1,720,963 research outputs found
Von Karman Vortex Induced Vibration in a Francis turbine: as reduced numerical model for estimating the shedding frequency.
No full textThe present paper presents a numerical analysis of the von Karman vortex shedding at the blades trailing edge of a recently installed small hydro Francis turbine manifesting very loud and high frequency acoustic pulsation when operating close to the rated discharge.
A reduced single passage numerical model is developed to reduce the computational effort of the simulation while ensuring high accuracy in the assessment of fluid flow.
The potentials of the numerical model are investigated by comparing the frequency spectrum of the experimentally acquired acoustic frequency and the numerical pressure signals allowing to confirm the nature of the vibrations affecting the machine. Thanks to the so demonstrated ability in the evaluation of the fluid flow, the proposed numerical model represents a valid alternative to the traditional correlation-bases approach for the evaluation of the von Karman shedding frequency with a less computational effort compared with a transient simulation of the entire machine
Von Karman Vortex Induced Vibration in a Francis Turbine: a reduced numerical model for estimating the shedding frequency
No full textThe present paper presents a numerical analysis of the von Karman vortex shedding at the blades trailing edge of a recently installed small hydro Francis turbine manifesting very loud and high frequency acoustic pulsation when operating close to the rated discharge. A reduced single passage numerical model is developed to reduce the computational effort of the simulation while ensuring high accuracy in the assessment of fluid flow. The potentials of the numerical model are investigated by comparing the frequency spectrum of the experimentally acquired acoustic frequency and the numerical pressure signals allowing to confirm the nature of the vibrations affecting the machine. Thanks to the so demonstrated ability in the evaluation of the fluid flow, the proposed numerical model represents a valid alternative to the traditional correlation-bases approach for the evaluation of the von Karman shedding frequency with a less computational effort compared with a transient simulation of the entire machine
Three-dimensional evolution of the flow unsteadiness in the S-shape of pump-turbines and its correlation with the runner geometry
No full textPump-turbines (RTP) are the most common mechanical equipment adopted in pumped-hydro power plants and, for grid balancing purposes, are required to sharply switch from pumping to generating mode, and to extend their operative, jeopardizing not only the machine operability but also its life. New design approaches to avoid the onset of unstable behaviours are still far from being defined, and control strategies for accelerating start-up/shut-down procedures are still not effective since these are based on semi-empirical approaches, due to the lack of identification of precursors of the unstable behavior. In this paper, a numerical analysis of the unstable behavior of an RPT during the transition from partial load up to the turbine-brake area was carried out. The fluid-dynamics in different operating points (partial load, run-away condition, turbine brake) was deeply investigated, identifying the rotor-stator mechanisms causing the 3D evolution of the flow field leading to the development of the unstable behavior. Three evolution phases (inception, growth and consolidation) were identified and clearly correlated with the runner geometry and with the S-Shape of the RPT characteristic curve. Customized signal processing strategies were adopted for spectrally characterizing each phase so as to identify potential triggers for new monitoring and control strategies. Moreover, for the first time, a clear fluid-dynamic explanation of the empirical results found in literature on the influence of the runner geometry is provided
Numerical investigation of the interaction between jet and bucket in a Pelton turbine
No full textThis article presents the numerical investigation of the interaction between the jet and the bucket in a Pelton turbine. Unsteady numerical analyses were carried out on a single jet Pelton turbine installed in the north of Italy. A two-phase inhomogeneous model was used.
Two different jet configurations were analysed and compared. In the first configuration, the interaction between the runner and an axial-symmetric jet characterized by a given velocity jet profile was investigated, whereas in the second configuration the runner was coupled with the needle nozzle and the final part of the penstock and the interaction between the jet and the bucket was analysed.
