Academic literature on the topic 'Draft tube'

Draft tube circulators have been used in various industrial processes. They contain an inner tube located inside the reactor. An axial impeller at the top part of the inner tube pushes the fluid downwards. This reactor concept uses less energy to achieve sufficient mixing in the system. Outotec has developed a novel draft tube reactor which exploits draft tube circulator principles and consists of horizontal and vertical sections. The aim of this work was to obtain information on the flow and solid resuspension behavior of the test tube reactor for the design and further development of the OKTOP®6000 reactor. Flow velocities were measured with an ultrasound measuring probe that utilizes the Doppler effect. The aim of the experiments was to gain information about the flow profile inside the reactor annulus from different measuring points. Flow velocity results were used to validate the computational fluid dynamic simulation results made earlier. Liquid-solid and liquid-solid-gas mixing was studied with electrical impedance tomography (EIT) devices, which measure conductivity difference in the media. Experiments were done with different solids contents, impeller rotation speeds and different gas feed rates to determine what kinds of mixing conditions achieve complete off-bottom suspension. Slurry samples were taken from the upper part of the reactor to obtain information on how much sand flows to the upper part of the reactor. Based on the flow velocity and CFD simulations, the flow inside the reactor annulus is highly turbulent and the flow profile is time dependent. There is a recirculation area at the bottom of the reactor and the flow is pulsating to the upper part of the reactor. Solids suspension tests show that the reactor concept achieves complete off-bottom suspension more easily without gas feed. The gas feeding system was not optimal and some of the gas flooded from the draft tube and disturbed the flow inside the draft tube. Solids suspension tests confirmed the fact that the flow profile was pulsating and time dependent. Flow velocity and solids suspension test results can be used to dimension and further develop the novel draft tube reactor OKTOP®6000, which is part of the Outotec reactor family<br>Imuputkireaktoreita on käytössä monissa erilaisissa teollisissa sovelluksissa. Reaktorin sisälle on asennettu sisäputki, jonka yläosassa on tyypillisesti sekoitin, joka saa aikaan liuoskierron sisäputken kautta reaktorin annulukseen. Sisäputken ansiosta sekoitukseen tarvittava energiamäärä on pienempi kuin perinteisissä sekoitusreaktoreissa. Outotec on kehittänyt sisäputkireaktoriteknologiaa hyödyntävän OKTOP®6000-reaktorin, joka koostuu vaaka- ja pystysuuntaisista osioista. Imuputkireaktorin avulla pyrittiin selventämään reaktorin suunnittelun ja jatkokehityksen avuksi virtausominaisuuksia sekä kiintoaineen suspengointiin liittyviä ilmiöitä. Virtausnopeuksia mitattiin Doppler-ilmiötä hyödyntävällä mittalaitteella eri kohdista reaktorin ulkokehää. Tarkoituksena oli saada tietoa virtausnopeuksista muutamassa mittauspisteessä reaktorin sisältä eri sekoittajan pyörimisnopeuksilla. Tuloksien avulla pyrittiin vahvistamaan virtauslaskennan tuloksia. Toisessa osassa reaktoriin lisättiin kiintoainetta ja tutkittiin sen liikkeelle lähtöä eri hiekkamäärillä, sekoituksen kierrosmäärillä sekä happisyöttömäärillä. Tarkoituksena oli saada tietoa milloin neste-kiintoaine- sekä neste-kiintoaine-kaasu-systeemeissä saadaan aikaan sellaiset sekoitusolosuhteet, että kaikki hiekka joko liikkuu reaktorin pohjassa ja mahdollisesti virtaa reaktorin yläosaan. Mittauksessa käytettiin apuna sähköimpedanssi tomografiasauvoja (EIT), jotka mittaavat sähkönjohtavuuseroja väliaineesta. Lisäksi reaktorin yläosaan virtaavan hiekan kiintoainepitoisuutta mitattiin yläosasta otettujen näytteiden avulla. Virtausmittausten ja -simulointien perusteella voidaan sanoa, että virtaus reaktorissa on hyvin turbulenttinen ja virtausprofiili vaihtelee eri ajanhetkillä. Reaktorin pohjaan muodostuu pyörteitä kun sisäputkesta tuleva virtaus muuttaa suuntaa. Virtaus kohti reaktorin yläosaa on hyvin sykkivää. Kiintoainekokeiden perusteella nähtiin, että hiekka pysyi liikkeessä kaikilla kierrosnopeuksilla ja hiekkamäärillä ilman kaasusyöttöä. Happisyöttö puolestaan häiritsi systeemiä ja sai hiekan laskeutumaan tietyssä pisteessä. Kaasun syöttöjärjestelmä ei ollut optimaalinen tälle reaktorikonseptille, sillä isommat kaasumäärät tulvivat ulos sisäputkesta virtaamatta nestevirtauksen mukana. Virtausmittaus- ja kiintoainekokeiden tuloksia voidaan käyttää OKTOP®6000-reaktorin jatkokehityksessä ja suunnittelussa

