Draft Tube Inlet Research Articles

A new strategy for reducing pressure fluctuation of Francis turbine by bionic modification of local components

Long term operation of hydroelectric units in low load conditions can induce large-scale blade vortices, swirling vortex ropes in the draft tube, and low-frequency high amplitude pressure fluctuation. These phenomena will cause adverse consequences such as excessive vibration of the unit and blade breakage of the runner. Taking inspiration from the protuberances of the leading edge of a humped whale flipper, the present study firstly proposes bionic modifications of guide vanes and draft tube to suppress high-amplitude pressure fluctuations for Francis turbine. Numerical simulations of transient flow characteristics of the prototype unit (PU), the bionic guide vane unit (BG), and the bionic draft tube unit (BDG) under two low load conditions are conducted. Results indicated that the bionic draft tube has a good effect on suppressing pressure fluctuations in the vaneless area and draft tube. Under the Q/QBEF = 0.41 working condition, BDG causes the main frequency amplitude of pressure fluctuation at the center point of the draft tube inlet to change from 8000.2 Pa to 390.9 Pa, a decrease of 95.11 %. Under the Q/QBEF = 0.57 working condition, BDG causes the maximum decrease rate of the main frequency amplitude in the draft tube to be 60.5 %. The reducing effect in BDG of monitoring points in the guide vane area has reached over 43 %. Under low load conditions, the vortices near the wall in the draft tube of BDG are intercepted by bionic structures, reducing the vortex scale and helping to prevent the generation of large swirling vortex ropes. The bionic guide vanes have a significant control effect on pressure pulsation in the the guide vane and vaneless regions, although perform poorly in the draft tube.

Internal flow characteristics in the draft tube of a Francis turbine under different working conditions

The Francis turbine stability is impacted significantly by the flow characteristics in its draft tube. In the case of a Francis turbine in a hydropower plant, numerical simulations were undertaken at the maximum and rated heads, respectively, under three output scenarios of 20%, 100%, and 120%. The flow characteristics in the turbine draft tube and the evolution mechanism of flow pattern were assessed, as well as the impact of opening on vortex rope. The results demonstrate that with the increasing of the guide vane opening, a progressive transformation is noted for the random large vortex rope located in the conical section at the draft tube inlet into uniformly distributed one. The opening influences the velocity and pressure greatly, and as the opening enlarges, enhancement of the flow pattern is noted in the draft tube. Meanwhile, as the head increases, the influence scope of the vortex rope becomes larger. This research can reveal the operating conditions not suitable for the Francis turbine, thereby offering guidance for ensuring the stability of power station operation.

The influence of the S shaped characteristics of high head pump-turbine on the minimum pressure of draft tube

Abstract The S shaped characteristics of pump-turbines have an important impact on the minimum pressure of draft tube. The quality of the S shaped characteristics is one of the key indicators to determine the success of model runner development, and this indicator is particularly important for ultra-high head pump-turbines. In order to delve deeper into the inherent relationship between the S-shaped characteristics of the runner and the optimization design and minimum pressure of draft tube, this article first defines a method for fixed-point quantitative analysis of the S shaped characteristic curve. Then, different experimental characteristic curves obtained from multiple optimized designs of the runner are used to calculate the hydraulic transition process. At the same time, the S shaped curve is quantitatively analyzed, and the relationship between the three is explored through continuous analysis. The results show that the bending degree of the equal opening lines in the middle at the S shape characteristics curves has the greatest influence on the minimum pressure at the draft tube inlet. The greater the ratio of unit flow difference to unit speed difference in the S shape characteristics curves of the equal opening line, the greater the runaway unit flow on the same equal opening line, the more favorable it is to increase the minimum pressure at the inlet of the draft tube. The optimal design of blade type and guide vane airfoil can improve the minimum pressure value of draft tube to a certain extent. The conclusions obtained in this paper can be used for reference in the research and development design of high head pump-turbine runner and the optimization calculation of transition.

Study on sensitivity evaluation of conveyance system to unit hydraulic transient characteristics in pumped storage plant

Abstract The minimum pressure of draft tube inlet under the condition of successive load rejection is the key factor restricting the unit hydraulic transient characteristics in pumped storage plant, and the layout of conveyance system is an important factor affecting the unit hydraulic transient characteristics. To explore the influence of the conveyance system layout on the unit hydraulic transient characteristics, the author proposed a new method to evaluate the sensitivity of conveyance system layout to the unit hydraulic transient characteristics. Furthermore, the simulation analysis of two case power stations is used to verify the new method.

