Axial Fan CFD analysis

This study shows smooth workflow of performing a complex CFD analysis of an axial fan using TCFD

TCFD Main Page - Download This Tutorial - Fan Results Report

TCFD - Turbomachinery CFD Axial Fan interface

TCFD is automated workflow

CFD SUPPORT introduces the new generation of CFD simulations. TCFD brings an extreme increase of productivity to CFD simulations. TCFD is extremely popular project, because it successfully merged benefits of an open-source (perpetual, unlimited users, jobs, and cores, customizable, ...) with benefits of commercial codes (professional support, well tested, ready for the industry, robust, accurate, automated, GUI, ...).

TCFD - Turbomachinery CFD scheme automated workflow

TCFD is fully automated, it can run the whole workflow by a single command: data input, new case is written down, mesh is created, case is set-up, case is simulated, results are evaluated and the results report is written down. Both GUI and batch mode. Data in - data out. TCFD is mainly focused on supporting the engineers in their real value added work. TCFD is fully automated and the beauty of TCFD is that it is the user who decides how deep to dive into a CFD or not at all. And all the options remain open at the same time.

The input data

This particular fan model is designed in CFturbo®. CFturbo® is a modern, powerful software for interactive design of turbomachinery. It's easy to use and enables the designer to either start from scratch or redesign existing geometries.

CFturbo mixed fan z+ view      CFturbo axial fan  general view      CFturbo  axial fan  z- view

The designed model data are exported from CFturbo. The surface model data in .stl file format together with physical inputs are loaded in TCFD. Other option would be loading an external mesh in OpenFOAM® mesh format, or loading an MSH mesh format (Fluent mesh format). This CFD methodology employs a multi component approach, which means the model is split into a certain number of regions. In TCFD each region can have its own mesh and individual meshes comunicate via interfaces.

CFturbo centrifugal fan meridian view      CFturbo centrifugal fan periodic view

The Mesh

In this particular case, the fan model is split into two components. The Impeller and the Volute. Each component has its own mesh. All the meshes are created automatically for each component within snappyHexMesh. Any number of model components is allowed.

Surface model axial fan z+ view STL model      Surface model axial Blower general view STL model      Surface model axial fan z- view STL model

Periodic segment or not?

The computational mesh can be created for whole impeller as well as for single blade periodic segment of the impeller. Periodic segment approach can save reasonable amount of the mesh cells, which is resulting in reduction of simulation CPU time. The full impeller approch is more robust and also allows the transient simulation on the same mesh.

Mesh Boundary Layer or not?

Another decision to take is to add "boundary layer" or not. Boundary layer is several layers of cells close to the walls to catch the velocity boundary layer. Boundary layer in the mesh usually gives more accurate results, but is paid by higher CPU time. By the experience the mesh with no boundary layer typically over-predicts the total efficiency in the order of 1% - and saves about 40% of CPU time.

Fine Mesh?

In any CFD simulation, there is always big question how fine mesh is needed for ceratin level of CFD results. In rotating machinery there is usually clear trend observed: the finer mesh leads to slightly higher efficiency.

CFD axial fan block rotor impeller    CFD axial Blower stator    CFD axial fan mesh view

Easy to test the mesh sensitivity

In practise the rough mesh with no boundary layer (CPU time: 4 core*hours/single point) can give the same results as the fine mesh with boundary layer (CPU time: 20 core*hour/point). So finally, the rough mesh effect can eliminate the lack of boudary layer. Anyway, with fully automated workflow it is easy to make many tests to callibrate to actual machine.

CFD axial fan mesh background mesh snappyHexMesh   

The component graph

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.

TCFD manifold tcfd tutorial component graph

CFD Simulation Set-up

  • Incompressible flow model
  • Steady-state flow model
  • Medium: Air
  • BEP Pressure ratio: ΔpTot = 1.033 [-]
  • Temperature at inlet: T = 40 [ºC]
  • Viscosity: μ = 1.831e-5 [Pa.s]
  • Rotation speed: 4500 [RPM]
  • BEP Flow Rate: 11520 [m3/h]
  • Interface: mixingInterface (axial averaging)
  • Turbulence Model: k-ω SST
  • Mesh: snappyHexMesh, hexadominant
  • Mesh Cells: 300327
  • Mesh Average y+ (full/segment): 85 [-]
  • CPU time (per point): 1.45 [core.hours]

For more details of CFD Simulation Set-up see TCFD Manual.

Running CFD Simulation

The simulation can be run on any number of parallel processors. Immediately after the simulation is started the user can follow the progress of all the important quantities: flow rates, residuals, efficiency, torque, flow number, pressure number or pressure difference. This run-time functions give the user valuable information of the convergence and also the availability to stop the simulation before its standard end.

CFD fan run time convergence monitor residuals OpenFOAM    CFD Blower run mass flow rate Torque Power    CFD fan Total Pressure Difference

The workflow computes complete characteristics point by point.

CFD fan run time convergence monitor residuals OpenFOAM    CFD Blower Total pressure averaged per interfaces    CFD axial Circumferential angle

Every simulation performed in TCFD has its report in .html format: Axial Fan Simulation Report Example.

Visual Postprocessing using Turbo Blade Post

The simulation results are examined in ParaView (included in any of OpenFOAM distributions). ParaView is CFD postprocessing tool providing all standard features for analyzing CFD data.

CFD axial fan CFD Cull frontface view impeller spiral interface      CFD Axial fan pressure surface cull front face paraview      CFD Axial fan impeller view

CFD Support developed special extension to ParaView for postprocessing rotating machinery: Turbo Blade Post which is special set of filters for ParaView to enable for example blade-to-blade view, or meridional average. Turbo Blade Post detailed manual is available on-line: Turbo Blade Post Manual

CFD Axial Fan Turbo Blade Post Pressure along blade      CFD Axial Fan Turbo Blade Post Unwraped Blade To Blade View

Within Turbo Blade Post the impeller mesh can be unwrapped to be able to slice the computed quantities of the same height along the blade. With such a unwrapped mesh it is also possible to plot quantities around the blade at the certain height.

CFD OpenFOAMCentrifugal Fan  Turbo Blade Post Unwraped Blades      CFD OpenFOAM Axial Fan Turbo Blade Post Pressure along blade      CFD OpenFOAM Axial Fan Turbo Blade Post Pressure along blade

Another Turbo Blade Post function is Meridional Average which creates a meridional plane of circumferential averages of simulated quantities.

CFD Turbo Blade Post Meridional Average Pressure      CFD OpenFOAM Turbo Blade Post Turbo Blade Post Meridional Average Relative Mach number

See also other TCFD simulation examples:

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