Numerical study on unidirectional fluid–solid coupling of Francis turbine runner

Sediment erosion poses a significant challenge affecting the efficiency and operational lifespan of Francis turbines. This study investigates the erosion characteristics and mechanisms of a Francis turbine with splitter blades under a small guide vane opening. Field measurements of actual sediment gradation were successfully integrated into numerical simulations. A comprehensive assessment of erosion characteristics was conducted for stay/guide vanes, main blades, and splitter blades. The results indicate pronounced erosion within the turbine runner, particularly severe on the suction side of main and splitter blades. Erosion on the stay/guide vanes primarily concentrates in the head region at a comparatively lower rate than that on the blades. Detailed analysis of particle impact velocity, impact angle, and impact number reveals that erosion mechanisms on the runner blades are predominantly governed by cutting pattern, whereas those on stay and guide vanes are primarily impingement pattern. These findings provide a theoretical reference for mitigating sediment erosion on Francis turbines with splitter blades in sediment-laden rivers.

Hydropower plants often work in off-design conditions to regulate the power grid frequency. Frequent transient operation of hydraulic turbines leads to premature failure, fatigue and damage to the turbine components. The speed-no-load (SNL) operating condition is the last part of the start-up cycle and one of the most damaging operation conditions of hydraulic turbines. Hydraulic instabilities and high-stress pressure fluctuations occur due to the low flow rate and unsteady load on the runner blades. Numerical simulations can provide useful insight concerning the complex flow structures that develop inside hydraulic turbines during SNL operation. Together with experimental investigations, the numerical simulations can help diagnose failures and optimize the exploitation of hydraulic turbines. This paper introduces the numerical model of a full-scale 10 MW Kaplan turbine prototype operated at SNL. The geometry was obtained by scaling the geometry of the corresponding model turbine as the model and prototype are geometrically similar. The numerical model is simplified and designed to optimize the numerical precision and computational costs. The guide vane and runner domains are asymmetrical, the epoxy layer applied to two runner blades during the experimental measurements is not modelled and a constant runner blade clearance is employed. The unsteady simulation was performed using the SAS–SST turbulence model. The numerical results were validated with torque and pressure experimental data. The mean quantities obtained from the numerical simulation were in good agreement with the experiment. The mean pressure values were better captured on the pressure side of the runner blade compared to the suction side. However, the amplitude of the pressure fluctuations was more accurately predicted on the suction side of the runner blade. The amplitude of the torque fluctuations was considerably underestimated.

For a Francis turbine, when operating at partial flow rate the fixed-pitch runner shows a strong swirl at the runner outlet which induces a helical vortex (so-called vortex rope) in the draft tube. The precessing vortex rope causes severe pressure fluctuation which effects the steady and secure operating of the turbine. Three-dimensional unsteady turbulent flow simulation with RNG k-ε turbulence model of complete flow passage of a model Francis turbine at partial discharge is performed. To verify the simulation, the model turbine is tested on the test rig at the Harbin Electric Machinery Co., Ltd. (HEC), China. An ideal result of the simulation is obtained. The simulation predicts the shape of the helical vortex successfully in the draft tube cone, and the predicted values of the pressure fluctuation frequencies and amplitudes agree well with the test data. The hydraulic disturbance method is introduced, i.e., injecting water with velocity circulation from the runner cone to reduce the pressure fluctuation in Francis turbine draft tube. The injected water with velocity circulation destroys the forming mechanism of vortex rope and eliminates the pressure fluctuation accordingly. The flow in the turbine with water injection is simulated, and it is indicated that with appropriate flow rate and velocity circulation water injection the pressure fluctuation in the draft tube is reduced effectively.

Sediment induced erosion in exposed components of hydraulic turbines is a major issue in most of the sediment laden hydropower projects. Gradual removal of material can change the components profile which can lead to change in flow characteristics and compromise in the efficiency as well. Among several components in Francis turbine, this study focuses on the runner sidewall gaps to develop a numerical model for erosion prediction. A prototype high head Francis runner with specific speed of 85.4 rpm has been considered as the reference case. A simplified numerical model based on the concept of rotating disc apparatus (RDA) is conceptualized to make analogy with the gap available between runner blades and guide vanes (GVs). Fillet gaps are introduced in the model to emulate the gaps available between runner blade and GV for real case scenario. Four samples with 90° separation of each are mounted on the disc rotating at 750 rpm. Sediment size of 150 μm diameter with particle mass flow rate based on the maximum amount of sediment passing through the reference turbine is used. Numerical calculation is done in ANSYS® with Tabakoff erosion model. Results give an indication of critical zones in the gap region. Erosion pattern, rate density and vorticity are compared with the actual turbine.

