Chirag Trivedi

Effect of transients on Francis turbine runner life: a review

The present electricity market and the injection of power generated using intermittent energy sources have brought instability in the operation of the power grid. This has resulted in frequent load variations, emergency shut-down and restart, total load rejections, and off-design operation of grid connected hydraulic turbines. The present paper reviews the available literature summarizing the effects of transients on Francis turbine investigated experimentally, numerically, and analytically. Transients create both steady and unsteady pressure loading on the runner blade, resulting in cyclic stresses and fatigue development in the runner. These effects shorten the runner life, increase cost of plant operation, and loss of power generation. The reviewed literature has shown that one start–stop cycle can shorten predefined refurbishment time up to 15 hours. Turbine start–stop cannot be avoided, but runner life may be improved by minimizing the unfavourable pressure loading on the blades during transients through strategic movement of guide vanes.

Transient Pressure Measurements on a High Head Model Francis Turbine During Emergency Shutdown, Total Load Rejection, and Runaway

The penetration of intermittent wind and solar power to the grid network above manageable limits disrupts electrical power grids. Consequently, hydraulic turbines synchronized to the grid experience total load rejection and are forced to shut down immediately. The turbine runner accelerates to runaway speeds in a few seconds, inducing high-amplitude, unsteady pressure loading on the blades. This sometimes results in a failure of the turbine components. Moreover, the unsteady pressure loading significantly affects the operating life of the turbine runner. Transient measurements were carried out on a scale model of a Francis turbine prototype (specific speed = 0.27) during an emergency shutdown with a transition into total load rejection. A detailed analysis of variables such as the head, discharge, pressure at different locations including the runner blades, shaft torque, and the guide vane angular movements are performed. The maximum amplitudes of the unsteady pressure fluctuations in the turbine were observed under a runaway condition. The amplitudes were 2.1 and 2.6 times that of the pressure loading at the best efficiency point in the vaneless space and runner, respectively. Such high-amplitude, unsteady pressure pulsations can affect the operating life of the turbine.

Experimental investigations of transient pressure variations in a high head model Francis turbine during start-up and shutdown

Penetration of the power generated using wind and solar energy to electrical grid network causing several incidents of the grid tripping, power outage, and frequency drooping. This has increased restart (star-stop) cycles of the hydroelectric turbines significantly since grid connected hydroelectric turbines are widely used to manage critical conditions of the grid. Each cycle induces significant stresses due to unsteady pressure loading on the runner blades. The presented work investigates the pressure loading to a high head (HP = 377 DP = 1.78 m) Francis turbine during start-stop. The measurements were carried out on a scaled model turbine (HM = 12.5 DM = 0.349 m). Total four operating points were considered. At each operating point, three schemes of guide vanes opening and three schemes of guide vanes closing were investigated. The results show that total head variation is up to 9% during start-stop of the turbine. On the runner blade, the maximum pressure amplitudes are about 14 kPa and 16 kPa from the instantaneous mean value of 121 kPa during rapid start-up and shutdown, respectively, which are about 1.5 times larger than that of the slow start-up and shutdown. Moreover, the maximum pressure fluctuations are given at the blade trailing edge.

Experimental Investigation of a High Head Francis Turbine During Spin-No-Load Operation

Water passes freely through a hydraulic turbine in the absence of power requirements or during maintenance of the transmission lines, spillways, or dam. Moreover, the turbine operates under no-load ...

Design and development of guide vane cascade for a low speed number Francis turbine

Guide vane cascade of a low speed number Francis turbine is developed for the experimental investigations. The test setup is able to produce similar velocity distributions at the runner inlet as that of a reference prototype turbine. Standard analytical methods are used to design the reference turbine. Periodic walls of flow channel between guide vanes are identified as the starting profile for the boundary of the cascade. Two alternative designs with three guide vanes and two guide vanes, without runner, are studied. A new approach, for the hydraulic design and optimization of the cascade test setup layout, is proposed and investigated in details. CFD based optimization methods are used to define the final layout of the test setup. The optimum design is developed as a test setup and experimental validation is done with PIV methods. The optimized design of cascade with one guide vane between two flow channels is found to produce similar flow conditions to that in the runner inlet of a low speed number Francis turbine.

Investigation of the unsteady pressure pulsations in the prototype Francis turbines during load variation and startup

This work investigates the unsteady pressure fluctuations in two prototype Francis turbines during load variation and start-up. Although hydraulic turbines are expected to experience such events over their lifetime, the resulting pressure amplitudes are so significant that they take a toll on a machines operating life. The interest of the present study is to experimentally measure and numerically characterize time-dependent pressure pulsations. Specific focus is on (1) how pressure pulsations of both synchronous and asynchronous types in vertical- and horizontal-axis turbines change when the load of a turbine changes from steady conditions, (2) what the pressure amplitudes during load change are, and (3) how quickly pressure amplitudes vary when a generator is synchronized to the power grid (load) during start-up. To this end, four pressure sensors were integrated in the draft tube cone. The results are quite interesting, especially during transition from the steady state to the transient load change. In...