Inner Flow Analysis of Kaplan Turbine under Off-Cam Conditions

Kaplan turbines are widely utilized in low-head and large flow power stations. This paper employs Computational Fluid Dynamics (CFD) to complete numerical calculations of the full flow channel under different blade angles and various guide vane openings, based on 25 off-cam experimental working conditions. The internal flow characteristics of the runner blade and draft tube are analyzed, and a discriminant number for quantitatively assessing the flow uniformity of the draft tube is proposed. The results indicate that low-frequency and high-amplitude pressure pulsations occur on the high- and low-pressure edge of the blade when the opening is small, with pulsations decreasing as the opening increases. The inner flow line of the draft tube is disturbed when both the blade angle and opening are small. Additionally, the secondary frequency of the draft tube inlet is double that of the vane passing frequency. The discriminant number of the flow inhomogeneity approaches 0 under optimal flow conditions. The number increases continuously with the decrease in efficiency, and the flow in the three piers of draft tube becomes more nonuniform. The research results provide a reference for enhancing performance and ensuring the operational stability of Kaplan turbines.

Influence of Runner Downstream Structure on the Flow Field in the Draft Tube of a Small-Sized Water Turbine

The flow in the draft tube of the water turbine is affected by the upstream flow and the inherent structure accompanied by various undesirable characteristics, affecting the efficient and stable operation of the water turbine. Changing the flow structure downstream of the runner is an important measure to reduce hydraulic loss in the draft tube and improve stability. In this study, three downstream structures of the runner, namely, the non-locking nut, small locking nut, and extended locking nut are numerically calculated and verified using experimental results. The unstable flow characteristics of the draft tube are analyzed using variations in swirling flow, backflow, pressure gradient, and vortex strip. The results show the non-negligible effect of the locking nut, which significantly reduces the rotational momentum flux at the draft tube inlet, accelerates the decay rate of the swirling flow, and suppresses the generation of axial low pressure. The small locking nut significantly reduces the pressure gradient, shortens the backflow zone, and decreases the backflow velocity. The extended locking nut reduces the backflow zone in some sections and reduces the vortex zone of the straight section but prolongs the backflow zone and increases the backflow velocity.

Numerical Simulation and Analysis of Hydraulic Turbines Based on BIM for Sustainable Development

Hydropower is considered to be an important way to achieve the sustainable development goal of human progress. The performance of turbines is very important to the safety and stability of hydropower stations. Most of the hydraulic turbine performance studies only use Computational Fluid Dynamics (CFD) for performance simulation, lacking the integration of Building Information Modeling (BIM) technology and CFD. Therefore, a performance analysis model of a Francis turbine based on BIM was put forward in this paper. The BIM software OpenBuildings Designer CONNECT Edition Update 10 was used to build the hydraulic turbine model, and then the BIM model was transferred to the CFD numerical simulation platform ANSYS through the intermediate format conversion. In the ANSYS environment, the numerical simulation of different working conditions was carried out with the help of Fluent 2021 R1 software. The numerical simulation results show that the fluid velocity gradient in the volute was 2~3 m/s under the three working conditions, which was relatively stable. The water flow could progress the guide vane mechanism at a higher speed, and the drainage effect of the volute was better. There were some negative pressure areas at the back of the runner blades and the inlet of draft tube, and the negative pressure value was as high as −420,000 Pa and −436,842 Pa under maximum head conditions, which were prone to cavitation erosion. It is proven that BIM supported the hydraulic turbine performance analysis and provided a geometric information model for hydraulic turbine CFD numerical simulation, meaning that the performance analysis model based on BIM is feasible. This study can expand the application value of BIM and provide guidance for the study of hydraulic turbine numerical simulation using BIM technology in combination with CFD methods.

Vortex breakdown in discharge cone of the Francis Turbine

Hydraulic turbines are usually operating at high efficiencies around 90%. It is possible to increase the efficiency by preventing flow characteristics such as failure, cavitation and vortex rope in the draft tube. In some cases, such as partial loads or overloads, pressure pulsations and vortex rope would occur in the draft tube. These undesired events would damage the components of the turbine and that also causes the efficiency to decrease. To eliminate these artifacts, it is decided to design a new component. Vortex Preventing Element, which is designed to eliminate vortex structures and pressure fluctuations, is located at the inlet of draft tube. Computational Fluid Dynamics analyses are performed for different designs having several stage numbers of vortex preventing elements. The preliminary results showed that the one stage vortex preventing element design creates more uniform flow in the draft tube and also increases the efficiency about 3%. Since more studies about the vortex preventing element are in progress, it could be said that the vortex preventing element can handle vortex phenomena in the draft tube and effects the efficiency of the Francis turbines.