The present research focuses on flow properties of the elbow draft tube. This element has a major function in low head turbines, since up to half of the losses may arise there at part load. The use of computational fluid dynamic (CFD) to redisign a draft tube necessitates detailed knowledged of the boundary conditions. They are generally not available and qualified guesses must be made. This applies in particular to the radial velocity at the inlet. A method to estimate this component in swirling flow from experimental values of the axial and tangential velocities is derived. The method uses a two dimensional non- viscous description of the flow, the Squire-Long formulation. It is tested against swirling flow in a diffuser and applied to the Turbine-99 draft tube flow. As several other boundary conditions are difficult to estimate and many input parameters are available to perform a simulation, the use of factorial design is proposed as an alternative to design simulations in a systematic, objective and quantitative way. The method allows the deternmination of the main and joint effects of input parameters on the numerical simulation. The input parameters may be experimental uncertainty on boundary conditions, unknown boundary conditions, grid and turbulence models. The method is applied to the Turbine-99 test case, where the radial velocity, the surface roughness, the turbulence length scale and the grid were the factors investigated. The inlet radial velocity is found to have a major effect on the pressure recovery. The flow in water turbines is highly unsteady due to the runner blade rotation, guide vanes and stay vanes. Unsteady pressure measurements on a Kaplan prototype point out unsteadiness in the high and low pressure regions of the turbine. Since model and prototype are not running in dynamically similar conditions, the influence of unsteadiness on the losses is of interest. The derivation of the variation of the mechanical energy for the mean, oscillating and turbulent fields point out the contribution of unsteadiness to the losses and the turbulent production. Application to turbulent channel flow reveals that the contribution is a function of the amplitude of the oscillation, the frequency and the friction velocity. Turbulent pulsating flow in a generic model of the rectangular diffuser found at the end of elbow draft tube is studied in detail with laser Doppler anemometry (LDA). Three frequencies, corresponding to the quasi-steady, relaxation and quasi-laminar regimes with an amplitude of about 10% are investigated. The results indicate no alteration of the mean flow by the excitation of a single frequency. Furthermore. the existence of the different regimes, as found in turbulent pulsating turbulent pipe and channel flows, is confirmed.<br>Godkänd; 2003; 20061108 (haneit)

The goal of the Turbine 99 Draft Tube experiments is to provide extensive experimental data on a well-defined sharp-heel draft-tube flow. The data bank has served as calibration data for the simulation challenge presented by the Turbine 99 workshop in Porjus in June 1999. This thesis gives some background on draft-tube flows in general and discusses in some detail the parameters and flow conditions relevant to the Turbine 99 draft tube. Some comments on the research and development conducted so far in the project and future plans are given at the end. In the three accompanying papers, details of the developments and the scientific results are presented: Paper 1. Presents the scope of the work and some initial results from the measurements Paper 2. Discusses the quality of the measurements Paper 3. Presents some of the results from the measurements<br>Godkänd; 2000; 20070318 (ysko)

Francis-99 is a series of upcoming workshops aimed at determining the state of the art of high head Francis turbine simulations. The scope of this essay is to provide empirical data to serve as validation for the CFD simulations.Axial and tangential velocity profiles were measured for a model of the Tokke runner while operating in part load, best efficiency, and full load, at two different locations in the draft tube cone: 64mm and 382mm below the draft tube inlet. The results were phase resolved in order to calculate normal Reynolds stresses at the different operating conditions, and the results have been compared with computer simulations. The resulting velocity profiles have been evaluated with respect to the governing equations, and the correlation between velocity profiles, pressure gradients and normal stresses has been discussed. The effect of head on the velocity profiles has been evaluated, and found to have only a small impact on the reduced velocity profiles, both axially and tangentially. The repeatability of the test rig was also tested, and found to be within acceptable limits.The reliability of the measured velocity profiles is considered high enough to be admissible as validation data for the Francis-99 workshop.The simulated results show good agreement with the measured velocity profiles for axial flow, but show great discrepancies for the tangential velocity profiles. How- ever, some similarities can be seen between the simulated and measured results, and further work is required in this area.

