Flow In Draft Tube Research Articles

Numerical Investigation of Cavitating Flow in Elbow Type Draft Tube

Cavitation is a complex flow phenomenon which causes the damage to the hydraulic turbine components. In this paper, an attempt has been made to carry out CFD-based flow analysis of cavitation of an elbow type draft tube of hydro turbine. Under different operating conditions the performance parameters of draft tube have been analysed in ANSYS CFX using shear stress transport (SST) turbulence model and Rayleigh Plesset equation for different values of cavitation number and mass flow rate. Results are presented in the form of head loss, head recovery coefficient, efficiency, cavitation rate and distribution of the vapour volume fraction of elbow type draft tube. Based on the CFD analysis, it has been found that at low values of cavitation number the efficiency is decreased, however, other parameters such as head loss, head recovery coefficients and cavitation rates are to be increased.

A partially-averaged Navier–Stokes model for the simulation of turbulent swirling flow with vortex breakdown

This paper presents the development and validation of a new partially-averaged Navier–Stokes (PANS) model which can successfully predict turbulent swirling flow with vortex breakdown. The proposed PANS model uses an extended low Reynolds number k–ε model as the baseline model. Furthermore, a new formulation for the unresolved-to-total turbulent kinetic energy ratio fk is developed using partial integration of the complete turbulence energy spectrum. Therefore, the present formulation of fk is believed to be superior to the previously used constant or computed values. The newly developed PANS model is used in unsteady numerical simulations of two turbulent swirling flows containing vortex breakdown, namely swirling flow through an abrupt expansion and flow in a draft tube of a hydraulic turbine operating under partial load. The present PANS model accurately predicts time-averaged and root-mean-square (rms) velocities in the case of the abrupt expansion, while it is shown to be superior to the Delayed Detached Eddy Simulation (DDES) and Shear Stress Transport (SST) k–ω models. Predictions of the reattachment length using the present model shows at least 14% and 23% improvements compared to the DDES and the SST k–ω models respectively. Also, transient features of the flow, e.g. vortex rope formation and precession, is well captured in the case of the complex draft tube flow. The frequency of the vortex rope precession, which causes severe fluctuations and vibrations, is well predicted by only 7% deviations from the experimental data.

Characterization of flow, mixing and particle suspension in alumina draft tube precipitators of taller aspect ratio

Alumina precipitation is a vital rate determining step in the entire Bayer process. Industry scale draft tube precipitator design (Agglomerators and Growth tanks) in Bayer alumina process has the task to uniformly suspend the alumina hydrate particles with minimum power consumption. Lane (2006) reported that during the operation, there always was a difference in particle concentrations in the overflow. In the previous work, computational fluid dynamics (CFD) modeling was carried out to investigate the reasons for the particle variations in two different aspect ratio tanks (H/T=1.96 and 2.57) using a uniform momentum source impeller model. Also, it was concluded that more frequent upward surging of turbulent flow in the smaller aspect ratio precipitator is responsible for the higher particle concentration in the overflow. It is important to note that the overflow particle concentration will be uniform if particles are uniformly suspended in the entire precipitator volume; else gradients in particle concentration may prevail in the alumina precipitators. Brown et al. (2014) validated CFD modeling of H/T≤2 aspect ratio tank using both uniform momentum source and frozen rotor impeller model. They observed differences in the draft tube region flow pattern between the uniform momentum source and frozen rotor impeller model. It is important to understand the impact of the actual draft tube flow pattern on the overall flow, mixing and particle suspension. Hence, in the present work, investigation has been made in the taller aspect ratio precipitator (H/T=2.57) using impeller rotation model. The CFD analysis is made for both batch and continuous models whereas the previous authors carried out the analysis in batch model. The present work brings out the importance of an actual impeller rotation model against the constant mass source model using CFD analysis. Also, the work depicts the important role of draft tube slots and operation mode on changes in the annular region flow pattern and particle suspension. The batch precipitator design exhibits a non-swirl flow pattern and the same design in the continuous operation mode reveals a swirl flow pattern in the annular region of the precipitator. The present study conveys the role of impeller rotation, draft tube slots and the effect of annular region flow pattern on mixing and particle suspension.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part I: Simulation of the Vortex Rope

