In turbine practice engineering, Draft tube downstream running under extreme water flow pressure and velocity. This is causing a vibrations and pressure variation during different operation frequencies. The practical challenge of obtaining a stabilized water flow is ongoing domain of research. In this paper, a proposition of initiating submerged weir in the downstream of Draft tube reaction turbine is inspected. The main goal of this research is to reduce the water flow pressure variation, velocity and shear distribution in accordance to the upstream water level influence. Two types of turbines including vertical Kaplan and Francis turbine units are examined. ANSYS CFX software tool is used to build three-dimension (3D) numerical models for the Kaplan and Francis turbines with building a submerged weir at the outlet of the Draft tubes at three deferent height suggestions. The influence of the proposed submerged weir is studied the flow through these turbines by considering the dimensions of their components including the penstock with inlets, spiral casing, shafts and blades, and the Draft tube with outlets. The findings of this research were tremendous proposition to solve the problem of negative pressure pulsation in Draft tube of Kaplan and Francis turbines types.

The effect of vortex flow in off-design conditions on the performance of the Draft tube in a horizontal Francis turbine has been investigated numerically.DTo validate the numerical approach, having experimental data in the Draft tube is the immediate objective of this research. Thus, a three-hole pressure probe has been designed and mounted inside the Draft tube for measuring the axial and tangential components of the velocity field. The specific speed of the turbine in the test rig was such that the radial component of the velocity at the inlet of the Draft tube could be neglected, which justifies implementation of the applied probe, especially at the inlet of the Draft tube.The velocity field is measured by traversing the probe at two sections of the inside of the Draft tube. The velocity field data at the inlet of the Draft tube are used as the boundary conditions of the 3D numerical analysis.It is well known that a high intensity vortex causes con-sider able degrees of anisotropy in stress and dissipation tensors, leading to a highly anisotropic eddy viscosity. Thus, all conventional eddy-viscosity-based models will no longer be valid under these conditions and Reynolds stresses in Navier-Stokes equations are modeled by using the RSM formulation.In this study, an emphasis is also placed on the influence of the inlet condition. Numerical results are in fairly good agreement with experimental data. Grid independency is carefully checked in the numerical approach.Several operating points, with different flow rates, under the constant head and rotational speed of the turbine, are investigated. For each case, by applying a numerical simulation at a constant mass flow rate, the pressure recovery factors are calculated in two ways, with and without the circumferential velocity component at the inlet of the Draft tube. These are necessary tools to find the influence of the vortex on the Draft tube efficiency.It is found that the vortex flow has an adverse effect on the performance of the Draft tube and may, consequently, decrease turbine production.

Draft tube of Francis type hydraulic turbine usually consists of: cone, elbow and diffuser. On the contrary, in some power stations an extra pipe should be added to the Draft tube at the bottom of cone because of installation limitation. In this paper, this special case has been numerically studied. To this end CFD analysis was applied to simulate all parts of hydraulic turbine. A homogeneous multiphase model with Rayleigh-Plesset cavitation model was applied for presence of cavitation. The results reveal that the additional tube causes pressure drop and severe cavitation at the trailing edge of runner blades. Also, results showed that the efficiency reduces in comparison with original hill-diagram of model test in which this extension was not considered. With the removal of the extension tube, the efficiency increased significantly. The comparison of pressure recovery factors along Draft tube, and theoretical investigation showed that the height of the Draft tube is an important parameter and addition of an extra pipe will cause reduction in Draft tube performance and increases the probability of occurrence of cavitation under the runner.

The attrition of 300 µ m natural zeolite particles was studied in a laboratory scale Draft tube spouted bed (DTSB) and spout-fluid bed (DTSFB). It has been shown that the attrition rate decreases with time and reaches to an almost constant value. The results show that the prevailing attrition mechanism under the conditions of this work is the surface abrasion which occurs due to the collisions between particles. It has been found that increasing the cone angle from 30º to 60º in the DTSB, causes a decrease in the extent of attrition. In addition, by increasing the spouting air velocity and the height of the entrainment zone in the DTSB, the extent of attrition increases due to a more energetic collision between particles as well as the increased circulation rate of solids. Increasing the auxiliary air velocity in the DTSFB increases the rate of attrition. A comparison between the attrition in the DTSB and DTSFB has been conducted and has indicated that applying the auxiliary air flow causes up to a 6 % increase in the extent of attrition. An empirical correlation is derived for evaluating the extent of the attrition in the DTSB and DTSFB. This empirical correlation is in good agreement with the experimental data.

Introduction: The Draft tube-spouted bed bioreactor with GAC particles immobilized with Pseudomonas syringae is being evaluated to study the effect of suspended biomass and biofilm thickness on the rate of denitrification. Though the biofilm thickness is not directly controlled in wastewater treatment by the diffusion limitation and consequent substrate penetration in the biofilm, biofilm thickness will probably have a significant impact on bacteriological activity. Materials and Methods: The reactor studies were accomplished to study the result of dilution rate on attached biomass, suspended biomass, and biofilm thickness with nitrate reduction under steady-state conditions. A spouted bed reactor with the growth media prepared was used to study the bio-denitrification using Pseudomonas syringae. Results: The study of the attached biomass on nitrate reduction indicated that, as the attached biomass increased from 0.35 g/g to 0.54 g/g at a 0.166/h dilution rate, the nitrate reduction percentage decreased from 98.18% to 88.2%. During the study, it was observed that the biomass and biofilm thickness increased and decreased, respectively, with a rise in influent nitrate concentration and dilution rate. The rise in dilution rate as well as influent nitrate concentration throughout the study increased the rate of suspended biomass. Conclusion: The nitrate reduction rate was high with higher rates of loading in a Draft tube spouted bed bioreactor, due to well-organized recirculation of the solids inside the reactor. The formation of biofilm thickness on solids is a significant character as it increases the nitrate reduction rate to meet the effluent standards.

