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Trivedi Chirag - One of the best experts on this subject based on the ideXlab platform.
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Interaction between trailing edge wake and vortex rings in a Francis turbine at Runaway condition: Compressible large eddy simulation
'AIP Publishing', 2018Co-Authors: Trivedi Chirag, Dahlhaug, Ole GunnarAbstract: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 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 vortical 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 decelerating flow at the outlet interacts with the secondary flow in the blade cascade
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Investigation and validation of a Francis turbine at Runaway operating conditions
'MDPI AG', 2016Co-Authors: Trivedi Chirag, Cervantes Michel, B. K. GandhiAbstract:Hydraulic turbines exhibit total load rejection during operation because of high fluctuations in the grid parameters. The generator reaches no-load instantly. Consequently, the turbine runner accelerates to high Speed, Runaway Speed, in seconds. Under common conditions, stable Runaway is only reached if after a load rejection, the control and protection mechanisms both fail and the guide vanes cannot be closed. The runner life is affected by the high amplitude pressure loading at the Runaway Speed. A model Francis turbine was used to investigate the consequences at the Runaway condition. Measurements and simulations were performed at three operating points. The numerical simulations were performed using standard k-", k-! shear stress transport (SST) and scale-adaptive simulation (SAS) models. A total of 12.8 million hexahedral mesh elements were created in the complete turbine, from the spiral casing inlet to the draft tube outlet. The experimental and numerical analysis showed that the runner was subjected to an unsteady pressure loading up to three-times the pressure loading observed at the best efficiency point. Investigates of unsteady pressure pulsations at the vaneless space, runner and draft tube are discussed in the paper. Further, unsteady swirling flow in the blade passages was observed that was rotating at a frequency of 4.8-times the Runaway runner angular Speed. Apart from the unsteady pressure loading, the development pattern of the swirling flow in the runner is discussed in the paper
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Investigations of Transient Pressure Loading on a High Head Francis Turbine
2015Co-Authors: Trivedi ChiragAbstract:This doctoral thesis includes six peer-reviewed publications. Beginning with an extensive literature review, this work discusses the study of turbine performance, investigations of the unsteady pressure loading during transient operations, and consequences at Runaway Speed. The liberalization of the electricity market and the introduction of intermittent power have changed the operating trends for hydraulic turbines. The first publication discusses how the operating trends have changed since 1981. The turbines are subjected to an increased number of transient operations, i.e., load variations, start-stop, and total load rejection. As a consequence, a turbine experiences high amplitude unsteady pressure pulsations during the transients, which significantly affect the runner life. Surprisingly, no investigations specifically on the unsteady pressure loading during the transients have been reported in the literature. The main objective of the current work was to investigate the unsteady pressure loading in a high head hydraulic turbine of the Francis type during transient operations. The test facility and a model Francis turbine available at NTNU were used for these investigations. The turbine was a scale (1:5.1) model of the prototype (head = 377 m, diameter = 1.78 m, power = 110 MW) operating at the Tokke power plant in Norway. The observed head for the model turbine was 12 m at the best efficiency operating point (BEP). The measurements were divided into two categories: (1) steady state and (2) transient. The turbine steady-state performance characteristics and a numerical study are discussed in the second publication. A constant efficiency hill diagram was prepared over the operating range, and the maximum hydraulic efficiency of 93.4% was observed at the BEP. Investigations on transient operation during load variation are discussed in the third publication. Analysis of the acquired pressure data revealed that the blades are subjected to high amplitude unsteady pressure loading. The amplitudes of the pressure pulsations are higher during load rejection than at load acceptance. The fourth publication discusses investigations of the turbine startup and shutdown processes. Two schemes, i.e., rapid and slow, were selected for the guide vane movements. It was observed that the rate of guide vane movement significantly affects the instantaneous amplitude of the pressure fluctuations. The fifth publication discusses the investigations carried out during emergency shutdown with a transition into total load rejection. Total load rejection is one of the most damaging transient conditions. The amplitudes of unsteady pressure pulsations were significant when the runner accelerated to the Runaway Speed. Separate measurements at Runaway conditions were carried out to investigate the consequences in detail. Investigations of the pressure loading during the steady-state Runaway Speed are discussed in the sixth publication. The measurements indicated that the blades experience high amplitude pressure pulsations that are more than two times the amplitudes observed for the BEP load. Thus, a high head hydraulic turbine is subjected to high amplitude pressure pulsations during the transients. The unsteady pressure loading can cause fatigue to the blades and affect the runner operating life. The current investigations may contribute to improved runner design to a certain extent. Ideally, the runner should be able to withstand extreme loading without much effect on the operating life and produce increases in the power grid reliability. The current work has been used to initiate a series of workshops, i.e., Francis-99 (www.francis-99.org), beginning in December 2014. The workshops will be jointly organized by LTU and NTNU. During the coming years, the Francis-99 workshops aim to determine the state-of-the-art of high head Francis turbine simulations (flow and structure) under steady and transient operating conditions and promote their development and open knowledge dissemination. The investigations presented in this thesis will be further explored in these workshops. The steady-state measurement data will be discussed in the first workshop scheduled for December 2014.Godkänd; 2014; 20140901 (chitri); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Chiragkumar Hasmukhlal Trivedi Ämne: Strömningslära/Fluid Mechanics Avhandling: Investigations of Transient Pressure Loading on a High Head Francis Turbine Opponent: Associate Professor Gabriel Ciocan, Université Laval, Québeck, Canada/project manager ALSTOM Power Hydro, Grenoble Cedex, France Ordförande: Professor Michel Cervantes, Avd för strömningslära och experimentell mekanik, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag 27 mars kl 09.30 Plats: E246, Luleå tekniska universite
Thomas Fluri - One of the best experts on this subject based on the ideXlab platform.
