Unstable S-shaped characteristics of a pump-turbine unit in a lab-scale model

Effect of interface condition on the hydraulic characteristics of a pump-turbine at various guide vane opening conditions in pump mode

Internal flow phenomena of a Pump–Turbine model in turbine mode with different Thoma numbers

S-shaped characteristics in turbine mode are prone to instabilities in associated transient processes. A single value of the speed factor corresponds to multiple values of the discharge factor, having the possibility of changing the operating point among the turbine, turbine brake, and reverse pump modes. Because of this characteristic, the S-shaped curves induce instability in transient processes. Understanding the hydraulic behavior of a turbine on the four-quadrant characteristic is important since it provides detailed performance information through the whole discharge range of the turbine. This study was numerically and experimentally investigated the scale effect on the S-shaped characteristics in the turbine transition region. The four-quadrant characteristic curves (full- and laboratory-scale) in the turbine mode were predicted by numerical simulations. To verify the predicted results, a laboratory-scale experiment was performed in the turbine, turbine brake, and reverse pump modes. Although the full-scale experiment was performed in the normal operating head range, the scale effect can be validated by comparing steady operating points between the two models. Based on the verified results, the internal flow and pressure pulsation characteristics were determined at the operating point in a specific transition region.

A Comparative Study of the Scale Effect on the S-Shaped Characteristics of a Pump-Turbine Unit

In recent times, optimization began to be popular in the turbomachinery field. The development of computational fluid dynamics (CFD) analysis and optimization technology provides the opportunity to maximize the performance of hydro turbines. The optimization techniques are focused mainly on the rotating components (runner and guide vane) of the hydro turbines. Meanwhile, fixed flow passages (stay vane, casing, and draft tube) are essential parts for the proper flow uniformity in the hydro turbines. The suppression of flow instabilities in the fixed flow passages is an inevitable process to ensure the power plant safety by the reduction of vortex-induced vibration and pressure pulsation in the hydro turbines. In this study, a CFD-based shape design optimization process is proposed with response surface methodology (RSM) to improve the flow uniformity in the fixed flow passages of a Francis hydro turbine model. The internal flow behaviors were compared between the initial and optimal shapes of the stay vane, casing, and the draft tube with J-Groove. The optimal shape design process for the fixed flow passages proved its remarkable effects on the improvement of flow uniformity in the Francis hydro turbine.

A CFD-Based Shape Design Optimization Process of Fixed Flow Passages in a Francis Hydro Turbine

This research aimed to establish a numerical analysis method for use in commercial CFD packages to evaluate a pump-turbine with different guide vane openings. Additionally, this study proposed an appropriate numerical method to investigate flow characteristics. A three-dimensional steady-state Reynolds-averaged Navier-Stokes (RANS) equation was solved to evaluate the internal flow characteristics in the turbine mode. A combination of a grid system, turbulence model, interface position, and interface condition is crucial for the effective prediction of the hydraulic performance of the pump-turbine. The reliability of the numerical analysis method was demonstrated by comparing numerical and experimental results. The error range of the reliability verification of the frozen rotor and stage average was approximately 1.5% -2.5% at the design flow rate. Overall, the numerical results indicated a high predictive power with high reliability within the operating range. 양수발전용 펌프터빈 해석기법 정립에 관한 연구 한국유체기계학회 논문집: 제22권, 제2호, 2019 23 것으로 예상되며 이를 보완하기 위한 3개소의 양수발전소가 추가될 예정이어서 양수발전시스템의 중요도가 지속적으로 증가하고 있다. 하지만, 현재 국내 양수발전용 펌프터빈은 설계기술 부족으 로 양수발전 주기기설비 전량을 외산에 의존하고 있으며, 이 로 인해 터빈의 동적성능에 큰 영향을 주는 천이구간에서의 운전상태파악 및 설비 문제발생시 체계적인 기술지원이 부 족한 실정이다. 본 연구는 이를 해소하고자 양수발전용 펌프터빈 이상 및 천이현상 대응 유동특성 예측기술개발 연구의 일환으로 수 행되었다. 범용의 비속도를 갖는 양수발전용 펌프터빈을 설 계하여 유동장을 확보하고 수치해석 수행시 발생할 수 있는 변수조건이 출력 값에 미치는 영향분석을 통해 해석기법을 합리적으로 결정하고 일관성 있게 표준화하고자 한다.

