Axial Fan Design and Simulation

This report presents a comprehensive axial fan analysis using TCAE simulation software.

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TCAE tutorial Axial fan FSI GUI interface

Introduction


This study shows a complex step-by-step analysis of a axial fan from its design to an advanced CFD & FEA simulation, including FSI and modal analysis. The simulation software used for this analysis is TCAE - a comprehensive simulation environment based on open-source. This particular axial fan which is used in this example is completely artificial, however, it was derived from a real existing fan, for which the comparison of the CFD and FEA results with measurement has been made. The goal of this study is to show in detail how to make a comprehensive analysis of the basic axial fan characteristics: efficiency, phi-psi, torque, power, pressure, stress, displacement, modal analysis, and many more.



Axial Fan FEA FSI Displacement Animation GIF


Axial Fan Design

A typical input for a detailed simulation analysis is a watertight (wet) surface model in form of STL surface. For CFD simulation, it is needed to have a closed watertight model (sometimes called waterproof, or model negative, or wet surface) of the fan inner parts where the air flows. For FEA simulation, it is needed to have a closed surface model of solid of the impeller in form of a single one STL surface.

Axial fan FSI general sketch x+ view Axial fan FSI general view Axial fan FSI general sketch y+ view

In general, there are multiple ways how the axial fan model can be created. The CAD model of the axial fan can be generally created in any CAD software manually or in an automated way via parametric model. Or, engineers can use a special dedicated software for turbomachinery design like for example CFturbo, Concepts NREC, or TURBOdesign Suite and create the CAD model and export STL surface. Alternatively, the surface model of a axial fan can be created in an open-source software like for example Salome, FreeCAD, or Open Cascade. In any case, a axial fan can be described with the help of a set of parameters that describe all the important shapes and measures of a fan.

Axial Fan general sketch parameters

CFDSUPPORT has developed a special geometry builder for axial fans, based on Salome, inside TCAE software module TCAD. This axial fan TCAD geometry builder reads the set of parameters and creates the CAD geometry in the STEP format, and extracts the STL surface out of it. STL surface is needed for the CFD & FEA simulation.

For example, according to the sketch, consider the following axial fan rotor (impeller) parameters:

  • axis = "x"
  • flow_direction = [1, 0, 0]
  • flange = True
  • CFD_domain = True
  • periodicity = True
  • fillet = False
  • hubCutIn = False
  • hubCutOut = False
  • onlyCFD = False
  • blade_rot = False
  • axis_pt=[0,0,0]
  • rot_angle = 12
  • n = 16
  • flange_center = [0.0, 0.0, 0.150]
  • Rp = 0.002
  • vp = 0.005
  • Dp = 0.035
  • Hub (-0.15 0 0.150) (0.08 0 0.150)
  • Shroud (-0.15 0 0.270) (0.08 0 0.270)


And following blade points (slices):
TCAE tutorial Blade Point CLoud z view TCAE tutorial Blade Point CLoud general view TCAE tutorial Blade Point CLoud general view


Such combination leads to a complete rotor (impeller) component CFD domain ready for the simulation:

TCAE Axial Fan CFD Simulation Domain Parametric


The rotor and stator components are created in a similar way. The full axial fan geometry would be following:

TCAE tutorial Axial fan FSI benchmark GUI interface


Axial Fan - CFD Preprocessing

At axial machines, the flow field is highly periodic and it is very common to simulate just a single blade periodic segment of the machine. It is very suitable because it drastically reduces the simulation time. Axial fans in the early design stage are typically simulated as periodic. In this case, let's consider a single rotor blade segment (out of 16 blades) with angle 22.5 degrees, and a single stator blade segment (out of 12 blades) with angle 30 degrees.


TCAE TCAD Axial Fan Components Spiral TCAE TCAD Axial Fan Components Impeller


For CFD simulation it is best to split the axial fan into several waterproof components because of rotation (some parts are rotating and some parts are not). Each component consists of a few or multiple STL surfaces. It is smart to split the surface model into multiple surfaces because it opens a wider range of simulation methods (mesh refinements, manipulation, boundary conditions, evaluation of results on model parts, ...). This particular axial fan seems reasonable to be split into two components: Rotor and Stator.