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maximum fluid power condition in solar chimney power plants an analytical approach
Solar Energy, 2006Co-Authors: Theodor W Von Backstrom, Thomas FluriAbstract:Abstract Main features of a solar chimney power plant are a circular greenhouse type collector and a tall chimney at its centre. Air flowing radially inwards under the collector roof heats up and enters the chimney after passing through a turbo-generator. The objective of the study was to investigate analytically the validity and applicability of the assumption that, for maximum fluid power, the optimum ratio of turbine pressure drop to pressure potential (available system pressure difference) is 2/3. An initial power law model assumes that pressure potential is proportional to volume flow to the power m, where m is typically a negative number between 0 and −1, and that the system pressure drop is proportional to the power n, where typically n = 2. The analysis shows that the optimum turbine pressure drop as fraction of the pressure potential is (n − m)/(n + 1), which is equal to 2/3 only when m = 0, implying a constant pressure potential, independent of flow rate. Consideration of a basic collector model proposed by Schlaich leads to the conclusion that the value of m is equal to the negative of the collector floor-to-exit efficiency. A more comprehensive optimization scheme, incorporating the basic collector model of Schlaich in the analysis, shows that the power law approach is sound and conservative. It is shown that the constant pressure potential assumption (m = 0) may lead to appreciable underestimation of the performance of a solar chimney power plant, when compared to the analyses presented in the paper. More important is that both these analyses predict that maximum fluid power is available at much lower flow rate and much higher turbine pressure drop than predicted by the constant pressure potential assumption. Thus, the constant pressure potential assumption may lead to overestimating the size of the flow passages in the plant, and designing a turbine with inadequate stall margin and excessive Runaway Speed margin. The derived equations may be useful in the initial estimation of plant performance, in plant performance analysis and in control algorithm design. The analyses may also serve to set up test cases for more comprehensive plant models.
Xiaoyi Chen - One of the best experts on this subject based on the ideXlab platform.
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investigation on water hammer control of centrifugal pumps in water supply pipeline systems
Energies, 2018Co-Authors: Wuyi Wan, Boran Zhang, Xiaoyi ChenAbstract:Water hammer control in water supply pipeline systems is significant for protecting pipelines from damage. The goal of this research is to investigate the effects of pumps moment of inertia design on pipeline water hammer control. Based on the method of characteristics (MOC), a numerical model is established and plenty of simulations are conducted. Through numerical analysis, it is found that increasing the pumps moment of inertia has positive effects both on water hammer control as well as preventing pumps rapid Runaway Speed. Considering the extra cost of space, starting energy, and materials, an evaluation methodology of efficiency on the increasing moment of inertia is proposed. It can be regarded as a reference for engineers to design the moment of inertia of pumps in water supply pipeline systems. Combined with the optimized operations of the valve behind the pumps, the pipeline systems can be better protected from accident events.
Romeo Susanresiga - One of the best experts on this subject based on the ideXlab platform.
