Draft Tube

An experimental investigation has been carried out in order to evaluate the detection of cavitation in actual hydraulic turbines. The methodology is based on the analysis of structural vibrations, acoustic emissions and hydrodynamic pressures measured in the machine. The proposed techniques have been checked in real prototypes suffering from different types of cavitation. In particular, one Kaplan, two Francis and one Pump-Turbine have been investigated in the field. Additionally, one Francis located in a laboratory has also been tested.

Numerical results for a gas-fluidized bed using a 2D Eulerian model including the kinetic theory for the particulate phase were provided. The circulation patterns for various operating conditions were discussed. Modeling parameters of drag function, algebraic and transport equations of granular temperature, frictional stress model, turbulent model and discretization scheme were investigated for a bed with different gas distributors and a slotted draft tube. CFD results showed that the drag model is an important hydrodynamics parameter for gas-fluidized beds with various gas distributors. Transport and algebraic equations for granular temperature should be utilized, respectively, for beds including partial and complete sparging at U g = 2.18 m/s. Frictional stresses play an important role for the beds containing partial sparging with and without draft tube. Discretization schemes should be examined to achieve better results. The Simonin and k-ε turbulent models can improve the CFD results at high gas velocities. Considering perforated plate distributor improves the results.

La technique de simulation des grandeséchelles (SGE) est utilisée pour simuler l'écoulement dans un aspirateur de centrale hydrauliqueà différents points de fonctionnement. La complexité de la géométrie et le haut nombre de Reynolds de l'écoulement nécessitent l'utilisation de méthodes spécifiques. Ainsi, nous avons développé un traitementà la paroi tenant compteà la fois du frottement pariétal mais aussi du gradient de pression. La technique utilisée pour permettre de générer des conditions d'entrée instationnaires réalistes,à partir de données expérimentales, estégalement détaillée. Les différents régimes d'écoulements résultant de cette configuration sont alors présentés.

Six laboratory scale biogas mixed anaerobic digesters were operated to study the effect of biogas recycling rates and draft tube height on their performance. The digesters produced methane at 0.40-0.45 L per liter of digester volume per day. A higher methane production rate was observed in unmixed digesters, while increased biogas circulation rate reduced methane production. However, different draft tube heights caused no difference in the methane production rate. Air infiltration (up to 15% oxygen in the biogas) was observed in the digesters mixed by biogas recirculation. Slight air permeability of tubing or leakage on the vacuum side of the air pump may have caused the observed air infiltration. The similar performance of the mixed and unmixed digesters might be the result of the low solids concentration (50 g dry solids per liter of slurry) in the fed animal slurry, which could be sufficiently mixed by the naturally produced biogas.

Fluid Dynamics for a cylindrical vessel stirred by a helical screw agitator. Simulations have been performed for a vessel geometry with and without a draft tube. Simulated flow patterns in the vessel have been examined and compared with the experimental work of previous authors. The power number and the circulation number have been evaluated, and interpreted in a similar manner to other works. The P O .Re constant, A, has been determined to be 295 for the geometry with the draft tube and 150 for that without the draft tube. These results are in the same range as previously reported values. The Metzner and Otto constant, k, has been evaluated to be 16.23 which is in excellent agreement with experimental results reported in the literature.

Flow behaviors of particles in a two-dimensional spouted bed with a porous draft plates are studied using a fluid dynamic computation with kinetic-frictional stresses models. Gas flow through the porous draft tube is simulated by Brinkman-Darcy-Forchheimer formulation. The distributions of concentration and velocities of particles are predicted. Simulated results predict the solid circulation rate and gas flux rate measured in the spouted bed with a porous draft tube . The solid circulation rate in the spouted bed with a porous draft tube is larger than that with a non-porous draft tube. The predicted bed pressure drop with the porous draft tube is high in comparison to the spouted bed with non-porous draft tube. The effect of the porosity of the porous draft tube on distributions of gas flux rate through the annulus and solid circulation rate are discussed.

