis of flow pattern in S-Shape region for low specific speed Francis turbine

In this research, a Francis turbine with low specific speed was analyzed by unsteady numerical simulation in case of S-Shape region appearing model test. It was found that S-Shape characteristics curve was caused by an increase of head. Firstly, the head was decreased in accordance with a decrease of flow rate. However, once a centrifugal force became larger, then a reverse flow appeared between guide vane and runner and the head was increased. And then, head was decreased with flow rate decreasing. In this research, operating conditions of flow rate were defined as three regions, first half of S-Shape region, just S-Shape region and last half of S-Shape region. The operating condition, where the head was decreased in accordance with the decrease of flow rate, was defined as the first half of S-Shape region. The operating condition, where the head was just being increased in accordance with the decrease of flow rate, was defined as the just S-Shape region. The operating condition after passing of the just S-Shape region was defined as the last half of S-Shape region. It was revealed that those three operating conditions showed a spectrum distribution of pressure fluctuation differently each other. A flow visualization analysis with numerical results was conducted to investigate the difference of pressure spectrum in three operating conditions. At the first half of S-Shape region, two groups of reverse flow were caused at chamber between guide vane and runner and revolving in same direction as runner rotation. At the just S-Shape, the reverse flows were circumferentially connected to build a long curved string. At the last half of S-Shape, the reverse flows were fully connected to build a round ring shape.

Optimization of Francis Turbines for Variable Speed Operation Using Surrogate Modeling Approach

Abstract Previous studies suggested variable speed operation (VSO) of Francis turbines as a measure to improve the efficiency at off-design operating conditions. This is, however, strongly dependent on the hydraulic design and, for an existing turbine, improvements can be expected only with a proper redesign of the hydraulic surfaces. Therefore, an optimization algorithm is proposed and applied to the runner of a low specific speed Francis turbine, with an optimization strategy specifically constructed to improve the variable speed performance. In the constrained design space of the reference turbine, the geometry of the replacement runner is parametrically defined using 15 parameters. Box–Behnken method was used to populate the design space with 421 unique samples, needed to train fully quadratic response surface models of three characteristic efficiencies defined by the proposed objective function. Computational fluid dynamics (CFD) was used to calculate the responses for each sample. The parametric study showed that the anticipated variation of the shape of the hill chart, needed to improve the variable speed performance of the turbine, is limited within a narrow range. The presented method is general and can be applied to any specific speed in the Francis turbine range, for both synchronous speed and variable speed optimization tasks.

Experimental investigation on the effect off near walls on the eigen frequency of a low specific speed francis runner

The importance to correctly predict the natural frequency of a turbine runner have been demonstrated several times. It is now common practice to include the added mass effect of the surrounding water when calculating the natural frequencies. In this paper the added mass effect of water on a simplified low specific speed francis turbine runner is experimentally investigated. Three cases are investigated. 1: The runner hanging in air. 2: The runner hanging in a tank of water. 3: The runner installed into the turbine housing. The measurements reveal a frequency reduction of about 40% when the runner is hanging in water. Installing the runner into the turbine housing does not significantly change the natural frequency of the main blade modes. Modes which vibrate heavily on the outside of the runner are visible in the water tank but becomes dampened when installed into the turbine housing.

The analysis of plasma acitretin and etretinate levels in patients treated with acitretin

A guide vane cascade is developed for the study of flow in the distributor of a low specific speed Francis turbine. Velocity and pressure measurements are done with Reynold's number 1.15 E+07, at 80% of BEP flow as in a reference prototype turbine. This work illustrates the development of test setup and focuses on investigation of PIV methods applied for the velocity measurements. Techniques developed for ‘in-situ’ calibration of PIV setup and methods applied for image processing are discussed in details. Approach to estimate total uncertainty in PIV measurements and minimum no of image pairs required for statistical convergence of velocity field is presented. Reference measurements are done along the plane of chord, from guide vane wall to its mid-span. Flow velocity exceeding 35 m/s, at the runner inlet of Francis turbine, is reported for the first time from such experimental studies. Flow phenomenon inside Francis turbine distributor are characterized and comparison are done with the cases for prototype turbines. The cascade setup is found to reproduce the flow conditions inside a Francis turbine distributor, except the rotor-stator-interaction. PIV methods are generalized for the cases of similar measurements and the results will be applicable to validate numerical studies.