Multi-objective control of transient process of hydropower plant with two turbines sharing one penstock under combined operating conditions

This paper studies the multi-objective control of transient process of hydropower plant with two turbines sharing one penstock under combined operating conditions (COCs). Firstly, the model of transient process of hydropower plant is presented. The illustration of four COCs is provided. Then, the multi-objective control strategy of transient process under COCs is designed. The decision variables and objective functions are selected. The control scheme and decision method are determined. Finally, the most favourable superposition time and the most unfavourable superposition time under COCs are determined. The results indicate that the objective function contains three objectives, i.e. spiral case outlet pressures, draft tube inlet pressures and rotational speeds of turbines. The most favourable superposition time under COC-1 can realize that the objectives reach control requirements simultaneously. The most favourable superposition time under COC-2 and the most unfavourable superposition time under COC-1 and COC-2 cannot realize that the objectives reach control requirements simultaneously, but can balance the objectives to the maximum extent. It is unnecessary to distinguish the most favourable superposition time and the most unfavourable superposition time strictly under COC-3. There is no the concepts of the most favourable superposition time and the most unfavourable superposition time under COC-4.

Reconstruction of proper numerical inlet boundary conditions for draft tube flow simulations using machine learning

This paper discusses an innovative strategy to determine appropriate mean and fluctuating inlet boundary conditions for the numerical flow simulations of a hydraulic turbine draft tube. Precise and comprehensive measurements at a draft tube inlet are hard to perform and thus rarely available. Moreover, its complex nature and strong sensitivity to imposed inlet boundary conditions render this flow particularly challenging to predict with numerical methods. The innovative strategy uses machine learning to properly reconstruct the incomplete or unknown inlet boundary conditions from available downstream flow information. Reynolds-Averaged Navier Stokes (RANS) and Large-Eddy Simulations (LES) turbulence modeling methods are used to investigate the draft tube and the numerical results are compared to experimental measurements. Reference simulations are first conducted with standard inlet boundary conditions commonly used in previous numerical works. For RANS, the innovative strategy significantly improves the mean velocity profiles and static pressure evolution inside the draft tube. For LES, its use with an artificial upstream extension allows a proper reconstruction of the upstream turbulent field and a more precise and realistic description of the draft tube head losses.

Multi-objective optimization of guide vane closure scheme in clean pumped-storage power plant with emphasis on pressure fluctuations

In this study, multi-objective optimization was conducted for the guide vane closure scheme during the load-rejection process of a pump turbine, considering pressure fluctuation, maximum rotational speed, maximum pressure at the volute, and minimum pressure at the draft tube inlet to decrease the low-frequency high-amplitude pressure fluctuations at conditions near no-load. Two quantitative indicators, minimum discharge (Qmin) and the ratio of rotational speed to hydraulic head (NDH) of the pump turbine, were proposed to represent the backflow-induced pressure fluctuations. Two objective functions were defined to optimize the guide vane closure scheme and two optimized guide vane closure schemes, OPS-Q (based on Qmin) and OPS-NDH (based on NDH), were developed. The enhancements resulting from these vane closure schemes were verified using a one-dimensional–three-dimensional coupling flow simulation method. NDH was revealed to comprehensively represent backflow-induced pressure fluctuations, whereas Qmin partially represented the backflow. Consequently, OPS-NDH suppressed the pressure fluctuations by appropriately increasing the maximum pressure at the volute and the maximum vacuum at the draft tube inlet within the allowance ranges of the pumped-storage power plant. OPS-Q suppressed the pressure fluctuations; however, the minimum pressure at the draft tube inlet could not be controlled to satisfy the design requirements of the pumped-storage power plant. The findings of this study can be used as a theoretical reference for suppressing the pressure fluctuations in a clean pumped-storage power plant under near-no-load conditions.

Multi-objective co-optimization of the guide vane closure law and rotor inertia in pumped-storage power system

Improving the transition safety and stability of pumped-storage hydropower plants supports the penetration of intermittent renewable energy into the grid. This paper presents the multi-objective optimization of the guide vane closure law and rotor inertia to reduce the rotational speed, water hammer pressure, and draft-tube vacuum by adopting genetic algorithms and a one-dimensional method of characteristic during load rejection transients. To reduce the increase in rotational speed to indirectly suppress pressure pulsations, a co-optimization of the guide vane closure law and rotor inertia is proposed. Results suggest that the above-mentioned co-optimization scheme can significantly reduce the increase in the rotational speed, water hammer pressure, and draft-tube vacuum and suppress the influence of pressure pulsations on the extreme pressure heads at the volute inlet and draft-tube inlet. Additionally, it can significantly reduce the transformation number of operation modes during pump-turbine load rejection transients.

