Week 10 - MRF project

Aim

The objective of the project is to create a MRF model by importing the model to Ansys Icepak and setup the physics & solve the thermal model.

Moving Reference Frame

The Moving reference frame approach is a steady state method used in CFD to model problems with rotating parts. The MRF is a moving/sliding mesh technique which can deal with strong interactions between the moving volume and the surrounding stationary volume. The volumetric region of mesh cells is created around the rotating body during the meshing. 

Geometry

The MRF tutorial.tzr file is unpack in the Icepak and the fluid region around the fan is created.

Model

Fluid A

Geom - Cylinder.

Type- fluid.

Plane - Y-Z.

Xc - 141.3 mm. 

Yc - 133.04 mm.

Zc - 130.06 mm.

Height - 20.

Radius - 34.

iRadius - 0.

Properties

The select use rotation to MRF option & Rotation - 6000rpm.

Fluid B

Geom - Cylinder.

Type- fluid.

Plane - Y-Z.

Xc - 141.3 mm. 

Yc - 133.04 mm.

Zc - 56.06 mm.

Height - 20.

Radius - 34.

iRadius - 0.

Properties

The select use rotation to MRF option & Rotation - 6000rpm.

Cabinet : It creates a fluid region around the model for which the governing equations are solved.

i) Geometry 

Shape - Prism.

Specified By - Start/length.

Start Xs - 232.6 mm. 

Start Ys - 84.84 mm.

Start Zs - -1.6 mm.

End XL - 405.3 mm.

End YL - 96.52 mm.

End ZL - 183.3 mm.

ii) Properties - The wall type for the cabinet is defined.

Min X - Wall.

Max X - Wall.

Min Y - Wall.

Max Y - Wall.

Min Z - default.

Max Z - Wall.

Assemble - 1

Select all the blocks and click on create --> assembly.

Under the meshing tab choose Mesh separately,

Multi level meshing

The assembly for the other 

• Macros --> Productivity --> Validation --> Automatic case check tool - It checks the assembly interface between the varies objects and assembly.    

Mesh Control

Cut plane along Z

Cut plane along Y

Cut plane along X

The multi level mesh is used to capture the fan blades with more accuracy.

Surface Mesh of fan blades

All the features of the fan have been captured with the Multilevel meshing of level 3.

Mesh Verification

Face alignment

• It is the measure of mesh quality defined by face alignment index = C0C1.f
• C0 & C1 are the centroids of two adjacent elements are normal vector to the face between the two elements.
• Range of face alignment 0 (bad) to 1 (good) & value greater than 0.1 (preferred) & value greater than 0.15 provide better result.

The min value of the model is 0.3523 which is ideal for the results.

Volume

• Preferable volume of mesh to be greater than 1e-13 for single precision solver & greater than 1e-15 for double precision.
• Max / Min cell volume should be less than 10^7 for single precision & must be less than 10^12 for double precicion since very small volume create divergence of the solution.

The volume of the mesh greater than 10^-13 & Max / Min cell ratio is around 4e6 which is in required range & the single precision can be used for this model. 

Skewness

It determines how close to the ideal & based on the equilateral volume. The value greater than 0.5 provide good cell quality.

The min value of the skewness is 0.2053 so only ideal element are formed.

Solver

The three dimensional steady state Navier stokes equations for the model are solved within the computational domain.

General Setup

• Variables solved - flow & temperature.
• Radiation - Off.
• Flow regime - Turbulent - The realizable two equation for the turbulent model.                 

The defaults value of the parameters are set to default.

Transient Setup

• Time variation - Steady

Solution Initialization

X velocity - 0.

Y velocity - 0.

Z velocity -0.

Temperature - ambient.

Turbulent kinetic energy - 1 m2/s2.

Turbulent dissipation rate - 1m2/s3.

Basic settings

No of iterations - 1000.

Convergence criteria

Flow - 1e-5.

Energy - 1e-7.

Joule heating - 1e-7.

Turbulent kinetic energy - 1e-7.

Turbulent dissipation rate - 1e-7.

Parallel Settings

Configuration - parallel.

Parallel options - 4 processors.

Advance Solver Setup

Results

The residuals for the following equations are plotted against the number of iterations:

• Continuity equation.
• X-momentum equation.
• Y-momentum equation.
• Z-momentum equation.
• Energy equation.

From the residual plot the solution has converged around 800 iterations.

Monitor points

These are probes set at the various location in the computational domain to determine the temperature variable. The temperature has reached the converged state around 600 iterations.

The velocity has reached the converged state around 400 iterations.

Results

Temperature along X plane

Temperature along Y plane

Temperature along Z plane

Velocity Plot

Particle tracer

The fan is rotated in clockwise direction and reverse flow occurs in the model & swirl effect is captured in the model

Conclusion

In this project the moving reference frame modelling enables for the more accurate prediction of the flow. The three dimensional steady-state governing equations for the model were solved for flow and temperature within the computational domain in the Icepak. 

• The maximum temperature within the computational domain is 64.662 deg.C.
• The maximum air velocity within the computational domain is 21.355 m/s.