Fluid Dynamics for a cylindrical vessel stirred by a helical screw agitator. Simulations have been performed for a vessel geometry with and without a draft tube. Simulated flow patterns in the vessel have been examined and compared with the experimental work of previous authors. The power number and the circulation number have been evaluated, and interpreted in a similar manner to other works. The P O .Re constant, A, has been determined to be 295 for the geometry with the draft tube and 150 for that without the draft tube. These results are in the same range as previously reported values. The Metzner and Otto constant, k, has been evaluated to be 16.23 which is in excellent agreement with experimental results reported in the literature.

An experimental investigation has been carried out in order to evaluate the detection of cavitation in actual hydraulic turbines. The methodology is based on the analysis of structural vibrations, acoustic emissions and hydrodynamic pressures measured in the machine. The proposed techniques have been checked in real prototypes suffering from different types of cavitation. In particular, one Kaplan, two Francis and one Pump-Turbine have been investigated in the field. Additionally, one Francis located in a laboratory has also been tested.

The accuracy of k-epsilon turbulence models for the swirling flow in the Turbine 99 draft tube is the subject of this work. The relation of the first-order upwind, second-order upwind, third-order upwind (QUICK) and Power Law schemes used with these models has been studied. As the turbulent flows are significantly affected by the presence of walls, the wall function and the near-wall models were tested for modeling the near-wall region. Two different grid concentrations near the wall y+1 and y+50 were used to study the flow behavior for case 1 of Turbine 99 Workshop III. Discussion is based on graphical results and by comparing numerical simulations and experiments in operational mode T (close to best efficiency). The results of this study indicate a very good representation of the flow at different cross sections by the RNG turbulence model but a poor level of convergence.

Keywords: turbulence models; hydraulic turbine; draft tube; CFD; visualization

This paper presents a front propagation method using the Eikonal equation, ∇ · ∇ = 1, in which, represents the smallest Euclidean distance field to the front to be propagated. The offset capturing approach consists in first calculating the field over a uniform Cartesian grid fully covering the front to be propagated, and then constructing the iso-curves or surfaces as the propagated result. The calculation of uses a 3D numerical scheme, the Fast Sweeping Scheme. Validation for accuracy of the method is presented using academic test cases. A real 3D industry application, draft tube with two piers, is successfully propagated and demonstrated using special boundary conditions to cope with inlet and outlet planes during front propagtion. This application involves the propagation of a front that exhibits both concave and convex shape regions, sharp corners, and local tangent plane surface discontinuities as well as a multi-connected domain. 227 than the local curvature radius in concave regions; and (2) a global self-intersection, when the distance between two distinct points on the curve or surface reaches a local minimum.

A novel circulating fluidized bed riser which features an enlarged bottom and a draft tube has been introduced and investigated in terms of its applicability for FCC. Multi-flow regimes were observed in this novel riser, which had a dense bottom and a dilute upper region. The flow pattern in the bottom part almost had no effect on the flow behavior in the upper part. The dense bed height could be adjusted easily. A funnelform draft tube enabled the solids to follow two flow patterns: an upward flow to the riser top and a downward flow from the annulus to the riser bottom. A solids distribution ratio was proposed to qualify this distribution relationship. Furthermore, the flow structure in the bottom region was depicted, and an obvious core-annulus countercurrent structure was observed.

Baffle Digester Draft tube Gas-lift Mixing a b s t r a c t Computational fluid dynamics simulations were used to study the effect of bottom configuration and a hanging baffle on the mixing inside a gas-lift digester filled with non-Newtonian sludge. The Navier-Stokes and continuity equations were solved numerically using commercially available finite element method-based solver. The results from this simulation were found to be in good agreement with previously reported experimental findings. At a gas recirculation rate of 84.96 l/h, the poorly mixed zones inside a flat bottom digester were about 33.6% of the digester volume, while in the case of digesters with 251 and 451 conical bottoms poorly mixed zones were about 31.9% and 29.6%, respectively. The power law viscosity index, n, did not have a significant effect on the mixing pattern under the conditions studied. Introduction of a hanging baffle in combination with a 451 hopper bottom resulted in reduction of the poorly mixed zone by a factor of three compared to a flat bottom without baffle configuration. Although the introduction of a hanging baffle was able to significantly reduce the size of the poorly mixed zones inside a gas-lift digester, further optimization of the digester geometry may lead to additional improvements.