TCAE TCAD Axial Fan Components Rotor TCAE TCAD Axial Fan Components Impeller


The model topology is always up to the user, there are no limitations on the number of components or individual surfaces. In any case, the final model for CFD simulation needs to be split into closed waterproof components. This axial fan is split into two components. First one is called rotor (this component is rotating). The second one is called stator (this component is fixed). Each component consists of individual STL files. Typically, they are the inlet, the outlet, and the wall. For example, within a simplest possible approach, the rotor component can consist just out of three STL surfaces called, for instance:

  • rotor-inlet.stl
  • rotor-outlet.stl
  • rotor-blade.stl
  • rotor-periodic-1.stl
  • rotor-periodic-2.stl
  • rotor-hub.stl
  • rotor-shroud.stl
TCAE TCAD Axial Fan Components Spiral


Or, within a detailed approach, the rotor component can consist out of the following parts:

  • rotor-inlet.stl
  • rotor-outlet.stl
  • rotor-blade-PS.stl
  • rotor-blade-SS.stl
  • rotor-blade-LE.stl
  • rotor-blade-TE.stl
  • rotor-periodic-1.stl
  • rotor-periodic-2.stl
  • rotor-hub.stl
  • rotor-shroud.stl
TCAE TCAD Axial Fan Components Spiral

Such structure of surface files allows much more possibilities for performing high tech simulations (mesh refinements, manipulation, boundary conditions, evaluation of results on model parts, ...). On the other hand, a simple structure if often sufficient and easier for simulation setup. For more details, see the TCAE documentation. Now the CFD model is ready for meshing with TMESH using snappyHexMesh open-source application.


Axial Fan - FEA Preprocessing

For FEA simulation, it is best to create a simple, single one, closed STL surface of the axial impeller solid, for instance rotor-blade_fea.stl. That has already been created with the TCAD geometry builder in the design stage.
TCAE TCAD Axial Fan impeller solid STL
Now the model is ready for meshing with TMESH using NetGen open-source application.

IMPORTANT NOTICE: The surface model has to be clean. The principle is always the same: the watertight surface model has to be created; all the tiny, irrelevant, and problematic model parts must be removed, and all the holes must be sealed up (the watertight surface model is required). The preprocessing phase is an extremely important part of the workflow. It sets all the simulation potential and limits. It should never be underestimated. Mistakes or poor quality engineering in the preprocessing phase, can be hardly compensated later in the simulation phase and postprocessing phase.



Axial Fan - CFD Meshing

The computational mesh for CFD is created in an automated software module TMESH, using the snappyHexMesh open-source application. All the mesh settings are done in the TCAE GUI.

TCAE Axial Fan Mesh GUI Graphical Interface

For each model component, a cartesian block mesh is created (box around the model), as an initial background mesh, that is further refined along with the simulated object. Basic mesh cell size is a cube defined with keyword "background mesh size". The mesh is gradually refined to the model wall. The mesh refinement levels can be easily changed, to obtain the coarser or finer mesh, to better handle the mesh size. Inflation layers can be easily handled if needed. For more details, see the TCAE documentation.

TCAE Axial Fan Mesh for CFD


Axial Fan - FEA Meshing

The computational mesh for FEA is created in an automated software module TMESH, using the NetGen open-source application. All the mesh settings are done in the TCAE GUI.

TCAE Axial Fan FEA Mesh GUI Graphical Interface

The closed STL model is meshed with just a little effort because there are just a few parameters to set. The most important parameters for FEA meshing are "h Max" and "h Min" which mean the maximal and minimal mesh edge in meters. The mesh is created with an automated algorithm. For more details, see the TCAE documentation.

TCAE Axial Fan Mesh for FEA


Axial Fan - CFD Simulation Setup

The CFD simulation is managed with TCAE software module TCFD. Complete CFD simulation setup and run is done in the TCFD GUI in ParaView. TCFD uses OpenFOAM open-source application.

TCAE Axial Fan CFD setup GUI


  • Simulation type: Fan
  • Time management: steady-state
  • Physical model: Incompressible
  • Number of components: 2 [-]
  • Wall roughness: none
  • Physical model: Incompressible
  • Speed: 3000 [RPM]
  • Outlet: Static pressure 0 [m2/s2]
  • Turbulence: RANS
  • Turbulence model: k-omega SST
  • Wall treatment: Wall functions
  • Turbulence intensity: 5%
  • Speedlines: 1 [-]
  • Simulation points: 5 [-]
  • Fluid: Air
  • Reference pressure: 1 [atm]
  • Dynamic viscosity: 1.8 × 10E-5 [Pa⋅s]
  • Air density: 1.2 [kg/m3]
  • CFD CPU Time: 2.0 core.hours/point
  • BladeToBlade: on
Any project simulated in TCFD has its component graph. The component graph shows how the components are organized - the model topology. What is the inlet, the outlet and how the components are connected via interfaces. A simple scheme of the component graph is shown below. The air flow enters the fan in the component suction via interface suction-inlet and leaves the fan from component spiral via interface spiral-outlet. This is a typical example of a very simple linear order of the flow through a combination of three components.

TCAE Axial Fan component graph




Axial Fan - FEA Simulation Setup

The FEA simulation is managed with TCAE software module TFEA. Complete FEA simulation setup and run is done in the TFEA GUI in ParaView. TFEA uses Calculix open-source application.