Numerical simulations and analysis of the vortex rope formation in a simplified draft tube of a model Francis turbine are carried out in this paper, which is the first part of a two-paper series. The emphasis of this part is on the simulation and investigation of flow using different turbulence closure models. Two part-load operating conditions with same head and different flow rates (91% and 70% of the best efficiency point (BEP) flow rate) are considered. Steady and unsteady simulations are carried out for axisymmetric and three-dimensional grid in a simplified axisymmetric geometry, and results are compared with experimental data. It is seen that steady simulations with Reynolds-averaged Navier–Stokes (RANS) models cannot resolve the vortex rope and give identical symmetric results for both the axisymmetric and three-dimensional flow geometries. These RANS simulations underpredict the axial velocity (by at least 14%) and turbulent kinetic energy (by at least 40%) near the center of the draft tube, even quite close to the design condition. Moving farther from the design point, models fail in predicting the correct levels of the axial velocity in the draft tube. Unsteady simulations are performed using unsteady RANS (URANS) and detached eddy simulation (DES) turbulence closure approaches. URANS models cannot capture the self-induced unsteadiness of the vortex rope and give steady solutions while DES model gives sufficient unsteady results. Using the proper unsteady model, i.e., DES, the overall shape of the vortex rope is correctly predicted and the calculated vortex rope frequency differs only 6% from experimental data. It is confirmed that the vortex rope is formed due to the roll-up of the shear layer at the interface between the low-velocity inner region created by the wake of the crown cone and highly swirling outer flow.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part II: Control of the Vortex Rope

Numerical simulations and investigation of a method for controlling the vortex rope formation in draft tubes are carried out in this paper, which is the second part of a two-paper series. As shown in the companion paper, formation of the vortex rope is associated with a large stagnant region at the center of the draft tube. Therefore, it is concluded that a successful control technique should focus on the elimination of this region. In practice, this can be performed by axially injecting a small fraction (a few percent of the total flow rate) of water into the draft tube. Water jet is supplied from the high-pressure flow upstream of the turbine spiral case by a bypass line; thus, no extra pump is needed in this method. It is shown that this method is very effective in elimination of the stagnant region in a simplified draft tube operating at two part-load conditions, i.e., at 91% and 70% of the best efficiency point (BEP) flow rate. This results in improvement of the draft tube performance and reduction of hydraulic losses. The loss coefficient is reduced by as much as 50% for the case with 91% of BEP flow rate and 14% for the case with 70% of BEP flow rate. Unsteady, three-dimensional simulations show that the jet increases the axial momentum of flow at the center of the draft tube and decreases the wake of the crown cone and thereby decreases the shear at the interface of the stagnant region and high velocity outer flow, which ultimately results in elimination of the vortex rope. Furthermore, reduction (by about 1/3 in the case with 70% of BEP flow rate) of strong pressure fluctuations leads to reliable operation of the turbine.

Flow determination of a pump-turbine at zero discharge

When starting up a reversible Francis pump-turbine in pump mode, the machine may operate at zero flow at a given gate opening. Besides reversal flow and prerotation in the draft tube cone, the onset of a fully separated flow in the vaned diffuser is observable at zero- discharge condition. In this paper, the occurrence of prerotation and reversal flow in the conical draft tube and the flow in one stay vane channel of a pump-turbine are examined experimentally and compared to numerical simulations. In order to assess the strongly three-dimensional flow in the stay vane channel, measurements with a 2D laser doppler velocimeter (LDV) were performed at various positions. The inlet flow in the draft tube cone, which becomes significantly at zero discharge in pump mode, is investigated by velocity measurements at two different positions. Pressure fluctuations in the draft tube cone induced by complex flow patterns are also recorded and analyzed. It is found that the swirl number at zero discharge does not significant differ from the values obtained at very low load pumping. Experimental investigations combined with CFD have shown that in the stay vane channel flow velocity components different from zero occur even at no discharge. Streamline plots show the fully separated flow structure.