Fluid Dynamics for a cylindrical vessel stirred by a helical screw agitator. Simulations have been performed for a vessel geometry with and without a draft tube. Simulated flow patterns in the vessel have been examined and compared with the experimental work of previous authors. The power number and the circulation number have been evaluated, and interpreted in a similar manner to other works. The P O .Re constant, A, has been determined to be 295 for the geometry with the draft tube and 150 for that without the draft tube. These results are in the same range as previously reported values. The Metzner and Otto constant, k, has been evaluated to be 16.23 which is in excellent agreement with experimental results reported in the literature.

Two-and three-phase hydrodynanuc studies were conducted m an air lift bubble column with an internal loop The bubble column had a conical dlstnbutor at the bottom, through which the gas was sparged, creating a hqmd or slurry circulation from the draft tube to the annular region The axial position of the draft tube was vaned with respect to the conical section The expenments were performed with the air-water system at ambient condltlons, usmg sihceous sand as the solid phase When the bottom of the draft tube was located precisely at the transltlon between the conical and cylmdncal regions, no hqmd circulation was observed When the draft tube was located m a lower posltlon, hqmd circulation started and its magmtude increased as the draft tube was further lowered mslde the conical region The results presented indicate that the hydrodynanuc behavlour of these systems can be effectively controlled by varying the axial position of the draft tube

Two dimensional Darcy and Ergun models are used to describe the hydrodynamics within the annulus of a spouted bed equipped with a draft tube. Experimental pressure and stream function data for water spouting are shown to be in good agreement with theoretical predictions. Verification of a general spout-annulus interfacial boundary condition is also established and it is shown that the entry region below the draft tube functions as a classical spouted bed. In addition, an experimental procedure for determining the location of the spout-annulus interface is presented. Fluid residence time distributions are calculated for a few cases of practical interest. Deux modeles dimensionnels de Darcy et d'Ergun sont utilists pour dCcrire I'hydrodynamique dans I'espace annulaire d'un lit jaillissant muni d'un tube de circulation. On montre que les donnkes experimentales sur la pression et la fonction de courant pour le jaillissement d'eau concordent bien avec les predictions theoriques. La verification d'une condition limite interfaciale jet-espace annulaire gCnCrale est Cgalement effectude et I'on montre que la region d'entrie au-dessous du tube de circulation fonctionne comme un lit jaillissant classique. En outre, on prksente une methode expirimentale pour determiner la localisation de I'interface jet-espace annulaire. Les distributions de temps de skjour des fluides sont calcultes pour quelques cas d'inttret pratique.

Over the last three years, work in the Particle Coating Laboratory at West Virginia University has focused on three main areas. Ž. The first area concerns the reversible agglomeration of cement to produce a granular product 2-10 mm that can be transported easily and can be broken down and hydrated to form a cement slurry with properties identical to virgin cement. This agglomeration process uses Ž. Ž. a binding agent consisting of calcium chloride CC and tartaric acid TA dissolved in methanol that can be considered an inert solvent. Ž. Ž. By adjusting the proportions of the cement set accelerating agent CC and the retarding agent TA a granular cement product can be formed that gives a cement slurry with essentially the same characteristics as that obtained from virgin cement. The resulting concrete also has the same compressive strength, obtained in a standard 3-day test, as virgin cement. Ž. The second research area concerns the formation of encapsulated brittle particles of ammonium persulfate AP that are used as viscosity breaking agents for fracturing fluids. In order to obtain a coat that under goes brittle fracture when subjected to a compressive Ž. load, a coating of a cross-linked acrylate polymer containing up to 80 wt.% of fine-15 mm silica was used. By varying the coating Ž. level of acrylate, the release of the ammonium persulfate using a standard leach test can be reduced to acceptably low levels-3%. By changing the fraction of silica in the coat, the release of the ammonium persulfate when the particles are subjected to a known Ž. compressive stress 13.8 MPa can be increased to approximately 70%. The particles formed by this process comprise of agglomerates of between 10 and 20 individually coated particles. When subjected to an applied load, these agglomerates fracture and the coating on the individual particles is sheared away thus releasing AP. These particles can be used as viscosity breaking agents in drilling well fracturing operations. The third project consists of the video imaging of particle movement in a semicircular fluidized bed typically used in coating operations. The particles of interest are 8-mm-diameter tablets. The technique used to capture particle velocity data utilizes two CCD cameras that are synchronized to capture images that are between 1 and 5 ms apart. The mapping of particle velocity within the spray region in the draft tube insert under a variety of conditions is currently underway. Preliminary data is presented and discussed.