A detailed analysis of the torque highlighted the influence of the shape of the water jet on the turbine losses and the influence of the stator on the efficiency of this type of hydraulic machines was shown. The numerical results were compared with the experimental data derived from the installation test of the turbine in order to validate the numerical analysis
Numerical analyses of a cavitating pelton turbine
No full textErosive wear of hydro turbine runners is a complex phenomenon, which depends upon many parameters and which leads to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. Consistently, the study of this wearing process is an important step to improve the impeller design, to avoid or minimize the rise of extraordinary maintenance. In the present paper the cavitation mechanics of a Pelton turbine was investigated using CFD analyses. A Pelton affected by pitting cavitation was taken as test case. The Pelton geometry was modelled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. The homogeneous approach was used to describe the multiphase flow composed by water, water vapour and air. Numerical results discriminated the vapour production processes during the cut in of the bucket on the water jet. The design and the part load flow rates were analyzed and the cavitation process compared. A simple procedure to identify the locations of higher damage risk was presented and verified on the test case runner
Characterization of the hydrodynamic instabilities in a pump-turbine operating at part load in turbine mode
No full textPump-turbines (RPT) nowadays represents the most common mechanical equipment adopted in the new generation of storage hydro plant. In order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources, RPT are forced to rapidly switch between the pumping and generating mode also extending their operation under off-design conditions in unstable operating areas. Because of the design criterion adopted for the development of a RPT, an unstable behavior represented by a typical S-shaped profile with a positive slope in the machine's characteristic can occur near to the runaway condition.
With the purpose of evaluating the evolution of the fluid field near to the no-load condition, an in-depth CFD analysis of the RPT model test of the Norwegian Hydropower Center is performed by retracing the machine's characteristic curve running through the flow-speed characteristic curve up to the turbine brake region for fixed guide vanes opening. To validate the numerical results, a comparison with the experimental results in terms of characteristic curves and pressure signals is performed.
The results allow to capture the 3D characteristics of the unsteady phenomena, progressively evolving in an organized rotating stall, highlighting also the influence of the flow rate change from partial loads to the turbine brake operation on their development. In order to characterize the pulsating nature of the instability phenomena developing in the runner and in the rotor-stator interaction, a time-frequency analysis is performed on the numeric pressure and torque signals. The combination between fluid-dynamic and time-frequency analysis makes it possible to identify and characterize three evolution phases: inception, growth and consolidation
Numerical Investigation of the Interaction between Jet and Buckets in a Pelton Turbine
No full textThis paper presents the numerical investigation of the interaction between the jet and the bucket in a Pelton Turbine. Unsteady numerical analyses were carried out on a single jet Pelton Turbine installed in the north of Italy. A two-phase inhomogeneous model was used.
Two different jet configurations were analyzed and compared. In the first configuration the interaction between the runner and an axial-symmetric jet characterized by a given velocity jet profile was investigated, whereas in the second configuration the runner was coupled with the needle nozzle and the final part of the penstock and the interaction between the jet and the bucket was analyzed.
A detailed analysis of the torque highlighted the influence of the shape of the water jet on the turbine losses and the influence of the stator on the efficiency of this type of hydraulic machines was shown. The numerical results was compared with the experimental data derived from the installation test of the turbine in order to validate the numerical analysis
Numerical Analyses of Cavitating Flow in a Pelton Turbine
No full textErosion and wear of hydraulic surfaces are frequent problems in hydraulic turbines, which lead to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. These circumstances have negative repercussions on the annual produced power due to the decay of the efficiency, the delivered power, and to the off line periods as result of ordinary and extraordinary hydraulic profiles maintenances. Consistently, the study of this wearing process is an important step to improve the impeller design, and to avoid or minimize the rise of extraordinary maintenance. While mechanical damages are well documented and studied, little information can be found on cavitation in Pelton turbines. In this paper, a CFD model was applied to study the cavitation mechanics on a Pelton turbine. A Pelton runner affected by pitting cavitation was taken as a test case. The bucket geometry was modeled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. Numerical results allowed us to highlight the different vapor productions during the cut-in water jet processes by the bucket. Furthermore, a simple procedure to identify the locations of higher damage risk was presented and verified in the test case runne
Numerical Analyses of a Cavitating Pelton Turbine
No full textErosive wear of hydro turbine runners is a complex phenomenon, which depends upon
many parameters and which leads to a decrease of the performance in time and/or in extreme
cases to the rotor mechanical failure. Consistently, the study of this wearing process is an important
step to improve the impeller design, to avoid or minimize the rise of extraordinary
maintenance.
In the present paper the cavitation mechanics of a Pelton turbine was investigated using
CFD analyses. A Pelton affected by pitting cavitation was taken as test case. The Pelton geometry
was modelled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS)
multiphase analyses. The homogeneous approach was used to describe the multiphase flow
composed by water, water vapour and air.
Numerical results discriminated the vapour production processes during the cut in of the
bucket on the water jet. The design and the part load flow rates were analyzed and the cavitation
process compared.
A simple procedure to identify the locations of higher damage risk was presented and
verified on the test case runner
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