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velocity and pressure fluctuations induced by the precessing helical vortex in a conical diffuser
27th IAHR Symposium on Hydraulic Machinery and Systems IAHR 2014; Hotel Omni Mont-RoyalMontreal; Canada; 22 September 2014 through 26 September 2014, 2014Co-Authors: Ardalan Javadi, A I Bosioc, Hakan Nilsson, Sebastian Muntean, Romeo SusanresigaAbstract:The flow unsteadiness generated in the draft tube cone of hydraulic turbines affects the turbine operation. Therefore, several swirling flow configurations are investigated using a swirling apparatus in order to explore the unsteady phenomena. The swirl apparatus has two parts: the swirl generator and the test section. The swirl generator includes two blade rows being designed such that the exit velocity profile resembles that of a turbine with fixed pitch. The test section includes a divergent part similar to the draft tube cone of a Francis turbine. A new control method based on a magneto rheological brake is used in order to produce several swirling flow configurations. As a result, the investigations are performed for six operating regimes in order to quantify the flow from part load operation, corresponding to Runaway Speed, to overload operation, corresponding to minimum Speed, at constant guide vane opening. The part load operation corresponds to 0.7 times the best efficiency discharge, while the overload operation corresponds to 1.54 times the best efficiency discharge. LDV measurements are performed along three survey axes in the test section. The first survey axis is located just downstream the runner in order to check the velocity field at the swirl generator exit, while the next two survey axes are located at the inlet and at the outlet of the draft tube cone. Two velocity components are simultaneously measured on each survey axis. The measured unsteady velocity components are used to validate the results of unsteady numerical simulations, conducted using the OpenFOAM CFD code. The computational domain covers the entire swirling apparatus, including strouts, guide vanes, runner, and the conical diffuser. A dynamic mesh is used together with sliding GGI interfaces to include the effect of the rotating runner. The Reynolds averaged Navier–Stokes equations coupled with the RNG k–e turbulence model are utilized to simulate the unsteady turbulent flow throughout the swirl generator.
B. K. Gandhi - One of the best experts on this subject based on the ideXlab platform.
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Investigation of a High Head Francis Turbine at Runaway Operating Conditions
MDPI AG, 2016Co-Authors: Chirag Trivedi, Michel J. Cervantes, B. K. GandhiAbstract:Hydraulic turbines exhibit total load rejection during operation because of high fluctuations in the grid parameters. The generator reaches no-load instantly. Consequently, the turbine runner accelerates to high Speed, Runaway Speed, in seconds. Under common conditions, stable Runaway is only reached if after a load rejection, the control and protection mechanisms both fail and the guide vanes cannot be closed. The runner life is affected by the high amplitude pressure loading at the Runaway Speed. A model Francis turbine was used to investigate the consequences at the Runaway condition. Measurements and simulations were performed at three operating points. The numerical simulations were performed using standard k-ε, k-ω shear stress transport (SST) and scale-adaptive simulation (SAS) models. A total of 12.8 million hexahedral mesh elements were created in the complete turbine, from the spiral casing inlet to the draft tube outlet. The experimental and numerical analysis showed that the runner was subjected to an unsteady pressure loading up to three-times the pressure loading observed at the best efficiency point. Investigates of unsteady pressure pulsations at the vaneless space, runner and draft tube are discussed in the paper. Further, unsteady swirling flow in the blade passages was observed that was rotating at a frequency of 4.8-times the Runaway runner angular Speed. Apart from the unsteady pressure loading, the development pattern of the swirling flow in the runner is discussed in the paper
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Investigation and validation of a Francis turbine at Runaway operating conditions
'MDPI AG', 2016Co-Authors: Trivedi Chirag, Cervantes Michel, B. K. GandhiAbstract:Hydraulic turbines exhibit total load rejection during operation because of high fluctuations in the grid parameters. The generator reaches no-load instantly. Consequently, the turbine runner accelerates to high Speed, Runaway Speed, in seconds. Under common conditions, stable Runaway is only reached if after a load rejection, the control and protection mechanisms both fail and the guide vanes cannot be closed. The runner life is affected by the high amplitude pressure loading at the Runaway Speed. A model Francis turbine was used to investigate the consequences at the Runaway condition. Measurements and simulations were performed at three operating points. The numerical simulations were performed using standard k-", k-! shear stress transport (SST) and scale-adaptive simulation (SAS) models. A total of 12.8 million hexahedral mesh elements were created in the complete turbine, from the spiral casing inlet to the draft tube outlet. The experimental and numerical analysis showed that the runner was subjected to an unsteady pressure loading up to three-times the pressure loading observed at the best efficiency point. Investigates of unsteady pressure pulsations at the vaneless space, runner and draft tube are discussed in the paper. Further, unsteady swirling flow in the blade passages was observed that was rotating at a frequency of 4.8-times the Runaway runner angular Speed. Apart from the unsteady pressure loading, the development pattern of the swirling flow in the runner is discussed in the paper