This work deals with the design and application of a neuro-fuzzy controller of the magma density of the fine crystals for the stabilisation of the crystal size distribution (CSD) of the product from an MSMPR (Mixed Suspension, Mixed Product Removal) continuous crystallizer. The cooling crystallization of potassium sulphate from aqueous solutions in a pilot-scale stirred vessel was investigated. A control strategy, based on the elimination of fines by using a combined sedimentation-dissolution device, is presented. The control scheme was successfully applied to a pilot-scale draft-tube (DT) crystallizer and yielded promising results. The proposed process control system allowed the reduction of the fines fraction by more than 30 %, while maintaining operation stability and short transient responses.

The investigation of the effect of salinity on the performance of airlift contactor was achieved using the 17 l internal loop airlift with height of 1.2 m, and 0.137 m diameter. Various draft tubes with different diameter sizes were provided to vary the ratio between downcomer and riser cross-sectional areas (A d /A r) from 0.061 to 1.01. The superficial gas velocity (u sg) was supplied in a range from 0.01 to 0.07 m/s and the salinity was adjusted from 0 to 45 ppt. The Sauter mean diameter of the bubble (d Bs) appeared to be smaller in saline water than in fresh water. Bubble size was regulated by two factors. The first one was the hydrophilic repulsive force which inhibited bubble coalescence whereas the second was the Laplace pressure which controlled the coalescence and breakup of bubbles. The range of pressure difference, P, acting on the bubble that promoted bubble coalescence was between 15 and 20 N/m 2 below which bubble coalescence was inhibited and above which bubble breakage prevailed. In saline water, d Bs decreased with u sg. This was caused by the collision and breakup of bubbles at high gas holdup which occurred at P greater than 20 N/m 2. Axial variation in d Bs was only observed at low u sg (less than 0.04 m/s) where bubbles in the bottom section of the airlift were larger than those in the middle and top sections. It was anticipated that the middle and top sections exhibited higher turbulent conditions than the bottom section at this low aeration rate. The effect of draft tube size was quite important where the smallest draft tube (smallest downcomer area) best promoted the breakup of the bubbles with a relatively high P of approximately 50-97 N/m 2. The effect of salinity on the overall volumetric mass transfer coefficient (k L a) was only apparent at high aeration rate where the fresh water provided a higher k L a than the saline water. In fact, the specific area (a) was high in the saline water systems, however, the mass transfer coefficient (k L) was higher in the fresh water system than saline water. Finally, a general correlation for the estimation of k L in the airlift system was proposed.

The vortex-ingesting stirred tank reactor uses a combination of a helical screw impeller and concentric draft-tube to entrain and recycle headspace gas and disperse it as bubbles in a liquid reaction solvent. This study presents an experimental characterisation of a 0.29 m diameter vortex-ingesting gas-liquid stirred reactor, operated with solids mass fractions from 0 to 25%, over a range of specific power inputs up to 3 W/kg and for a variety of tank geometries. As was found in previous studies the most important geometric variable is the draft-tube top clearance, since this has the most direct effect on the gas ingestion rate. The presence of low solids fractions causes a significant increase in the minimum speed required for vortex ingestion of the gas. At low impeller speeds and low solids mass fractions, accumulation of particles in the annulus surrounding the draft-tube reduces the pumping capacity of the impeller. This mechanism also reduces the gas holdup and gas-liquid mass transfer coefficient, although not always in a monotonic fashion. In this design of reactor, aeration results in only a small decrease in the specific power input and so the just-suspended condition is not much affected by the presence of gas.

Fed-batch culture of Bacillus thuringiensis in a modified airlift reactor has been developed by using adaptive control of glucose concentration in the reactor. The glucose concentration was estimated via a correlation equation between carbon dioxide production rate and glucose consumption rate. The estimated glucose concentration as the output variable was fed back to computer for calculation of substrate addition. The modified reactor was an airlift reactor with a net draft tube. The airlift reactor had high oxygen transfer rate and low shear stress which were important factors for production of thuringiensin. Fed-batch culture of Bacillus thuringiensis in the modified airlift reactor provided significant improvement of thuringiensin production.