Hydroelectricity plays an important role to balance the stability of grid network.  In order to improve the stability of presently high loaded grids, hydropower plants are being operated over a wide range of operations and experiencing frequent start-stop, load rejection, and load acceptance. The turbines need to sustain sudden change in their operating condition to balance the grid frequency. Francis turbines have been widely used because of their wider operating range and higher stability in operation during rapid load variation. This has resulted in severe damage to the turbines as they are not normally designed to operate under such transient conditions. Several low and high frequency pressure fluctuations prevail during transients operating conditions. Generally, wall pressure measurements are performed which may not provide sufficient information to investigate the flow instabilities related to these fluctuations. Thus, the main objective of the present work is to simplify and perform optical measurements in a turbine during transient operating conditions to investigate the flow field. The measurements have been performed at the Water Power Laboratory using a high head model Francis turbine. The turbine is a 1:5.1 scale down model of a prototype operating at the Tokke Power Plant, Norway. The model runner diameter, net head, and discharge at the best efficiency point (BEP) were 0.349 m, 12 m, and 0.2 m3 s-1, respectively. A total ten pressure sensors were mounted at different locations namely, turbine inlet, vaneless space, and draft tube. The data were acquired at a sampling rate of 5 kHz. The instruments and sensors have been calibrated according to guidelines available in IEC standards. The determined total uncertainty in the measurement of hydraulic efficiency was ±0.15% at BEP condition. The velocity measurements in the draft tube cone were performed using a 2D PIV system and the images were sampled at a rate of 40 Hz.      Steady state measurements were carried out considering the realistic design and off-design operating conditions of the prototype turbine. Therefore, the angular speed of the runner was maintained constant for all steady state conditions during the measurements. The maximum hydraulic efficiency (92.4%) was observed at nED = 0.18, QED = 0.15, and a = 9.8º, which is named BEP. It is observed that the turbine experiences significant pressure fluctuations at the vaneless space, runner, and the draft tube. The fluctuations due to rotor-stator interaction (RSI) were observed to be most dominating at high load condition, however, fluctuations due to the rotating vortex rope (RVR) at part load (PL) condition. Two different modes (synchronous and asynchronous) modes of vortex rope are observed at PL condition of the turbine. An asymmetry in the flow leaving the runner was detected at both design and off-design conditions, with a stronger effect during off-design operating condition. Numerical simulations of the model turbine were carried out at PL operating condition. The simulations were performed using two turbulence models, standard k-ε and SST k-ω, with high-resolution advection scheme. The numerical pressure values obtained with both standard k-ε model and SST k-ω showed a small difference with the experimental values. The amplitudes of numerical pressure values were higher (~2.8%) in the vaneless space and lower (~5.0%) in the draft tube than the experimental values. The frequencies of the RSI and RVR were well captured in the turbine but the amplitudes were overestimated.   During load rejection from BEP to PL, the plunging mode of the vortex rope was observed to appear first in the system than that of the rotating mode. Whereas during the load acceptance from PL to BEP, both the modes were observed to disappear simultaneously from the system. In the velocity data, the axial velocity only contributed to the development of the plunging mode and radial velocity to the rotating mode. The region of low velocity, stagnation point, flow separation, recirculation, oscillating flow and high axial velocity gradients were well captured in the system during the transients. The induced high-velocity gradients during the load acceptance from BEP to HL was observed to develop a vortex core in the draft tube. During startup and shutdown, the guide vanes angular position was moved from one to another steady state condition to achieve the minimum load condition of the turbine. At this condition, the generator of the turbine was magnetized at the synchronous speed during startup and shutdown, respectively. The frequency of wave propagation was observed to vary with the runner angular speed during startup and complete shutdown of the turbine. Comparatively high-pressure fluctuations in the draft tube were observed during the guide vane movement from the high discharge conditions. Some unsteady phenomena such as the formation of dead velocity zone, backward flow, and flow oscillations were observed during startup and shutdown of the turbine.   The current work has been also used to continue a series of workshops, i.e., Francis-99. The first workshop was held on December 2014 with the cooperation of LTU and NTNU. The measurements performed in this work were used for the second workshop which was held on December 2016. The investigations presented in this thesis will be further explored in the third workshop scheduled for December 2018.