Numerical Study of Aerodynamic Characteristics of a Symmetric NACA Section with Simulated Ice Shapes

To develop a numerical model of icing on wind turbine blades, a CFD simulation was conducted to investigate the effect of critical ice accretions on the aerodynamic characteristics of a 0.610 m chord NACA 0011 airfoil section. Aerodynamic performance coefficients and pressure profile were calculated and compared with the available measurements for a chord Reynolds number of 1.83x106. Ice shapes were simulated with flat plates (spoiler-ice) extending along the span of the wing. Lift, drag, and pressure coefficients were calculated in zero angle of attack through the steady state and transient simulations. Different approaches of numerical studies have been applied to investigate the icing conditions on the blades. The simulated separated flow over the sharp spoilers is challenging and can be seen as a worst test case for validation. It allows determining a reliable strategy to simulate real ice shapes [1] for which the detailed validation cannot easily be provided.

Interaction between trailing edge wake and vortex rings in a Francis turbine at runaway condition: Compressible large eddy simulation

The present study aims to investigate the unsteady flow phenomenon that produces high energy stochastic fluctuations in a highly skewed blade cascade. A complex structure such as a turbine is operated at runaway speed, where the circumferential velocity is dangerously high, and the energy dissipation is so significant that it takes a toll on the operating life of a machine. Previous studies showed that a large vortical structure changes the spatial location very quickly and interacts with the secondary flow attached to the blade pressure-side. The temporal inception of the rings dissipates the energy of a wide frequency band and induces heavy vibration in the mechanical structure. The focus of the present study is to experimentally measure and numerically characterize the time-dependent inception of vortex rings in the blade cascade. The experimental data are used to verify and validate the numerical results obtained from the large eddy simulation. Flow compressibility is considered to obtain more accurate amplitudes of unsteady pressure pulsations associated with the wave propagation and reflection. The following three aspects are of particular focus: (1) How the wake from a guide vane interacts with the stagnation point of a blade, (2) how vortex rings are developed in a blade cascade, and what are the temporal characteristics, and (3) how the decelerating flow at the runner outlet interacts with the secondary flow in the draft tube.The present study aims to investigate the unsteady flow phenomenon that produces high energy stochastic fluctuations in a highly skewed blade cascade. A complex structure such as a turbine is operated at runaway speed, where the circumferential velocity is dangerously high, and the energy dissipation is so significant that it takes a toll on the operating life of a machine. Previous studies showed that a large vortical structure changes the spatial location very quickly and interacts with the secondary flow attached to the blade pressure-side. The temporal inception of the rings dissipates the energy of a wide frequency band and induces heavy vibration in the mechanical structure. The focus of the present study is to experimentally measure and numerically characterize the time-dependent inception of vortex rings in the blade cascade. The experimental data are used to verify and validate the numerical results obtained from the large eddy simulation. Flow compressibility is considered to obtain more accurat...

Simplified hydrodynamic analysis on the general shape of the hill charts of Francis turbines using shroud-streamline modeling

The paper presents a simplified one-dimensional calculation of the efficiency hill-chart for Francis turbines, based on the velocity triangles at the inlet and outlet of the runners blade. Calculation is done for one streamline, namely the shroud streamline in the meridional section, where an efficiency model is established and iteratively approximated in order to satisfy the Euler equation for turbomachines at a wide operating range around the best efficiency point (BEP). Using the presented method, hill charts are calculated for one splitter-bladed Francis turbine runner and one Reversible Pump-Turbine (RPT) runner operated in the turbine mode. Both turbines have similar and relatively low specific speeds of nsQ = 23.3 and nsQ = 27, equal inlet and outlet diameters and are designed to fit in the same turbine rig for laboratory measurements (i.e. spiral casing and draft tube are the same). Calculated hill charts are compared against performance data obtained experimentally from model tests according to IEC standards for both turbines. Good agreement between theoretical and experimental results is observed when comparing the shapes of the efficiency contours in the hill-charts. The simplified analysis identifies the design parameters that defines the general shape and inclination of the turbines hill charts and, with some additional improvements in the loss models used, it can be used for quick assessment of the performance at off-design conditions during the design process of hydraulic turbines.

Time-Dependent Effects of Glaze Ice on the Aerodynamic Characteristics of an Airfoil

The main objective of this study is to estimate the dynamic loads acting over a glaze-iced airfoil. This work studies the performance of unsteady Reynolds-averaged Navier-Stokes (URANS) simulations in predicting the oscillations over an iced airfoil. The structure and size of time-averaged vortices are compared to measurements. Furthermore, the accuracy of a two-equation eddy viscosity turbulence model, the shear stress transport (SST) model, is investigated in the case of the dynamic load analysis over a glaze-iced airfoil. The computational fluid dynamic analysis was conducted to investigate the effect of critical ice accretions on a 0.610 m chord NACA 0011 airfoil. Leading edge glaze ice accretion was simulated with flat plates (spoiler-ice) extending along the span of the blade. Aerodynamic performance coefficients and pressure profiles were calculated and validated for the Reynolds number of 1.83 × 106. Furthermore, turbulent separation bubbles were studied. The numerical results confirm both time-dependent phenomena observed in previous similar measurements: (1) low-frequency mode, with a Strouhal number –0.02, and (2) higher frequency mode with a Strouhal number –0.69. The higher frequency motion has the same characteristics as the shedding mode and the lower frequency motion has the flapping mode characteristics.