The vector form o f the Ergun equation was used to numerically predict gas phase streamlines, residence time distributions, chemical conversions and pressure drops in the annulus o f a side-outlet spouted bed with an internal draft tube. By varying the position o f the side-outlet, it was demonstrated that there exists an optimum location for this outlet at which conversion is a maximum and which shows a greatly improved gas phase residence time distribution in the annulus compared with a conventional spouted bed unit. On a utilise la forrne vectorielle de I'equation d'Ergun afin de predire numeriquement les lignes de courants de la phase gazeuse, les distributions de temps de sejour, les conversions chimiques et les pertes de charge dans I'espace annulaire d'un lit jaillissant a sortie laterale et muni d'un tube d'aspiration interne. En variant la position de la sortie laterale, il a ete demontre qu'il existe un emplacement optimal pour cette sortie auquel la conversion est maximum et qui montre une distribution de temps de sejour de la phase gazeuse dans I'espace annulaire grandement amelioree comparativement a un systeme a lit jaillissant classique.

Internal loop reactors and bubble columns are used extensively in the bio-and petrochemical industry. Simulation of these reactors has traditionally been based on dispersion and cell type models. For design and scaleup purposes however. these models are not adequate and more detailed knowledge of the flow structure within the reactors is necessary. This paper presents the results obtained from a two phase fluiddynamic model and how these results compare with experimental data. At ISCRE 11 a two fluid model based on a first principles approach was presented. To describe turbulence the single.phase k-E model was used. In this work a modified turbulence model based on the k-E structure is used. The model takes into account the turbulent kinetic energy and dissipation caused by interaction between the gas and liquid phases. Furthermore the basic two fluid model has been modified to take into account terms arising from the volume fraction fluctuations including pressure gradients. For experimental verification local gas fractions have been measured using the five point conductivity method. The system investigated is air/water. Measurements have been performed both for an ordinary bubble column and an internal loop reactor. both with largest inner diameter 0.288m. Furthermore the model results for axial liquid turbulent intensity and axial liquid velocity profiles are compared with experimental data available in the literature for an ordinary bubble column. The new turbulence model is shown to be a significant improvement over the single phase one previously used.

The present study summarizes results of mixing characteristics in a draft tube airlift bioreactor using ERT. This technique offers the possibility for noninvasive and nonintrusive visualization of flow fields in the bioreactor and has rarely been utilized previously to analyze operating parameters and mixing characteristics in this type of bioreactors. Several operating parameters and geometric characteristics were examined. In general,

Two-and three-phase hydrodynanuc studies were conducted m an air lift bubble column with an internal loop The bubble column had a conical dlstnbutor at the bottom, through which the gas was sparged, creating a hqmd or slurry circulation from the draft tube to the annular region The axial position of the draft tube was vaned with respect to the conical section The expenments were performed with the air-water system at ambient condltlons, usmg sihceous sand as the solid phase When the bottom of the draft tube was located precisely at the transltlon between the conical and cylmdncal regions, no hqmd circulation was observed When the draft tube was located m a lower posltlon, hqmd circulation started and its magmtude increased as the draft tube was further lowered mslde the conical region The results presented indicate that the hydrodynanuc behavlour of these systems can be effectively controlled by varying the axial position of the draft tube

A model has been built to predict the evolution of sand drying in a conical spouted bed with a non-porous draft tube. Three regions have been considered in the model, i.e., spout, annulus and fountain, and unsteady-state mass balances have been written for water in the solid and gaseous phases. The model has been validated by comparing its results with the experimental ones obtained in a previous study and it allows predicting the moisture content evolution of both the air and the sand during the drying process.

