1,720,966 research outputs found
Design of Reliable and Efficient Banki-Type Turbines
Full text linkA new shape for the external surface of the Crossflow turbine blades is proposed, which
allows for the preservation of hydraulic efficiency in spite of a significant maximum blade
thickness providing mechanic robustness and reliability. The final shape of the blades is assessed
using an iterative solution for two uncoupled models: a 2D computational fluid dynamic (CFD)
and a structural 3D finite element method (FEM) analysis of a single blade. Application of the
proposed methodology to the design of a power recovery system (PRS) turbine, a new
backpressure Crossflow-type inline turbine for pressure regulation, and energy production in a
real Sicilian site follows
MAST-RT0 solution of 3D Navier Stokes equations in very irregular domains. Preliminary results in the laminar case
No full textA new numerical methodology to solve the 3D Navier-Stokes equations for incompressible fluids within complex boundaries and unstructured body-fitted tetrahedral mesh is presented and validated with three literature and one real-case tests. We apply a fractional time step procedure where a predictor and a corrector problem are sequentially solved. The predictor step is solved applying the MAST (Marching in Space and Time) procedure, which explicitly handles the non-linear terms in the momentum equations, allowing numerical stability for Courant number greater than one. Correction steps are solved by a Mixed Hybrid Finite Elements discretization that assumes positive distances among tetrahedrons circumcentres. In 3D problems, non-Delaunay meshes are provided by most of the mesh generators. To maintain good matrix properties for non-Delaunay meshes, a continuity equation is integrated over each tetrahedron, but the momentum equations are integrated over clusters of tetrahedrons, such that each external face shared by two clusters belongs to two tetrahedrons whose circumcentres have positive distance. A numerical procedure is proposed to compute the velocities inside clusters with more than one tetrahedron. Model preserves mass balance at the machine error and there is no need to compute pressure at each time iteration, but only at target simulation times
EXTERNAL RECIRCULATION IN PRS TYPE TURBINE: EXPERIMENTAL AND NUMERICAL RESULTS.
No full textCavitation is a relevant phenomenon for structural safety and noise level in hydraulic turbines, occurring when water pressure falls below the vapor value at a given temperature. In this case bubbles of vapor grow inside the liquid and move along with it. When the pressure returns above the vapor value the bubble collapses, and the pressure can locally achieve very high values, up to 7000 bars (Kumar & Saini, 2010). Moreover, if the bubble was confined also by the solid wall of a blade, the solid particles suspended in the fluid can be transported by the fluid ones and hit the solid wall at very high velocity, generating erosion. Cavitation is also the source of high frequency noise, very disturbing for humans.
Cavitation has been extensively studied mainly in reaction turbines, especially Francis and Kaplan types (Luo &Tsujimoto, 2016, Turi et al., 2019; Alligne et al., 2014). Adhikari et al. (2016) carried on a numerical analysis occurring at the tip of the blades of a Crossflow turbine. The conclusion was that, at least in the case study, cavitation occurred only at rotational velocity greater than the design one.
Power Recovery System (PRS) turbine is a reaction turbine with a rotor like the Crossflow one, that can be used instead of the Crossflow in the case of outlet pressure greater than zero. PRS has a very simple design and can provide hydraulic regulation with the use of a mobile flap that can reduce the rotor inlet area. Due to its simplicity and small size, PRS can be easily installed by replacing a short part of the pipeline where the hydraulic jump is available.
In the following paper it is first shown, by means of CFD simulations, that low pressure and cavitation can easily show up in PRS turbines, especially when the downstream outlet pressure is very low, and that pressure attains the minimum value immediately after the tip of the nozzle, according to the rotational direction. The physical explanation is that the momentum Пout of the water volume trapped in the blade channel, immediately after the tip of the nozzle, cannot be balanced by the Пin, inlet one and this leads to strong negative pressure gradients.
A possible countermeasure is the use of external recirculation by means of a small pipe connecting the outlet pipe with a rectangular opening of the case immediately after the tip of the nozzle. The connection leads to a small recirculation flow and to a pressure rise in the target area. The length of the arc between the end of the nozzle and the rectangular opening is a fraction of the arc of the blade channel, so that an inlet momentum is always provided to the blade channel up to the end of the rectangular opening.