Francis turbine is the most widely used model in hydropower station, and the hydraulic design of runner blades affects the efficiency of the unit. This paper discusses how to complete the hydraulic design of Francis turbine runner blades according to the given parameters. In hydraulic design of Francis turbine runner, firstly, the structural scheme is determined according to the given parameters. On the basis of referring to excellent hydraulic model, a Francis turbine runner blade is designed according to the given head. By analyzing and calculating the data, the geometric parameters and axial projection of the Francis turbine runner flow channel are determined, and the axial flow network is drawn and checked, so as to complete the calculation of axial vortex line and blade drawing. Finally, the airfoil is thickened by a certain thickening principle.

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

The variable demand on the energy market requires great flexibility in operating hydraulic turbines. Therefore, turbines are frequently operated over an extended range of regimes. Francis turbines operating at partial load present pressure fluctuations due to the vortex rope in the draft tube cone. This phenomenon generates strong vibrations and noise that may produce failures on the mechanical elements of the machine. This paper presents the failure analysis of a broken Francis turbine runner blade. The failure appeared some months after the welding repair work realized in situ on fatigue cracks initiated near to the trailing edge at the junction with the crown, where stress concentration occurs. In order to determine the causes that led to the fracture of the runner blade, the metallographic investigations on a sample obtained from the blade is carried out. The metallographic investigations included macroscopic and microscopic examinations, both performed with light and scanning electron microscopy, as well as EDX – analyses. These investigations led to the conclusion, that the cracking of the blade was caused by fatigue, initiated by the surface unevenness of the welding seam. The failure was accelerated by the hydrogen embrittlement of the filling material, which appeared as a consequence of improper welding conditions. In addition to the metallographic investigations, numerical computations with finite element analysis are performed in order to evaluate the deformation and stress distribution on blade.

Now days drilling of fiber reinforced plastics become a challenging task in terms of obtaining defect free holes. Quality of drilling mainly depends on cutting parameters such as spindle speed, feed rate, diameter of the drill etc. Recently Finite element analysis (FEA) is found to be the most reliable method to investigate the effect of such parameters on quality of hole in drilling of composites. Present study focuses on the drilling of Epoxy resin with and without carbon fiber reinforced with 6% and 10% volume of Si3N4 composites as per the Taguchi’s L27 standard orthogonal array using HSS tool. FEA is carried out for each combination of cutting parameters given by taguchi’s table. Simulation results such as stress distribution, strain and deformation in the material are correlated with the with the drilled hole performance characteristics like Cylindricity, circularity, delamination and surface roughness. Simulation results depict the influence of thrust force and torque on the degree of deformation, strength, elasticity and stress distribution in the work material during drilling. From the simulation, it is found that deformation, and equivalent stress in the material decreases with increase in the % of Si3N4 reinforcement resulting in minimum surface roughness. The effect of feed rate is more on the value of thrust force resulting in maximum principal stress, equivalent stress and directional deformation in the material, which leads to delamination at the entry and exit of the hole. Further, influence of spindle speed is more on the value of torque resulting in maximum principal stress, equivalent stress and directional deformation in the material, which leads to maximum value of Cylindricity.

This work presents an experimental study about the application of acoustic emission (AE) techniques in the monitoring of cavitation erosion mass loss in small Francis turbines. A vertical Francis turbine test bench is specially devised to perform some experiments designed to evaluate the influence of small surface mass losses on turbine blades in the acoustic emission signals. An AE wideband transducer is employed in the test bench instrumentation system. In order to evaluate the AE levels associated with the turbine erosion stages, a small defect is introduced into the turbine runner. This defect is intended to simulate a small mass loss in the turbine runner. The measurements of the AE signals are performed in the Turbine Francis model at two situations: 1) turbine without defect, which means that the runner blades are free of apparent geometric imperfections; 2) turbine with defect, which is represented by a small hole drilled into a runner blade. The AE transducer is installed on the turbine draft tube and the AE measurements are performed at several operating conditions. The preliminary results obtained for the AE amplitude in this investigation show that the small defect introduced into a runner blade causes variations in the AE levels measured in the experiments, confirming that there is a large potential for the application of AE monitoring techniques in the accurate evaluation of cavitation wear on hydraulic turbines in field.