Current and impending legislation for public health and environmental compliance regarding effluent are becoming very stringent. For unrestricted domestic effluent reuse and environmental friendly disposal, the need for advanced treatment processes has attracted great attention during the last decade. Membrane-based treatment technologies for low quality water have exhibited improved performance as well as a reduction of operating and maintenance expenses. Membrane technology also provides a preferable alternative for wastewater reclamation in small and isolated communities. This study presents a long term study of an airlift membrane bioreactor (MBR) for domestic wastewater treatment using a hollow fibre membrane module of ZW-10 under ambient desert conditions (Kiryat Sde-Boker, Israel). In this system, the crossflow velocity across the membrane surface was induced by a cylindrical draft tube to reduce the membrane fouling. Under optimal operating conditions, the system was stable and efficient without external chemical cleaning during the experimental period. The system produces a high quality permeate, and the removal efficiencies of COD and BOD 5 were around 94% and 99%, respectively. Nitrate concentration in the effluent (permeate) was higher (by 40%) than that in the reactor mixed liquor, probably due to extra nitrification carried out by Nitrospira bacteria on the membrane biofilm. The system shows potential and promising applications for localized, decentralized sewage treatment facilities.

Baffle Digester Draft tube Gas-lift Mixing a b s t r a c t Computational fluid dynamics simulations were used to study the effect of bottom configuration and a hanging baffle on the mixing inside a gas-lift digester filled with non-Newtonian sludge. The Navier-Stokes and continuity equations were solved numerically using commercially available finite element method-based solver. The results from this simulation were found to be in good agreement with previously reported experimental findings. At a gas recirculation rate of 84.96 l/h, the poorly mixed zones inside a flat bottom digester were about 33.6% of the digester volume, while in the case of digesters with 251 and 451 conical bottoms poorly mixed zones were about 31.9% and 29.6%, respectively. The power law viscosity index, n, did not have a significant effect on the mixing pattern under the conditions studied. Introduction of a hanging baffle in combination with a 451 hopper bottom resulted in reduction of the poorly mixed zone by a factor of three compared to a flat bottom without baffle configuration. Although the introduction of a hanging baffle was able to significantly reduce the size of the poorly mixed zones inside a gas-lift digester, further optimization of the digester geometry may lead to additional improvements.

An experimental investigation has been carried out in order to evaluate the detection of cavitation in actual hydraulic turbines. The methodology is based on the analysis of structural vibrations, acoustic emissions and hydrodynamic pressures measured in the machine. The proposed techniques have been checked in real prototypes suffering from different types of cavitation. In particular, one Kaplan, two Francis and one Pump-Turbine have been investigated in the field. Additionally, one Francis located in a laboratory has also been tested.

It has already been proved that the soft polyurethane foam particles are very effective carriers for immobilizing fungi and/or mycelia. However, the classic three phase fluid&d bed bioreactor was not effective enough for the production of antibiotics like penicillin due to the lower value of the mass transfer rate of oxygen. In order to improve this parameter, we introduced a draft tube in the reactor thus obtaining a gas-liquid-solid draft tube spouted bed bioreactor. The effect of both design and operational parameters on the mass transfer rate of oxygen was studied experimentally. It was shown that the volumetric mass transfer rate of oxygen in the new bioreactor was increased between 1.5 and 3 times as compared to a conventional three phase fluidixed bed. Indications for scaling-up of the bioreactor for industrial use were also obtained.

Current and impending legislation for public health and environmental compliance regarding effluent are becoming very stringent. For unrestricted domestic effluent reuse and environmental friendly disposal, the need for advanced treatment processes has attracted great attention during the last decade. Membrane-based treatment technologies for low quality water have exhibited improved performance as well as a reduction of operating and maintenance expenses. Membrane technology also provides a preferable alternative for wastewater reclamation in small and isolated communities. This study presents a long term study of an airlift membrane bioreactor (MBR) for domestic wastewater treatment using a hollow fibre membrane module of ZW-10 under ambient desert conditions (Kiryat Sde-Boker, Israel). In this system, the crossflow velocity across the membrane surface was induced by a cylindrical draft tube to reduce the membrane fouling. Under optimal operating conditions, the system was stable and efficient without external chemical cleaning during the experimental period. The system produces a high quality permeate, and the removal efficiencies of COD and BOD 5 were around 94% and 99%, respectively. Nitrate concentration in the effluent (permeate) was higher (by 40%) than that in the reactor mixed liquor, probably due to extra nitrification carried out by Nitrospira bacteria on the membrane biofilm. The system shows potential and promising applications for localized, decentralized sewage treatment facilities.