A new start-up law of pump turbines with misaligned guide vane devices

The frequency of high head pump turbines easily fluctuates under the low head no-load condition, which makes it difficult to achieve the synchronous grid connection. In this study, a new start-up strategy of linkage between misaligned guide vane (MGV) devices and conventional guide vanes is proposed, and the optimal arrangement scheme of MGV devices is selected through experiments. Based on the characteristic method, improved Suter transform and the characteristics of MGV devices, the calculation model of transition process is established. The simulated rotational speed of emergency shutdown process fits well with the field measured data. Under start-up conditions, the newly proposed control law can effectively reduce the fluctuation amplitude of frequency, draft tube inlet pressure and volute inlet pressure, and shorten the synchronization time. Additionally, the newly proposed law can significantly improve the no-load stability of the pump turbine under different head conditions, especially low head conditions.

Evolution and influence of pump-turbine cavitation during load rejection transients of a pumped-storage plant

Load rejection is one of the most dangerous transients in pumped-storage plants, during which cavitation in pump-turbines is normally inevitable. In this study, we applied the one-dimensional and three-dimensional coupled computational fluid dynamics method to simulate the 100% load rejection transient process of a middle-head plant, analysed the evolution and influence of cavitation cavities, and evaluated the safety. During the guide-vane closing process, a spiral-shaped cavitation cavity under the runner cone and nine wedge-shaped cavitation cavities inside the blade channel outlets occur successively. Although no water column separation happens in the draft tube inlet, the collapses of the wedge-shaped cavities induce obvious impacts on the runner, causing axial force fluctuations, to which attention should be paid in the plant design phase. This study further validated the feasibility of the coupled method and revealed the mechanism of the dynamic cavitation phenomenon, which provides references for research and engineering design.

Time-frequency domain characteristics analysis of a hydro-turbine governor system considering vortex rope excitation

Understanding and simulating the dynamic characteristics of the hydro-turbine governor system is essential to guarantee the safety of the hydro-power systems. This study proposes a modification method of the water head at the draft tube inlet, which is to discretize the draft tube and introduce the excitation term of the vortex rope. The proposed approach is applied for the modeling of the hydro-turbine governor system. The proposed model (Model I) is verified by means of a comparison with a second model (Model II) of the hydro-turbine governor system model without considering the vortex rope and verified against the experimental data during the load reduction. The model developed is accurate providing errors of the water head, inlet water head and flow of the hydro-turbine for Model Ⅰ are within ±3.3%, ±2% and ±5%, respectively. The proposed model overcomes the limitations of Model Ⅱ in the time-domain and frequency-domain since that only provides the average pressure at the draft tube, which do not reflect the pressure fluctuation and energy fluctuation. Instead the proposed model allows to capture and model the energy fluctuation during the load reduction as shown in the experimental results. The proposed model promotes the development of the hydro-turbine governor system model.

Turbine Test Rig to Investigate Flow Instabilities in Draft Tube

Time-dependent phenomena in a reaction turbine such as rotor-stator interactions (RSI) and rotating vortex rope (RVR) contribute to pressure fluctuations, vibrations, and even failure of the machine. A small scale test rig is being developed to investigate RSI and RVR and to study RVR mitigation methods. The test rig is designed for a maximum available head and discharge of 8 meters and 0.05 m3/s, respectively. Provisions are made to operate the test rig in open and closed loops. The test rig is flexible to operate for the turbines of high specific speed (high head Kaplan turbine or very low head Francis) to low specific speed (high head Francis). The runner of the test rig is 200 mm in diameter at draft tube inlet. The turbine is connected with a variable speed generator to run up to 1000 rpm. This paper presents the design and arrangements of the test rig components according to the head and discharge conditions. Scale down design calculations for a model are performed using IEC 60193.