The proposed change has been numerically and experimentally tested on a 5kW PRS prototype, installed in the experimental loop of the hydraulic lab of the University of Palermo
Numerical analysis of a new cross-flow type hydraulic turbine for high head and low flow rate
Full text linkCross-flow turbines have recently been proposed for energy recovery in aqueducts when the outlet pressure is greater than zero, owing to their constructive simplicity and good efficiency within a large range of flow rates and head drops. In the case of high head drop (higher than 150 m) and relatively small discharge (lower than 0.2 m3/s), the traditional design of these turbines leads to very small widths of the nozzle and the runner; as a consequence, friction losses grow dramatically and efficiency drops down to very low values. Standard Pelton turbines require zero outlet pressure and cannot be used as alternatives. A new counter-pressure hydraulic turbine for high head and low flow rate, called the High Power Recovery System (H-PRS) is proposed. H-PRS presents a different geometry to reduce friction losses inside the nozzle and the runner by widening the two external walls. Several curved baffles are proposed to guide the fluid particles inside the nozzle and to guarantee the right velocity direction at the inlet surface of the runner. Computational Fluid Dynamics (CFD) 3D transient analyses are carried out to measure H-PRS efficiency for different operating conditions and to compute its characteristic curve for different positions of the regulating flap
MAST-RT0 SOLUTION OF 3D NAVIER STOKES EQUATIONS ON UNSTRUCTURED MESHS. PRELIMINARY RESULTS IN THE LAMINAR CASE
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A NEW SOLVER FOR NON-ISOTHERMAL FLOWS IN NATURAL AND MIXED CONVECTION
No full textMost thermal fluid flow of real-life practical problems fall in the category of low Mach-number or incompressible flow (e.g., industrial flows inside ducts, or around stationary/moving objects, flows in biological/biomedical problems, or atmospheric flows). Several numerical techniques have been proposed for simulation of thermal flows, Finite Difference (FDM), Finite Element (FEM), Finite Volume (FVM) and Lattice Boltzmann (LBM) methods. Unlike the FVMs and FEMs, the classical FDMs show some difficulties in handling irregular geometries. Conventional formulation of FEMs (e.g., Galerkin FEMs) suffers from the lack of local mass balance, recovered by modified formulations (Narasimhan & Witherspoon, 1976). In the Discontinuous Galerkin FEMs (DG-FEMs) high-order accuracy is achieved using discontinuous high-order polynomial approximation within the computational elements. The main drawback of DG-FEMs are the high computational cost and variable storage requirement, compared to the classical FEMs and FVMs. LBMs were originally proposed to simulate weakly compressible flows, and only recently they have been applied to incompressible flows (e.g., Guo et al., 2000).
We present a new FVM solver for the solution of incompressible non-isothermal flows in natural and mixed convection over unstructured triangular meshes. The Incompressible Navier-Stokes Equations (INSEs) are solved along with the Energy Conservation Equation (ECE). Fluid velocity and temperature are coupled in the buoyancy term of the momentum equations, assuming small variations of the fluid density due to temperature, according to the Oberbeck–Boussinesq approximation (e.g., Patel & Chhabra, 2019). Two numerical procedures, recently proposed to solve 1) the INSEs (Aricò et al., 2021) and 2) the transport problem of a passive scalar (Aricò & Tucciarelli, 2007), are adjusted to solve the governing equations of the present problem. Unstructured meshes easily discretize irregular geometries, and do not require interpolation operations between the underlying Cartesian mesh and the irregular boundaries as in the Immersed Boundary methods. Local refinements can be easily performed, avoiding unnecessary mesh refining in large portions of the domain
Numerical and experimental investigation for helical savonius rotor performance improvement using novel blade shapes
No full textWind energy is extensively invested as a renewable and clean energy source. Savonius wind rotor, as an energy converter, has the merit of being appropriate for specific applications. The current paper centers on the performance enhancement of a helical Savonius wind rotor through the blade shape modification. The basic objective is to assess and compare, numerically and experimentally, the performance of two rotors with novel blade shapes named delta bladed and two-stage delta bladed shapes in relation to a helical Savonius rotor. Numerical study was conducted using Ansys Fluent software. 3-D unsteady simulations were carried out through the use of the SST k-ω turbulence model based on the finite volume method solver. Aerodynamic flow and performance characteristics were investigated. Static and dynamic experimental tests were undertaken on a wind tunnel. An improvement in Cp by 22.58 % and 29.5 % for the two-stage delta bladed rotor and 19.35 % and 16.4 % for the delta bladed rotor over the helical Savonius rotor was recorded, respectively, numerically and experimentally. In addition, the self-starting ability as well as the aerodynamic flow characteristics of the helical rotor were enhanced with the novel blade shapes
Energy recovery from rectangular weirs in wastewater treat-ment plants
Full text linkHydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges and small head jumps, are usually screw or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) treat-ment plant, a traditional Kaplan solution is compared with two other ones: a Hydrostatic Pres-sure Machine (HPM) located in the upstream channel and a cross-flow turbine located in a specif-ic underground room downstream the same channel
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