The importance of renewable energy sources for the electrical power supply has grown rapidly in the past decades. Their often unpredictable nature however poses a threat to the stability of the existing electric grid. Hydroelectric powerplants play an important role in regulating the integration of renewable energy sources into the network by supplying on-demand load balancing as well as primary and secondary power network control. Therefore, the operating ranges of hydraulic machines has to be continuously extended, which potentially produces undesirable flow phenomena involving cavitation. An example is the formation of a gaseous volume in the swirling flow leaving a Francis turbine runner at off-design operating conditions. At high load, this so called vortex rope is shaped axisymmetrically and may enter a self-excited oscillation, measurable through significant fluctuations of the pressure throughout the system and the mechanical torque transferred to the generator. The main objective of the present work is the identification of the physical mechanisms governing this self-sustained, unstable behavior by measurement. Furthermore, the key parameters of numerical approaches using one-dimensional hydroacoustic flowmodels or CFD require experimental validation. For this purpose, the measurements provide a comprehensive data base of various flow and system parameters at varying operating conditions. Two test cases are studied, a small scale hydraulic circuit with a micro-turbine as well as a reduced scale physical model of an existing Francis turbine. On the first test case, the study of the flow rate fluctuations up- and downstream of the oscillating vortex rope in the draft tube, together with the volume of the cavity, revealed the destabilizing effect of the flow swirl in the draft tube inlet. The second test case accurately simulates the behavior of an actual hydraulic power plant. Investigations range from a local study of the flow field in the draft tube cone bymeans of LDV, PIV, high speed visualization and wall pressure measurements to a global analysis, considering the response of the hydraulic and mechanical system to the excitation by the vortex rope oscillation. Among the main observations is a periodical variation of the flow swirl in the draft tube, synchronized with the pressure oscillations. This is likely to be caused by a cyclically appearing volume of cavitation on the runner blades, modifying the relative flow angle at the outlet. The interaction of the blade cavitation and the vortex rope oscillation via the flow swirl is found to play a crucial role in the occurrence of self-excited pressure oscillations in Francis turbines.

Francis turbines are subject to various types of cavitation flow depending on the operating condition. To enable a smooth integration of the renewable energy sources, hydraulic machines are now increasingly required to extend their operating range, especially down to extremely low discharge conditions called deep part load operation. The inter-blade cavitation vortex is a typical cavitation phenomenon observed at deep part load operation. However, its dynamic characteristics are insufficiently understood today. In an objective of revealing its characteristics, the present study introduces a novel visualization technique with instrumented guide vanes embedding the visualization devices, providing unprecedented views on the inter-blade cavitation vortex. The binary image processing technique enables the successful evaluation of the inter-blade cavitation vortex in the images. As a result, it is shown that the probability of the inter-blade cavitation development is significantly high close to the runner hub. Furthermore, the mean vortex line is calculated and the vortex region is estimated in the three-dimensional domain for the comparison with numerical simulation results. In addition, the on-board pressure measurements on a runner blade is conducted, and the influence of the inter-blade vortex on the pressure field is investigated. The analysis suggests that the presence of the inter-blade vortex can magnify the amplitude of pressure fluctuations especially on the blade suction side. Furthermore, the wall pressure difference between pressure and suction sides of the blade features partially low or negative values near the hub at the discharge region where the inter-blade vortex develops. This negative pressure difference on the blade wall suggests the development of a backflow region caused by the flow separation near the hub, which is closely related to the development of the inter-blade vortex. The development of the backflow region is confirmed by the numerical simulation, and the physical mechanisms of the inter-blade vortex development is, furthermore, discussed.