The Draft tube is one of the main components that integrate a turbine, since it has the function of recovering the residual kinetic energy after the runner by the pressure energy. The search for a Draft tube design that increases the efficiency of the turbine is always an engineering challenge. The hydromechanics components geometry optimization can be accomplished through the integration of optimization methods and CFD tools. In this work, the geometric optimization of a double diffuser Draft tube of a Bulb turbine applied to ultra-low heads is presented, with the objectives of maximizing the pressure recovery coefficient, Cp, and increasing the hydraulic efficiency of the turbine, ηh. These improvements would make it possible to reduce the longitudinal length of the Draft tube, thereby, making an easier insertion of this kind of turbines in water transport systems, with pressures around 3 [mH2O]. The optimization methodology was performed in the meridional plane, using twelve geometric variables in the Draft tube through the integration of optimization methods and computational fluid dynamics. The optimized geometry obtained showed an increase in the Cp value of 0. 71516, from the original geometry, to 0. 83080. The results were extended to the 3D flow analysis, where the optimized turbine showed efficiency gains of 82% to 84%, when compared to the original turbine considering that its total length was reduced and its geometry simplified, resulting in a more compact and versatile equipment. The study also concluded that the applied methodology can be extended to other similar optimization problems in the design of hydraulic machines‎.

Based on previous studies, prediction of flow in the Draft tube, as a sample of complex flow phenomena, has been shown to be of vital importance in modeling hydroturbine flows.In this research, numerical solutions are presented for the turbulent flow through the Draft tube of Masjed-e- Soliman HEPP., a new project constructed during recent years in the south of Iran [5]. The special features of the subject flow, such as rapid curvature, separation, swirl, and vortices require exact physical model investigation and also great challenges in numeric and turbulence modeling aspects.Unfortunately, as the physical measurements provided by the supplier are incomplete, no physical data are available. Thus, using known software (with light background) and grid independence study are two other ways for verification of results. However, this paper demonstrates the capability of CFX-tascflow, a commercial CFD software Widely used for complex flow prediction, and NS3D, a new CFD code developed by LHM. Reynolds-Averaged Navier-Stockes (RANS) equations are solved in both of them and standard k-ε and standard k- w turbulence models are examined In the end, the flow structures are discussed using various graphical post processing results.

The hydraulic turbines, especially Francis turbines, frequently run at part load (PL) conditions to meet the dynamic energy needs. The flow field at the runner exit changes significantly with a change in the operating point. At PL, flow instabilities such as the Rotating Vortex Rope (RVR) form in the Draft tube of the Francis turbine. The present paper compares the features of the velocity and vorticity field of the Francis turbine Draft tube at the best efficiency point (BEP) and PL operations using the Proper Orthogonal Decomposition (POD) of the 2D-PIV data. The POD analysis decomposes the flow field into coherent and incoherent structures describing the spatiotemporal behavior of the flow field. A visual representation of the coherent structures and the turbulent length scales in the flow field is extracted and analyzed for BEP and PL, respectively. The study highlights the salient features of the Draft tube flow field, which differentiate the BEP and PL operation. The fast Fourier transform of the temporal coefficients confirms the presence of RVR frequency (0.29 times the runner frequency) at PL. The phase portraits of different modes elucidate the relationship between different harmonics of the RVR frequency at PL.

For the purpose of automatic generation control (AGC), a portion of the propeller hydro-turbine units in China is adjusted to operate within a restricted range of 75%-85% load using computer-controlled AGC strategies. In engineering applications, it has been observed that when a propeller hydro-turbine unit operates under off-design conditions, a large-scale vortex rope would occur in the Draft tube, leading to significant pressure fluctuations. Injecting air into the Draft tube to reduce the amplitude of pressure fluctuations is a common practice, but its effectiveness has not been proven on propeller hydro-turbine units. In this study, a CFD model of a propeller hydro-turbine was established, and 15 cases with different guide vane openings (GVO, between 31° and 45°) under unsteady conditions were calculated and studied. Two air admission measures were introduced to suppress the vortex rope oscillation in the Draft tube and to mitigate pressure fluctuations. The reason for the additional energy loss due to air admission was then explained by the entropy production theory, and its value was quantified. This study points out that when injecting air, it is necessary to first consider whether the air will obstruct the flow in the Draft tube. Finally, based on simulation and experimental data under various load conditions, pressure fluctuation analysis (based on fast Fourier transform, FFT) was conducted to assess the effectiveness of air admission measures. This study can provide an additional option for balancing unit efficiency and stability when scheduling units using an AGC strategy.