Numerical Analysis of the Turbine 99 Draft Tube Flow Field Provoked by Redesigned Inlet Velocity Profiles.

In recent years, several investigations on hydraulic turbine draft tube performance have shown that the hydrodynamic flow field at the runner outlet determines the diffuser efficiency affecting the overall performance of the turbine. This flow field, for which the principal characteristics are the flow rate and the inlet swirling flow intensity, is mostly developed on turbines designed for low head (high specific velocity) and operated away from their best efficiency point. To identify factors of the flow field responsible for loosing draft- tube efficiency, the correlations between the flow pattern along the diffuser and both swirl intensity and flow rate have been examined. An analytical representation of inlet flow field has been manipulated by a Multi Island Genetic Algorithm through the automatic coupling of multidisciplinary commercial software systems in order to obtain redesigned inlet velocity profiles. This loop allowed determining the profile for which the minimum energy loss factor was reached. With different flow field patterns obtained during the optimization process it was possible to undertake a qualitative and quantitative analysis which has helped to understand how to suppress or at least mitigate undesirable draft tube flow characteristics. The direct correlation between the runner blade design and the kinematics of the swirl at the draft tube inlet should suppose the perfect coupling at the runner-draft tube interface without compromising the overall flow stability of the machine.

Reduced order model of draft tube flow

Swirling flow with compact coherent structures is very good candidate for proper orthogonal decomposition (POD), i.e. for decomposition into eigenmodes, which are the cornerstones of the flow field. Present paper focuses on POD of steady flows, which correspond to different operating points of Francis turbine draft tube flow. Set of eigenmodes is built using a limited number of snapshots from computational simulations. Resulting reduced order model (ROM) describes whole operating range of the draft tube. ROM enables to interpolate in between the operating points exploiting the knowledge about significance of particular eigenmodes and thus reconstruct the velocity field in any operating point within the given range. Practical example, which employs axisymmetric simulations of the draft tube flow, illustrates accuracy of ROM in regions without vortex breakdown together with need for higher resolution of the snapshot database close to location of sudden flow changes (e.g. vortex breakdown). ROM based on POD interpolation is very suitable tool for insight into flow physics of the draft tube flows (especially energy transfers in between different operating points), for supply of data for subsequent stability analysis or as an initialization database for advanced flow simulations.

Draft tube flow phenomena across the bulb turbine hill chart

In the framework of the BulbT project launched by the Consortium on Hydraulic Machines and the LAMH (Hydraulic Machine Laboratory of Laval University) in 2011, an intensive campaign to identify flow phenomena in the draft tube of a model bulb turbine has been done. A special focus was put on the draft tube component since it has a particular importance for recuperation in low head turbines. Particular operating points were chosen to analyse flow phenomena in this component. For each of these operating points, power, efficiency and pressure were measured following the IEC 60193 standard. Visualizations, unsteady wall pressure and efficiency measurements were performed in this component. The unsteady wall pressure was monitored at seven locations in the draft tube. The frequency content of each pressure signal was analyzed in order to characterize the flow phenomena across the efficiency hill chart. Visualizations were recorded with a high speed camera using tufts and cavitation bubbles as markers. The predominant detected phenomena were mapped and categorized in relation to the efficiency hill charts obtained for three runner blade openings. At partial load, the vortex rope was detected and characterized. An inflection in the partial load efficiency curves was found to be related to complex vortex rope instabilities. For overload conditions, the efficiency curves present a sharp drop after the best efficiency point, corresponding to an inflection on the power curves. This break off is more severe towards the highest blade openings. It is correlated to a flow separation at the wall of the draft tube. Also, due to the separation occurring in these conditions, a hysteresis effect was observed on the efficiency curves.

