Water Wheel CFD Simulation
The Water Wheel is an example of Pelton Wheel turbines. The wheels spin in two different phases of air and water.
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The water wheel is an example of Pelton turbines. Most water wheels are mounted vertically on a horizontal axis, and can also be mounted horizontally on a vertical shaft. The wheels are perpendicular to certain parts of the turbine due to reduced friction force and increased nozzle thrust. The other part of the turbine is out of the water, therefore the wheels are in two different phases of water and the air, rotating around its axis.
Water Wheel CFD Simulation
In order to solve the fluid flow equations, the averaged form of the Navier-Stokes equation is used. For coupling the velocity and pressure equations the Simple algorithm is applied. For momentum and turbulence equations discretization, the first-order method is used. To discrete two-phase volume fraction equations for the exact estimation of the interface between water and air, the Compressive model is applied. The two-phase model also uses is the VOF model.
Physical Properties of the Water Wheel Problem
The turbine’s diameter is 0.7 m and the boundary of the free surface is actually 0.2 m below the center of the water wheel. Water velocity is considered to be 3 to 5 m/s depending on the average river velocity. And accordingly, the rotational speed of the turbine should be calculated with no drag or bump in the flow. In this simulation, the rotational speed of the turbine is 60 rpm.
Water Wheel Geometry and Mesh
The geometry of the turbine is designed by SOLIDWORKS and divided into smaller sections to improve the geometry and mesh of the turbine. The geometry is divided into two general sections of rotary (Rotor) and one stationary section (Stator). The rotor part consists of the turbine and the cylinder that is located around the turbine. The diameter of the cylinder can be between 1.12 and 1.2 times larger than the diameter of the turbine. The stationary part actually surrounds the rotary cylinder.
ICEM software was used to generate the mesh. The rotor section is first meshed, unstructured. Parts such as the turbine leading-edge, use a larger number of meshes (smaller elements), due to the complexity of the flow and the high gradient in that area. The structured mesh is used for the stationary zone. Structured mesh causes the number of mesh to decrease and the quality of the mesh to be very high. After meshing the two parts separately by two different methods, the two parts are coupled together so that the geometry is fully meshed.
The boundary conditions used in the present work for the studied geometry, include the inlet boundary condition defined as Mass-Flow-Inlet and for air and water separately. Also, the Pressure-Outlet boundary condition for both outlets is specified. The symmetry boundary condition defined on the sides and top of the computational domain. The turbine is also introduced as the Wall.
The most important part of boundary conditions in turbomachinery solutions is to specify the Interface surfaces; which we need to connect the rotor and stator surfaces so that they fit perfectly together and have approximately similar meshing.
The rotation of the impeller at each time step can have different values depending on the significance. In this simulation, we have 3 degrees of rotation of the impeller at each time step, which should be considered much smaller for more accurate simulations.
For the present problem, the flow of the water wheel is turbulence and the solution will not produce good results with the laminar flow model application. Due to the nature of the turbulence of the flow around turbine blades and the importance of the turbulence kinetic energy, a turbulence model should be used along with the two-phase models. Choosing the appropriate turbulence model in this process can have a significant impact on the accuracy of the results. In this regard, several types of turbulence models were investigated, among which, considering the advantages and disadvantages of each and the type of flow, the K-Epsilon model of the normalized groups was selected as the best turbulence model. The RNG model yields better results than Standard mode for conditions where the curvature of the pathlines is intense.
For the present issue, we should use the MESH MOTION. In fact, stationary and rotary meshes slide with common boundaries (INTERFACE) and simulate the problem.
All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.