Horizontal Axis Tidal Turbine Performance CFD Simulation (Validation)
The present project validates the “Performance of horizontal axis tidal current turbine by blade configuration” article using ANSYS Fluent.
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The present project simulates the rotational water flow around the blades of the horizontal axis water turbine using ANSYS Fluent software and the result is compared and validated with the results of the article “Performance of horizontal axis tidal current turbine by blade configuration”. There are two areas around the blades for water flow; Thus, an area is considered as a cylindrical shape just around the blades and a rectangular cube area with a larger scale around this cylinder.
The water flow in the outer rectangular cube space travels as a horizontal transfer flow at a velocity of 1 m.s-1 to the body of the water turbine; Therefore, by colliding the water flow to the turbine blades and creating a torque force on the blades, a rotational motion is obtained in the turbine blades, which causes a rotational flow for the surrounding water around blades in the cylindrical region. The frame motion technique is used to simulate the rotation of the turbine blades; Thus, it is assumed that the blades are fixed and the water around the blade rotates relative to the fixed blades.
Therefore, for the cylindrical region, the frame motion mode is defined by defining a rotational speed of 191 rpm around the central horizontal axis of the turbine.
Tidal Turbine Geometry & Mesh
The present model is designed in three dimensions; Thus, the sections related to the turbine blades are in the form of airfoil type S814, the coordinates of the points around its curvature are obtained from the airfoil tools site and the output is taken in the form of a notepad file. Due to the fact that the airfoil section of the blades decreases or increases at different blade sections (at different distances from the central axis of the turbine) by a certain scale (based on the length of the airfoil chord), each airfoil section as a set of points with coordinates are imported and drawn in SOLIDWORKS software at a certain angle and distance from the central axis.
These sections, which include 16 sections, are then imported to the Design Modeler software for integrated blade design. In design modeler software, modeling is done in such a way that for the desired turbine, 3 blades are drawn and in the space around the turbine blades, a special cylinder is created to make a circulating water flow and a rectangular cube space is designed as a space for free water flow. Geometrical information about turbine blades, including the chord size of each airfoil section of the blade and its angle of inclination with respect to the central axis, is presented in Table 3 of the mentioned paper.
The figure below shows the geometry.
The meshing of the model was done using ANSYS Meshing software and the mesh type is unstructured. To increase the accuracy of meshing, the boundary layer mesh is used on the surfaces of the turbine blades and the element number is 4270222. The following figure shows the mesh.
Tidal Turbine CFD Simulation Setting
To simulate the present model, several assumptions are considered:
- We perform a pressure-based solver.
- The simulation is steady. Because the present water turbine is of the horizontal axis type and as a result, time will not affect the hydrodynamic forces.
- The gravity effect on the fluid is ignored.
A summary of the defining steps of the problem and its solution is given in the following table:
|Models (Tidal Turbine)
|Boundary conditions (Tidal Turbine)
|Velocity inlet||Inlet type|
|Pressure outlet||Outlet type|
|0 Pa||gauge pressure|
|stationary wall||all walls|
|Solution Methods (Tidal Turbine)
|Second order upwind||pressure||Spatial discretization|
|Second order upwind||momentum|
|Second order upwind||turbulent kinetic energy|
|Second order upwind||turbulent dissipation rate|
|Initialization (Tidal Turbine)
|-1 m.s-1||velocity (z)|
Paper Results Validation
At the end of the solution process, the amount of turbine power (P) is calculated based on the amount of torque applied to each of the turbine blades (T) and, consequently, the amount of pressure coefficient applied to its blades (Cp) is obtained by the software. Then, it has been compared and validated with similar values in the article. This comparison and validation process is based on the data in Table 2 of the article. In fact, some of the data in the table are considered as input data or reference values and based on them, the final value of torque and pressure coefficient is obtained.
The formulas related to power and pressure coefficient based on the article are as follow, and the comparison of the results of the present CFD work with the results of the paper is presented in the table below.
Turbine Power Formula (tidal turbine)
Formulation of Pressure Coefficient on Turbine Blades
Comparison and Validation with the Article
|design parameters (Tidal Turbine)
||symbol||paper||present CFD Simulation
|estimated power coefficient||Cp||0.4||0.34|
|rated current velocity [m.s-1]||Urated||1||1|
|sea water density [kg.m-3 ]||ρ||1025||1025|
|turbine diameter (m)||D||0.5||0.5|
|angular speed (rpm)||ω||191||191|
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.