Les aspirateurs de turbines hydrauliques jouent un rôle crucial dans l’extraction de l’énergie disponible. Dans ce projet, les écoulements dans l’aspirateur d’une turbine de basse chute ont été simulés à l'aide de différents modèles de turbulence dont le modèle DDES, un hybride LES/RANS, qui permet de résoudre une partie du spectre turbulent. Déterminer des conditions aux limites pour ce modèle à l’entrée de l’aspirateur est un défi. Des profils d’entrée 1D axisymétriques et 2D instationnaires tenant compte des sillages et vortex induits par les aubes de la roue ont notamment été testés. Une fluctuation artificielle a également été imposée, afin d’imiter la turbulence qui existe juste après la roue. Les simulations ont été effectuées pour deux configurations d’aspirateur du projet BulbT. Pour la deuxième, plusieurs comparaisons avec des données expérimentales ont été faites pour deux conditions d'opération, à charge partielle et dans la zone de baisse rapide du rendement après le point de meilleur rendement. Cela a permis d’évaluer l'efficacité et les lacunes de la modélisation turbulente et des conditions limites à travers leurs effets sur les quantités globales et locales. Les résultats ont montrés que les structures tourbillonnaires et sillages sortant de la roue sont adéquatement résolus par les simulations DDES de l’aspirateur, en appliquant les profils instationnaires bidimensionnels et un schéma de faible dissipation pour le terme convectif. En outre, les effets de la turbulence artificielle à l'entrée de l’aspirateur ont été explorés à l'aide de l’estimation de l’intermittence du décollement, de corrélations en deux points, du spectre d'énergie et du concept de structures cohérentes lagrangiennes. Ces analyses ont montré que les détails de la dynamique de l'écoulement et de la séparation sont modifiés, ainsi que les patrons des lignes de transport à divers endroits de l’aspirateur. Cependant, les quantités globales comme le coefficient de récupération de l’aspirateur ne sont pas influencées par ces spécificités locales.<br>Draft tubes play a crucial role in elevating the available energy extraction of hydroturbines. In this project, turbulent flows in the draft tube of a low-head bulb turbine were simulated using, among others, an advance hybrid LES/RANS turbulent model, called DDES, which can resolve portions of the turbulent spectrum. Providing appropriate inflow boundary conditions for such models is a challenging issue. In this regard, different inflow boundary conditions were tested, including axisymmetric 1D profiles, and unsteady 2D inflow profiles that take runner blade wakes and vortices into account. Artificial fluctuation at the inlet section of the draft tube was also included to mimic the turbulence existing after the runner. Simulations were conducted for two draft tube configurations of the BulbT project. For one of them, intensive comparisons with experimental data were done for two operating conditions, one at part load and another in the sharp drop-off portion of the efficiency hill after the best efficiency point. This allowed to assess the effectiveness and shortcomings of the adopted turbulence modeling and boundary conditions through their effects on the global and local quantities. The results showed that the runner-related vortical structures and wakes are appropriately resolved using stand-alone DDES simulation of the draft tube flows. This is achieved by applying unsteady 2D inflow profiles along with adopting low dissipation scheme for the convective term. Furthermore, the effects of applying artificial turbulence at inlet were explored using separation intermittency, two-point correlation, energy spectrum and Lagrangian coherent structure concepts. These analyses revealed that the type of inflow boundary conditions modifies the details of the flow and separation dynamics as well as patterns of the transport barriers in different regions of the draft tube. However, the global quantities such as recovery coefficient are not influenced by these local features.