Investigations of Compressible Turbulent Flow in a High-Head Francis Turbine

Trivedi, Chirag

doi:10.1115/1.4037500

Cited by 34 publications

(13 citation statements)

References 51 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…16(a) and 16(b). Equations 7and (8) were used to estimate the synchronous and asynchronous components of the pressure pulsations, respectively. The amplitude of the synchronous component was almost identical to the amplitude observed at locations S1 and S3.…”

Section: -14mentioning

confidence: 99%

“…4 Furthermore, hydraulic turbines experience unstable flow conditions and high amplitude pressure pulsations during the transient cycles. [5][6][7][8][9][10] The pressure amplitudes are primarily dependent on the instantaneous rate of the guide vanes' movements and the runner's speed. 11,12 For the load acceptance cycle, the power output from the hydraulic turbine increases by opening the guide vanes.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Trivedi

Gogstad

Dahlhaug

2017

Journal of Renewable and Sustainable Energy

Self Cite

View full text Add to dashboard Cite

speed. For the start-up cycle, the guide vanes are initially opened by a few percentages from a completely closed position. As the discharge to the runner increases, the runner starts spinning at a synchronous speed to that of the corresponding turbine. The generator then is coupled to the grid network at a minimum load. After synchronization, the power output is further increased to the set value by increasing the discharge to the runner. Unsteady pressure measurements observed during the transient cycles of a Francis turbine model indicated that a turbine could experience more than twice the pressure amplitudes observed at the best efficiency point (BEP). 13-15 Pressure measurements in a draft tube indicated that high amplitude pressure oscillations occur during the load variation and start-stop cycles. The oscillations were nearly three times higher than that of the normal operating conditions. The literature indicated that the repeated transients affect the operating life of the turbine components. 16-19 During a transient condition, the turbine passes through a rapid pressure variation and experiences low-cycle fatigue. 20,21 The repeated fatigue loading to the runner blades initiate cracks where the blades are welded. 22 Pressure pulsations developed in a draft tube are primarily related to the vortex breakdown. The pressure pulsations are composed of two different phenomena that occur simultaneously at the same frequency, which may be synchronous (axial) and asynchronous (radial) types. 23,24 The synchronous component may have equal phase and amplitude in the runner and the draft tube. The pressure may be considered as a plane wave propagating to the hydraulic system through the draft tube. The asynchronous component is a pressure pattern that develops at the runner downstream and that rotates about the circumference of the draft tube. The rotation period is dependent on the circumference and the runner angular speed. The synchronous component may not be present at high load conditions. In a straight/conical draft tube, only asynchronous type pulsations exist. 25 Both synchronous and asynchronous types of pressure pulsations cause different impacts on turbine operation. It is important to analyze and distinguish such pulsations properly. During the steady-state operating condition of a turbine, the pulsations are periodic and may repeat over certain periods of time. 26-29 However, during load variation and start-stop, the pressure pulsations are non-repeating and largely dependent on the runner's instantaneous speed as well as guide vanes' apertures at that time. Recently, velocity measurement conducted by Goyal et al. 30,31 indicated that the vortex structure in the draft tube evolves over a certain time as the runner accelerates/decelerates. The study showed that, unlike a steady-state load, 32-35 amplitudes of both types of pressure pulsations vary with time as the load changes from one operating point to another. The frequency of synchronous pulsations appeared before the asynchronous pulsations during...

show abstract

Section: -14mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Trivedi

Gogstad

Dahlhaug

2017

Journal of Renewable and Sustainable Energy

Self Cite

View full text Add to dashboard Cite

show abstract

“…The LES is expected to be able to answer the research questions associated with the challenging flow conditions present in the turbine blade cascade. Previous work 35,41 showed the benefit of compressible flow simulations over the incompressible flow in the Francis turbine. The flow compressibility accounts for the effect of pressure waves and reflection from the guide vanes and the draft tube similar to the actual turbine.…”

Section: Introductionmentioning

confidence: 99%

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

Trivedi

Dahlhaug

2018

Physics of Fluids

Self Cite

View full text Add to dashboard Cite

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.

show abstract

“…The highest amplitude corresponds to the runner blade passing frequency, and the second highest amplitude is the RVR frequency. These two frequencies were also observed in the calculation of Francis pump-turbines performed by Iliev et al [35]; whereas, only 15.0 times that of the runner rotational frequency was reported in some of the papers by Trivedi et al [36][37][38]. Based on the simulation results in this study, it can be concluded that the pressure wave of RVR is able to travel upstream (from the draft tube domain to the runner domain, and then to vaneless space), although the amplitude of fluctuation decreases, indicating that the oscillating intensity gradually recedes.…”

Section: In Vaneless Spacementioning

confidence: 67%

Investigation of Pressure Fluctuation and Pulsating Hydraulic Axial Thrust in Francis Turbines

et al. 2020

View full text Add to dashboard Cite

Under the circumstances of rapid expansion of diverse forms of volatile and intermittent renewable energy sources, hydropower stations have become increasingly indispensable for improving the quality of energy conversion processes. As a consequence, Francis turbines, one of the most popular options, need to operate under off-design conditions, particularly for partial load operation. In this paper, a prototype Francis turbine was used to investigate the pressure fluctuations and hydraulic axial thrust pulsation under four partial load conditions. The analyses of pressure fluctuations in the vaneless space, runner, and draft tube are discussed in detail. The observed precession frequency of the vortex rope is 0.24 times that of the runner rotational frequency, which is able to travel upstream (from the draft tube to the vaneless space). Frequencies of both 24.0 and 15.0 times that of the runner rotational frequency are detected in the recording points of the runner surface, while the main dominant frequency recorded in the vaneless zone is 15.0 times that of the runner rotational frequency. Apart from unsteady pressure fluctuations, the pulsating property of hydraulic axial thrust is discussed in depth. In conclusion, the pulsation of hydraulic axial thrust is derived from the pressure fluctuations of the runner surface and is more complicated than the pressure fluctuations.

show abstract

Investigations of Compressible Turbulent Flow in a High-Head Francis Turbine

Cited by 34 publications

References 51 publications

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

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

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

Investigation of Pressure Fluctuation and Pulsating Hydraulic Axial Thrust in Francis Turbines

Contact Info

Product

Resources

About