On the physical mechanisms governing self-excited pressure surge in Francis turbines

The required operating range for hydraulic machines is continually extended in an effort to integrate renewable energy sources with unsteady power outputs into the existing electrical grid. The off-design operation however brings forth unfavorable flow patterns in the machine, causing dynamic problems involving cavitation, which may represent a limiting factor to the energy production. In Francis turbines it is observed that the self-excited oscillation of a vortex rope in the draft tube cone prevents the delivery of maximum power when required. This phenomenon is referred to as full load pressure surge and has been the object of extensive research during the past decades. Several contributions deepened its understanding through measurement and simulation of the local flow properties and the global stability parameters. The draft tube pressure level and the runner outlet swirl are identified as key variables in the modelling of the vortex rope dynamics. Recently, a cyclic appearance of blade cavitation has been observed at overload conditions in a multiphase numerical simulation coupling the runner and the draft tube. From the analysis of the simulation it becomes obvious that the cyclic appearance of blade cavitation has a direct effect on the runner outlet swirl, thus introducing an additional interaction mechanism that is not accounted for in formerly published models. For the presented work, the results of this numerical study are confirmed experimentally on a reduced scale model of a Francis turbine. Several wall pressure measurements in the draft tube cone are performed, together with high speed visualizations of the vortex rope and the blade cavitation. The flow swirl is calculated based on Laser Doppler Velocimetry measurements. A possible mechanism explaining the coupling between the self-excited pressure and vortex rope oscillation and the cyclic appearance of the blade cavitation is proposed. Furthermore, the streamwise propagation speed of the flow swirl in the draft tube is calculated. The results offer important insights in the physics of high load pressure surge and contribute to the further development of numerical draft tube flow and stability models.

Analysis of RANS turbulence models by calculating the steady-state flow in the Turbine-99 draft tube

Analysis of RANS turbulence models by calculating the steady-state flow in the Turbine-99 draft tube

Evaluation of a Francis turbine draft tube flow at part load using hybrid RANS-LES turbulence modelling

It is well known that part-load operation of Francis turbines is dominated by time-dependent phenomena. A high swirl level at the turbine runner outlet constitutes a complex inlet boundary condition for the draft tube flow at this condition. The draft tube flow simulation using a quasi-steady approach of typical two-equation turbulence modelling (k-ω SST) can provide good agreement between measurements and computational calculation for the optimum point. Global main structures of the flow as well as secondary flows can be well predicted. For part load condition this hypothesis is quite questionable due to different time scales which are associated to the different effects that can be observed at this condition. Steady state simulations do not predict the physics of the flow accurately. This work presents an evaluation of the part-load simulation results when applying a hybrid RANS-LES turbulence modelling approach using the finite volume method. Firstly, steady state results employing the k-ω SST turbulence model are presented. Then transient calculations employing both k-ω SST and SAS SST, a 2nd generation hybrid turbulence model, are also presented. In this model, a spatial filtering based on the grid size is not the main criteria applied to determinate the smallest calculated scales of the flow, but the instabilities of the flow field itself are the trigger for the reduction of eddy viscosity production. The effects of the mesh resolution on the solved turbulent structures are also evaluated. The numerical results are compared to time-averaged velocity profile data obtained by LDA (Laser-Doppler-Anemometry) measurements at the inlet of the draft tube diffuser of an nq=55 1/min Francis turbine. Velocity components in flow direction and transversal to it are evaluated. The investigation shows that the steady state results present considerable deviations when compared to those measurements. The transient calculations using the unsteady k-ω SST approach shows some improvement, but the tendencies are similar to steady state calculations. Calculations employing the SAS SST model shows overall better concordance between simulations and measurements. The results calculated with a finer grid model do not present significant improvement on the velocity profiles prediction when compared with the coarser grid one, although allowing the calculation of smaller eddies.