Along with recent advancements in novel materials and manufacturing processes, the interest in morphing wings has increased in order to further improve the aerodynamic performance of flying bodies. The morphing wing can be tailored to deliver superior performance, compared to its non-morphing counterparts, for multiple operating conditions and in varying flows. In particular, the morphing wing is implemented for drag reduction and lift enhancement, and hence, the maneuverability, adaptability, and capability of the morphing wing can encompass an even wider spectrum by changing the wing shape. In this research, an existing morphing UAV wing design, Spanwise Morphing Trailing Edge (SMTE), actuated by bending Macro Fiber Composites (MFCs), is considered to generate the spanwise sinusoidal variations on the trailing edge of the morphing wing. A comparative aerodynamic study of the morphing wing by varying the spatial frequency (i.e., number of waves along the span) and the phase shift (i.e., wave shape along the span) at different angles of attack is conducted through analytical approaches and numerical Computational Fluid Dynamic (CFD) simulations, which are validated with previous experimental measurements. The analytical approach uses the three-dimensional (3D) Prandtl lifting line theory, and the CFD modeling in turbulence flow solves the 3D Reynolds-Averaged Navier-Stokes (RANS) equations with the k-ω Shear Stress Transport (SST) turbulence model. Note that the numerical simulations of a morphing wing focus on the pre-stall condition to estimate the aerodynamic performance. This work extends a prior study about a nominal flight condition testing a morphing wing at multiple flight conditions to evaluate multi-point 1 performance. The results show that there are governing aerodynamic efficiency zones of the lift-to-drag ratio, endurance, and aircraft range within a zone of angles of attack. Therefore, the morphing wing is shown to have a good aerodynamic performance as compared to the non-morphing wing.

In this study, the performance of a pump-jet propulsor (PJP) with pre-swirl stator in open water is numerically investigated. Both full-scale and model-scale configurations are considered. The Reynolds-averaged Navier–Stokes equations and shear stress transport k−ω turbulence model are used in the numerical calculation. The computational domain is discretized using structured grids, and a rotating grid is affixed to the rotor to deal with the relative motion between the rotor and stationary components. The mesh quality is determined based on a grid uncertainty analysis. The numerical method is validated using model-scale experimental data. The simulation results reveal the influences of the scale size on the hydrodynamic performance and the distributions of the velocity, pressure and vorticity under three advance coefficients. With the increase in the advance coefficients, the scale influences on the efficiency become more obvious, and the efficiency of the full-scale PJP is always higher than that of the model-scale PJP. The full-scale configuration is found with a more significant instability in the gap vortex development, because it presents larger interaction between tip leakage vortex (TLV) and the inner wall of the duct. As the main velocity increases, the TLV shedding is delayed. Finally, the development process of gap vortices is analyzed for the difference operation conditions.

The influence of micro air jets on mixing augmentation of fuel in cavity flameholder at supersonic flow

In this study, the flow behaviors and throttle effects of high-pressure air flow through orifices were experimentally investigated in a pipe system and numerically simulated using Reynolds-averaged Navier–Stokes equations with the shear stress transport (SST) k−ω turbulence model. Three orifice configurations with porosities (orifice hole area to pipe area ratio) of β = 28.4%, 7.1%, and 1.8% were comparatively evaluated under four different inflow pressures, i.e., pin= 1, 2, 3, and 4 bar. The numerical results agreed well with the corresponding experimental measurements. A primary recirculation with a smaller secondary recirculation region was formed immediately behind the trailing face. In the presence of supersonic flow, the streamwise velocity exhibited periodic oscillations, increasing at the expansion wave and decreasing at the shock wave region. The throttle effects were highly sensitive to the orifice porosities and the inflow conditions. In the β = 28.4% configuration, shock waves and expansion waves were evident under different inflow pressures and alternately dominated the downstream flow, leading to periodic streamwise temperature variations, with the minimum temperature reaching −160 °C at pin = 4 bar. For the orifice with β = 7.1%, the expansion and shock waves appeared at pin≥ 2 bar, and continuously expanded with intensified flow fluctuations at pin = 3 and 4 bar, with the minimum temperature reaching −120 °C. For the orifice with β = 1.8%, the impact of shock waves on the temperature field was barely observable under pin = 1 and 2 bar due to the small porosity. When pin≥ 3 bar, similar temperature patterns with larger-porosity orifices were observed downstream of the orifice trailing face but were smaller in size.