Flow measurements around guide vanes of Francis turbine: A PIV approach

A guide vane cascade is developed for the study of flow in the distributor of a low specific speed Francis turbine. Velocity and pressure measurements are done with Reynold's number 1.15 E+07, at 80% of BEP flow as in a reference prototype turbine. This work illustrates the development of test setup and focuses on investigation of PIV methods applied for the velocity measurements. Techniques developed for ‘in-situ’ calibration of PIV setup and methods applied for image processing are discussed in details. Approach to estimate total uncertainty in PIV measurements and minimum no of image pairs required for statistical convergence of velocity field is presented. Reference measurements are done along the plane of chord, from guide vane wall to its mid-span. Flow velocity exceeding 35 m/s, at the runner inlet of Francis turbine, is reported for the first time from such experimental studies. Flow phenomenon inside Francis turbine distributor are characterized and comparison are done with the cases for prototype turbines. The cascade setup is found to reproduce the flow conditions inside a Francis turbine distributor, except the rotor-stator-interaction. PIV methods are generalized for the cases of similar measurements and the results will be applicable to validate numerical studies.

Sediment erosion induced leakage flow from guide vane clearance gap in a low specific speed Francis turbine

Opportunities of future hydropower developments in Asia comes with challenges of handling sediments in rivers. Hard minerals in flow causes turbine parts to erode with several undesirable effects. In Francis turbines, sediment erosion causes an increase of clearance gap between guide vane walls and cover plates. Due to inherit pressure difference between guide vane surfaces, a leakage flow arises from the clearance gap. A guide vane cascade is developed to study the characteristics of the leakage flow in a low specific speed Francis turbine. Velocity and pressure measurements are done at 80% of BEP flow as that in a reference prototype turbine. Cases with five different sizes of clearance gaps are investigated. Strong cross-wise jet-like leakage flow is observed from the clearance gap. A vortex filament developed due to mixing of leakage flow with the main flow is found to hit the hub at runner inlet. The existence of a critical clearance gap size for which the leakage velocity and its effects are maximum is revealed. Interpretations of the experimental results show a close match with the observations of eroded turbine parts from a power plant.

Velocity and pressure measurements in guide vane clearance gap of a low specific speed Francis turbine

In Francis turbine, a small clearance gap between the guide vanes and the cover plates is usually required to pivot guide vanes as a part of governing system. Deflection of cover plates and erosion of mating surfaces causes this gap to increase from its design value. The clearance gap induces the secondary flow in the distributor system. This effects the main flow at the runner inlet, which causes losses in efficiency and instability. A guide vane cascade of a low specific speed Francis turbine has been developed for experimental investigations. The test setup is able to produce similar velocity distributions at the runner inlet as that of a reference prototype turbine. The setup is designed for particle image velocimetry (PIV) measurements from the position of stay vane outlet to the position of runner inlet. In this study, velocity and pressure measurements are conducted with 2 mm clearance gap on one side of guide vane. Leakage flow is observed and measured together with pressure measurements. It is concluded that the leakage flow behaves as a jet and mixes with the main flow in cross-wise direction and forms a vortex filament. This causes non-uniform inlet flow conditions at runner blades.

Investigation on internal flow of draft tube at overload condition in low specific speed Francis turbine

The cavitating vortices causes the unsteady phenomena like the pressure fluctuation, the noise and the vibration in the draft tube at the overload condition which is the far operating point from the design point. Because the full load was normally near the design point, there were few troubles due to cavitating vortices at the full load. Today, however, the design point is sometimes set to lower load to achieve the high efficiency from the partial load to the full load in low specific speed Francis turbines, which have good performance to a change in a discharge. Then, the full load is relatively further from the design point. As the result, the potential for the cavitating vortices at the full load is increased. To control of the unsteady phenomena at the full load, the study focused on the cavitating vortices at the overload condition is important. This paper presents the unsteady behavior of the cavitating vortices at the overload condition with the scaled model of specific speed NQE=0.083. On the experimental approach, the pressure pulsation in the upper draft tube was measured and the unsteady behavior of cavitating vortices was taken movies with a high speed camera. On the numerical approach, Computational Fluid Dynamics (CFD) adopting a two-phase unsteady analysis was carried out. The pressure fluctuation and the velocity distribution of two runners, an original and a newly designed, were compared.