Simulation of flow in a simplified draft tube: turbulence closure considerations

Numerical simulations of turbulent flow in a simplified draft tube are carried out. The main emphasis of these simulations is on the investigation of different turbulence closure models in predicting the flow behavior. Both steady and unsteady simulations are performed for axisymmetric as well as three-dimensional grid in an axisymmetric geometry, and results are compared with available experimental data. It is seen that steady simulations, performed with FLUENT and OpenFOAM, using Reynolds-Averaged Navier-Stokes (RANS) models for the axisymmetric and three-dimensional flow geometries give the same symmetric results. These steady, symmetric results underpredict the levels of turbulent kinetic energy (by at least 40%) and axial velocity (by at least 14%) near the centerline of the draft tube. No significant improvement is achieved applying various turbulence RANS models such as realizable k-ε, RNG k-ε, SST k-ω and v2−f model. Unsteady simulations are also performed using both URANS and hybrid URANS/LES (DES) turbulence closure approaches. It is seen that URANS models cannot capture the self-induced nature of the vortex rope and result in steady solutions due to steady boundary conditions while hybrid URANS/LES model can capture unsteady features of the flow. Importance of time-dependent inlet boundary conditions is discussed and it is shown that predictions are improved (at least 10%) by applying time-dependent boundary conditions which include turbulence fluctuations at inlet. Finally, results of two methods of vortex rope identification (iso-pressure surface and Δ-criterion) are compared.

Effects of inlet boundary conditions, on the computed flow in the Turbine-99 draft tube, using OpenFOAM and CFX

The flow in the Turbine-99 Kaplan draft tube was thoroughly investigated at three workshops (1999, 2001, 2005), which aimed at determining the state of the art of draft tube simulations. The flow is challenging due to the different flow phenomena appearing simultaneously such as unsteadiness, separation, swirl, turbulence, and a strong adverse pressure gradient. The geometry and the experimentally determined inlet boundary conditions were provided to the Turbine-99 workshop participants. At the final workshop, angular resolved inlet velocity boundary conditions were provided. The rotating non-axi-symmetry of the inlet flow due to the runner blades was thus included. The effect of the rotating angular resolution was however not fully investigated at that workshop. The first purpose of this work is to further investigate this effect. Several different inlet boundary conditions are applied – the angular resolved experimental data distributed at the Turbine-99 workshop, the angular resolved results of a runner simulation with interpolated values using different resolution in the tangential and radial directions, and an axi-symmetric variant of the same numerical data. The second purpose of this work is to compare the results from the OpenFOAM and CFX CFD codes, using as similar settings as possible. The present results suggest that the experimental angular inlet boundary conditions proposed to the workshop are not adequate to simulate accurately the flow in the T-99 draft tube. The reason for this is that the experimental phase-averaged data has some important differences compared to the previously measured time-averaged data. Using the interpolated data from the runner simulation as inlet boundary condition however gives good results as long as the resolution of that data is sufficient. It is shown that the difference between the results using the angular-resolved and the corresponding symmetric inlet data is very small, suggesting that the importance of the angular resolution is small. The results from OpenFOAM and CFX are very similar as long as the inlet data resolution is fine enough. CFX seems to be more sensitive to that resolution.

Interaction of a pulsating vortex rope with the local velocity field in a Francis turbine draft tube

Acoustic resonances in Francis turbines often define undesirable limitations to their operating ranges at high load. The knowledge of the mechanisms governing the onset and the sustenance of these instabilities in the swirling flow leaving the runner is essential for the development of a reliable hydroacoustic model for the prediction of system stability. The present work seeks to study experimentally the unstable draft tube flow by conducting a series of measurements on a reduced Francis Turbine model. The key physical parameters and their interaction with the hydraulic and mechanical system are studied and quantified. In particular, the evolution of the axial and tangential velocity components in the draft tube cone is analysed by means of Laser Doppler Anemometry. Combined with the calculation of the instantaneous vortex rope volume based on flow visualization and the measurement of the pressure fluctuations, the nature of the auto-oscillation in the draft tube flow is investigated.

Numerical prediction of a draft tube flow taking into account uncertain inlet conditions

The swirling turbulent flow in a hydroturbine draft tube is computed with a non-intrusive uncertainty quantification (UQ) method coupled to Reynolds-Averaged Navier-Stokes (RANS) modelling in order to take into account in the numerical prediction the physical uncertainties existing on the inlet flow conditions. The proposed approach yields not only mean velocity fields to be compared with measured profiles, as is customary in Computational Fluid Dynamics (CFD) practice, but also variance of these quantities from which error bars can be deduced on the computed profiles, thus making more significant the comparison between experiment and computation.