TCAE Axial Fan CFD setup GUI


  • Beam material: steel
  • Material density: 7800 kg/m3
  • Material structure: isotropic
  • Young modulus: 2.1E11 Pa
  • Poisson ratio: 0.3
  • Fixed radius: 100 [mm]
  • Finite element order: second
  • FEA CPU Time: 0.02 core.hours/point


Axial Fan - TCAE Simulation run


The TCAE simulation run is completely automated. The whole workflow can be run by a single click in the GUI, or the whole process can be run in the batch mode on a background. Modules used are TCAD, TMESH, TCFD, and TFEA. The simulation is executed in the steady-state mode, for five volumetric flow rate values of 7.48793, 6.54555, 5.78475, 4.98318, and 4.5046032 m3/s resulting from special "Outlet Vent" boundary condition. TCFD includes a built-in post-processing module that automatically evaluates all the required quantities, such as efficiency, torque, forces, force coefficients, flow rates, pressure, velocity, and much more. All these quantities are evaluated throughout the simulation run, and all the important data is summarized in an HTML report, which can be updated anytime during the simulation, for every run. All the simulation data are also saved in tabulated .csv files for further evaluation. TCFD is capable of writing the results down at any time during the simulation. The convergence of basic quantities and integral quantities is monitored still during the simulation run. The geometry was created onetime using TCAD in the preprocessing phase. First, the TMESH is executed to create the volume meshes for CFD & FEA. Then the CFD simulation is executed and evaluated. After that, in the FSI step, the pressure field is integrated to create the force field which is prescribed as a load for the FEA simulation. Finally, the FEA simulation is executed and evaluated.

TCAE simulation run interface GUI




Axial Fan - Postprocessing - Integral Results

The simulation results are evaluated automatically. Every simulation run in TCAE has its own unique simulation report. The integral results both for CFD and FEA are written down in the following HTML or PDF reports:

CFD Simulation Report


FEA Simulation Report



TCAE Axial Fan residuals     TCAE Axial Fan phi psi     TCAE Axial Fan efficiency     TCAE Axial Fan efficiency in iterations     TCAE Axial Fan torque     TCAE Axial Fan torque     TCAE Axial Fan axial force     TCAE Axial Fan total pressure     TCAE Axial Fan total pressure profile     TCAE Axial Fan velocity magnitude profile     TCAE Axial Fan volumetric flow rate     TCAE Axial Fan volumetric flow rate    






Axial Fan - Postprocessing - Volume Fields

All the integral results are stored in the .CSV files and are available for further postprocessing if needed. The volume fields are postprocessed in open-source visualization tool ParaView. ParaView provides a wide range of tools and methods for CFD & FEA postprocessing and results' evaluation. There are available countless useful filters and sources, for example: Calculator, Contour, CLip, Slice, Threshold, Glyph (Vectors), Streamtraces (Streamlines), and many others.

Axial Fan CFD slice total pressure




Axial Fan CFD streamtraces


Axial Fan FEA impeller stress


Axial Fan FEA impeller displacement

Axial Fan FEA impeller stress     Axial Fan FEA impeller displacement




Axial Fan - Turbo Postprocessing

Meridional Average

For turbomachinery engineers, it is typically important to see the results, for example, total pressure or velocity, circumferentially averaged and projected on the meridian plane. This method is called the Meridional Average. This meridional average projection avoids the holes (blades) and shows how the total pressure (energy) or velocity are distributed along the meridian (a 2D interpretation of flow through the fan).

Axial Fan CFD FUll View     Axial Fan CFD FUll View     Axial Fan CFD FUll View     Axial Fan CFD FUll View     Axial Fan CFD FUll View


Blade to Blade View

Blade to blade view is a special transformation method that transforms the rotational object (and the CFD results) into the dimensionless hexahedron of edges 2phi x 1 x 1. The blade-to-blade view offers a unique perspective for an inspection of the flow between the blades on a plane at a fixed relative distance between hub and shroud surfaces. Especially, leading and trailing edges (flow angles) are of the main interest here.

Axial Fan CFD Relative Velocity Blade to Blade View Axial Fan CFD Relative Velocity Blade to Blade View Axial Fan CFD Relative Velocity Blade to Blade View Axial Fan CFD Relative Velocity Blade to Blade View Axial Fan CFD Relative Velocity Blade to Blade View


Results Animation

Below, there is an animation of the rotating axial fan (right-hand side) and the deformation (left-hand side) of the impeller due to the inertia and axial forces and forces from the airflow. FYI: The speed of rotation is approximately 1200 times slower than the real rotation. The deformation movement is displayed 200 times higher to enhance the details.



Axial Fan FEA FSI Stress Animation GIF




Conclusion


• It has been shown how to make a comprehensive CFD & FEA analysis including FSI of the axial fan in a single one automated workflow.

• TCAE showed to be a very well suited tool for CFD, FEA, and FSI engineering simulations.

• More information about TCAE can be found on CFD SUPPORT website: https://www.cfdsupport.com/tcae.html

• Questions will be answered via email info@cfdsupport.com.




References

[1] TCAE Manual
[2] TCAE Training



See also other TCAE simulation examples:

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