Numerical investigation of the flow phenomena around a low specific speed Francis turbine's guide vane cascade

Guide vanes of Francis turbines convey a significant influence on the flow field at the inlet of the runner. This influence is in the form of pressure pulsation, caused due to rotor-stator-interaction. A guide vane cascade containing a single blade passage was developed to predict the flow field experimentally. Firstly, this paper investigates flow phenomena around the guide vane cascade through computational techniques. A numerical model is prepared with three different turbulence models. The velocity distribution obtained from these models are compared with experimental results at two circumferential midspan locations. Secondly, the influence of increasing the clearance gap on the flow is studied. Such gaps are expected to increase when the flow containing eroding particles passes through the turbine. This paper also shows that the pressure difference between the pressure and the suction side of guide vane influences the leakage flow through the gap. Hence, reduction of the pressure gradient will reduce leakages through clearance gaps, hereby condensing the subsequent effect of pressure pulsations and erosion. This study also shows that the effect of the gap is prominent in the near wall regions which are close to the gap, whereas it dissipates gradually towards the midspan.

Experimental and numerical investigation of unsteady behavior of cavitating vortices in draft tube of low specific speed Francis turbine

At both partial and full load of Francis turbines, the unsteady behavior of cavitating draft tube vortices occurs and leads to undesirable matters such as power house vibration, noise and power swing in some cases. This paper presents the investigation of the interaction between the flow pattern at runner outlet and the unsteady behavior of cavitating vortices in draft tube with experimental and numerical approaches. On the experimental research, the pressure pulsation in the draft tube is measured and the unsteady behavior of cavitating vortices is taken pictures with a high speed camera in the model test. On the numerical research, by Computational Fluid Dynamics (CFD) adopting a two-phase unsteady analysis, the analysis domain from the guide vane to the draft tube is carried out for investigating the interaction between the runner outlet flow and the vortex pattern. The pressure pulsation at the upper draft tube and the unsteady behavior of cavitating vortices obtained from CFD results are similar to those obtained in the model test. Detailed analysis of CFD results at overload indicates the repeat of expansion and contraction of cavitating vortices, which were shaped helical vortices with opposite direction of runner rotation, and the corresponding flow pattern in every time step of the pressure pulsations.

Research of performance prediction to energy on hydraulic turbine

Refer to the low specific speed Francis turbine blade design principle and double-suction pump structure. Then, design a horizontal double-channel hydraulic turbine Francis. Through adding different guide vane airfoil and and no guide vane airfoil on the hydraulic conductivity components to predict hydraulic turbine energy and using Fluent software to numerical simulation that the operating conditions and point. The results show that the blade pressure surface and suction surface pressure is low when the hydraulic turbine installation is added standard positive curvature of the guide vane and modified positive curvature of guide vane. Therefore, the efficiency of energy recovery is low. However, the pressure of negative curvature guide vane and symmetric guide vane added on hydraulic turbine installations is larger than that of the former ones, and it is conducive to working of runner. With the decreasing of guide vane opening, increasing of inlet angle, flow state gets significantly worse. Then, others obvious phenomena are that the reflux and horizontal flow appeared in blade pressure surface. At the same time, the vortex was formed in Leaf Road, leading to the loss of energy. Through analyzing the distribution of pressure, velocity, flow lines of over-current flow in the the back hydraulic conductivity components in above programs we can known that the hydraulic turbine installation added guide vane is more reasonable than without guide vanes, it is conducive to improve efficiency of energy conversion.