Numerical simulation of pressure fluctuations in a large Francis turbine runner

The pressure fluctuation caused by unsteady flow in runner is one of the main reasons of vibration for a large Francis hydraulic turbine. It directly affects the steady operation of the hydraulic turbine unit. The existing research of the pressure fluctuation in hydraulic turbine mainly focuses on the unsteady flow in draft tube. Accurate distribution of pressure fluctuations inside a runner is not very clear. In this paper, the numerical method for predicting the pressure fluctuations in runner is investigated and the numerical simulation is performed for a large Francis hydraulic turbine. It is proved that the combination of shear-stress transport(SST) k-ω turbulence model and pressure-implicit with splitting of operators(PISO) algorithm could give more reliable prediction of pressure fluctuations in runner. The frequencies of pressure fluctuations in runner are affected by the flow in guide vane and the flow in draft tube. The first dominant frequency is significantly determined by the flow in draft tube, especially at part load condition. This frequency is approximately equal to one-third of the runner rotating frequency. The evident second dominant frequency is exactly equal to the guide vane passing frequency. The peak-to-peak amplitudes of pressure fluctuations in runner at small guide vane open angle are larger than that at large open angle at the same operating head. The amplitudes at points on blade pressure surface are generally greater than that on suction surface. The research results could be used to direct the hydraulic design and operation stability improvement of a large Francis hydraulic turbine.

Experimental Investigation of Draft Tube Inlet Velocity Field of a Propeller Turbine

The draft tube of reaction hydraulic turbines is subject to numerous investigations since it accounts for a significant portion of the energy recovery. But even with up-to-date computational fluid dynamics methodologies, simulating the draft tube flow remains highly challenging since it is a diverging swirling flow that may undergo flow separations and become dominated by unsteady secondary flows. Within the framework of a collaborative research project on the flow dynamics of a propeller turbine model, the flow at the inlet region of the draft tube was studied using 2D-laser Doppler velocimetry (2D-LDV). Measurements were used to detect and characterize the flow structures at three operating conditions: partial discharge, near best efficiency, and full-load conditions. The paper presents analysis based on phased-averaged velocity fields to yield information on fluctuations and dominant frequencies according to runner positions. The main features detected are the flow nonuniformity at the runner exit and the secondary flow structures associated with the runner hub wake. Those results are part of a larger database aimed at providing test cases for the validation of numerical simulation strategies.

Optimization methodology assessment for the inlet velocity profile of a hydraulic turbine draft tube: part I—computer optimization techniques

In recent years, numerical and experimental investigations on the draft tube performance have confirmed the importance of the inlet swirling flow created by the runner vanes. The results indicate that it is still a challenge to get the optimal flow distribution at the draft tube inlet which gives the best machine performance over a range of operation points. Consequently, there is a need to adjust the runner-draft tube coupling to minimize the losses arising from the inlet flow distribution. This paper focus on establishing an optimization methodology for maximizing the draft tube performance as a function of the inlet velocity profile. The overall work is divides into two parts: The part one establish the inlet velocity parametrization, the numerical optimization set-up and the objective function definition. The part two validate the numerical CFD draft tube model. These steps are represented by the coupling of the commercial softwares MATLAB, FLUENT and iSIGHT. It is considered that this proved methodology will help to find a inlet velocity profile shape which will be able to suppress or mitigate the undesirable draft tube flow characteristics.

Prediction and experimental verification of vortex flow in draft tube of Francis turbine based on CFD

The three-dimensional unsteady turbulent flow in the draft tube of a Francis turbine was calculated by using the SIMPLE algorithm with the body-fitted coordinate and tetrahedral grid system with the help of FLUENT software. This calculation was based on the Navier-Stokes equations and LES (large-eddy simulation) model. The flowing situations in the draft tube under three operating conditions of this turbine were successfully calculated. The PIV (particle image velocimetry) was used to measure the distribution of flow field at the draft tube cone section at 4 horizontal sections from the outlet of turbine runner. Specialized image analysis software was used to process the experimental results. Through comparison, the results of numerical simulations were in good coincidence with the experimental results.