Establishment of Numerical Analysis Method of Pump-Turbine for Pumped Storage

A spiral casing is an important component of Francis hydro turbine for the even distribution of kinetic energy along the stay and guide vanes. The fluid flow around the runner is dependent on the flow condition of a spiral casing. The shape of the casing plays an important role in proper flow distribution. In this study, the optimization of the shape of a spiral casing is based on a steady-state flow analysis. Numerical optimization is performed using response surface methodology (RSM) and multiobjective genetic algorithm (MOGA). The flow uniformity and head loss in the spiral casing are selected as objectives for the optimal design of the spiral casing. The optimal design is selected from the solution acquired by RSM and MOGA. Moreover, the flow characteristics in the initial and optimal designs of the spiral casing are compared. The flow conditions in the optimal design improve significantly with the optimal design of the spiral casing. Thus, the inlet conditions for the stay vane are improved with the optimal design of the spiral casing.

Improvement of flow behavior in the spiral casing of Francis hydro turbine model by shape optimization

Sediment erosion is the wear phenomenon from the hard abrasive particle found in the rivers. Sediment erosion is an inevitable process in the hydropower. It deteriorates the turbine shape and structure and increasing the maintenance expenses. There are various methods to improve the runner to operate against the sediment erosion. Firstly, the turbine material can be modified to the harder material but it will be costly and difficult for the manufacturing. Secondly, regular maintenance of the turbine needs to be done but it is the tedious task and hinder the power generation from the hydropower. Thirdly, the critical area of the runner blade surface which is vulnerable to sediment erosion need to identify and modification of the turbine shape according to it by altering the design parameter of the blade. The modification such as a change in thickness, blade angle and coating is possible for minimizing the effect of sediment erosion. The determination of the vulnerable area in the runner for the sediment erosion is a difficult task considering the many parameters included in sediment erosion phenomenon. The parameter such as sediment size, sediment shape, sediment concentration, sediment velocity, sediment hardness, impact angle and operating condition of hydropower. The precise prediction of the sediment erosion in the turbine is the difficult task due to the synergetic effect of parameters. The accurate and precise indication of the sediment erosion required both experimental and computational analysis. After identification of the susceptible area of sediment erosion, modification and optimization will be performed to reduce the effect of the sediment erosion without falling in the performance of the turbine.

https://robots.iopscience.iop.org/article/10.1088/1755-1315/240/4/042001/pdf

Numerical studies on sediment erosion due to sediment characteristics in Francis hydro turbine

The oscillating flow is a problem occurring in the stay vane passage of hydro turbines. The improper shape of the stay vane causes wake formation and sheds large eddies in the trailing edge of the stay vane. The wake at the trailing edge of the stay vane produces pressure fluctuation in the stay vane passage, which leads to noise and vibration during the turbine operation. Therefore, the improper shape of the stay vane may cause flow oscillation in the stay vane passage. Thus, a proper shape design of the stay vane considering the oscillating flow is necessary to mitigate the flow instability. Consequently, experiment and CFD analyses showed that the initial stay vane (ISV) shape causes recirculation and pressure fluctuation. An optimum design methodology is adopted to improve the flow behavior around the stay vane. Optimization of the stay vane is also conducted by using two objective functions (turbine efficiency and flow uniformity) and 12 design variables. The optimal stay vane (OSV) shape is attained by a multi-objective genetic algorithm. Finally, the flow behavior in the stay vane passage with ISV and OSV is compared by experiment and CFD analyses. Thus, the flow instabilities are mitigated with OSV. The installation of OSV improved the flow angle distribution, secondary flow, and pressure fluctuation in the Francis hydro turbine.

Ujjwal Shrestha

Papers

A CFD-Based Shape Design Optimization Process of Fixed Flow Passages in a Francis Hydro Turbine

Establishment of Numerical Analysis Method of Pump-Turbine for Pumped Storage

Improvement of flow behavior in the spiral casing of Francis hydro turbine model by shape optimization

Numerical studies on sediment erosion due to sediment characteristics in Francis hydro turbine

Suppression of flow instabilities in the stay vane passage of the Francis hydro turbine model by design optimization