Study on the influence of tip clearance of tidal tubular turbine on pressure pulsation of draft tube

With the development of clean and renewable energy, especially tidal energy, people pay more and more attention to the performance optimization of tubular turbine. Tip clearance of a tubular turbine affects the flow field at the draft tube inlet, and then affects the overall performance of the turbine. In this paper, a full-passage model is established for a bulb tubular turbine. When the tip clearance is 2mm, the variation of pressure pulsation in the different sections of draft tube and the wall surface in different working conditions are monitored. And the effect of tip clearance on the pressure pulsation in the diffusion section is analyzed. The results show that the pressure pulsation is significantly enhanced near the inlet wall of draft tube under the effect of clearance leakage flow. The main frequency amplitude of pressure pulsation on the draft tube wall decreases along the flow direction, indicating that the effect of tip clearance leakage flow on the draft tube wall decreases gradually. The amplitude of pressure pulsation under deviation condition is much larger than the optimal condition, indicating that the effect of clearance leakage vortex on the wall of diffusion section is stronger under the deviation condition. The research results provide reference for the draft tube design of the tubular turbine and the safe and stable operation of the unit.

Numerical Simulation Study on Hydraulic Transients in Hydropower Station with Trigeminy Surge Tank

A static and dynamic model for hydraulic transient calculation of three-throttled-orifice surge tank was established based on the basic equations of surge tank, by using structure matrix method (matrix analysis of structure?). Control conditions in an engineering practice were calculated by a simplified model and a three-throttled-orifice model respectively. The results indicate that the maximum pressure at the volute end, the maximum rising rate of unit speed and the water level extremum of the surge tank are basically identical by using these two models, but the minimum pressure at draft tube inlet is significantly different. The three-throttled-orifice model was found capable of calculate the flow between throttled orifices at constant state and the pressure behavior at draft tube inlet during the transition accurately. The selection of the resistance factor is discussed in the end, and it is proved that the resistance factor is allowed to have a certain tolerance in the transition calculation, which will not obviously affect the maximum pressure at draft tube inlet and water level extremum of the surge tank. This research can provide reliable reference for the transient calculation of three-throttled-orifice surge tank in similar engineering practice.

Reduced Numerical Modeling of a High Head Francis Turbine Draft Tube at Part Load

In the present study, a reduced model of the Francis-99 model turbine was investigated numerically at part load operating condition. The reduced model consists of a standalone draft tube domain of the Francis-99 model turbine. Numerical studies performed in the past on nearly complete hydro-turbine models (inclusive of the spiral casing, distributor domains, runner, and draft tube) reportedly consist of a large number of computational grids. This may increase the computational costs and data storage required to perform numerical analysis, which could be a setback for future research on new design concepts and optimization study of the draft tube domain. The reduced model was developed by mapping the phase averaged axial, radial, and tangential velocity profiles from the runner exit to the inlet of the standalone draft tube domain. Additionally, turbulent kinetic energy (k) and turbulent eddy dissipation (ε) variables were also considered for better flow prediction inside the draft tube domain. Two methods for mapping inlet boundary conditions were considered in the present study. In the first method, the entire planar profile of the runner-draft tube interface was considered. In the second method, the variables along a radial profile at the runner exit were considered with an axis-symmetric flow assumption over the entire draft tube inlet plane. The numerical results obtained from the Francis-99 reduced model turbine were validated against the numerical model of the NVKS Francis-99 model turbine (with available structured mesh) that was also analysed using the passage flow numerical technique and available experimental results. The results were found to be in reasonable agreement, with each other. The present study could be useful for the future mitigation study of rotating vortex rope by modifying the draft tube domain.

Investigation on mutual traveling influences between the draft tube and upstream components of a Francis turbine unit

Abstract Owing to an inconsistent load demand, hydraulic turbines are often operated under off-load range. Resultant flow instabilities comprise low-frequency pressure fluctuations, which are dependent on the operating condition and unavoidable high-frequency fluctuations evolving from the interactional design concept of hydrofoils. Through the analysis of averaged flow parameters within the runner, this article investigates the mechanism of reciprocity in flow instabilities between draft tube and upstream components. Two recommended operating conditions are used to approximate harmless traveling disturbances, and two part-load discharges are discussed accordingly. The CFD commercial code ANSYS CFX has been used for unsteady flow simulation, where the Shear Stress Transport (SST) turbulence model has been adopted. The used numerical scheme has been validated through experiments in terms of pressure pulsation characteristics, and findings display a good agreement. It has been found that low-frequency disturbances, as instabilities taking source from the fluid structure itself, can override strong flow gradients and exert a higher backward influence farther than high-frequency perturbations. The low-frequency component and its harmonic strengthened by the wall impact feedback are less sensitive to the backward damping. The blade passing frequency dominates the draft tube inlet to influence the local pressure recovery; but it is rapidly diffused and tapped by the low-frequency core instability.