Conjugate Heat Transfer Analysis on a graphics card using Ansys Fluent

Conjugate Heat Transfer Analysis on a graphics card

Objective:

1. Run the simulation by varying the velocity from 1m/sec to 5m/sec for at least 3 velocities and discuss the results.
2. Find out the maximum temperature and heat transfer coefficient attained by the processor.
3. Prove that the simulation has achieved convergence with appropriate images and plots.
4. Identify potential hotspots on the model.

Steps in Simulation:

Open Ansys Workbench. Drag and drop Fluid Flow (Fluent) onto Project Schematic.

Here, we will see Steps we need to follow to solve a particular engineering system from a CFD standpoint.

1. Geometry: Spaceclaim, where we import the geometry.
2. Mesh: Ansys Mesh, where Boundary naming, element sizing and mesh generation with required grid size will be done.
3. Setup: Ansys Fluent, where actual case setup will be done.
4. Solution: Ansys Fluent, where runtime results visualization done.
5. Results: CFD-Post, where Post Processing of Results will be done.

CAD Cleanup:

Import Graphics card geometry into Spaceclaim, Spaceclaim is CAD cleanup software that can be used to prepare CAD models for simulation setup. It could be for CFD or FEA.

As we can see above figure is the Solid model. We have Printed Circuit Board, Fins, Processor and Enclosure in which the graphics card is placed. Processor is the main Heat generating component which transfers its heat to the PCB and Fins.

Mesh Generation

Baseline Mesh:

• Name Selection: Inlet, Outlet and Symmetry

Refined Mesh:

Mesh size=5mm

Assign Body sizing:

PCB- element size=1mm, Fins-element size =0.5mm, Processor-element size 0.5mm

As the Processor generates the heat and it transfer to the fins hence we are refining the Processor to get much accurate Heat transfer coefficient, temperature values.

Fluent Setup:

Setting up physics: Select k-epsilon Standard terbulent model

Solver: Pressure-based.

Assign Time as Steady-State Solver.

Material Selection:

For Fluid- air

For solid- aluminium.

Properties of Material:

For Air: Density = 1.22 kg/m^3

Viscosity = 0.0000181 kg/m.s

For Aluminium: Density= 2710kg/m^3

Cell zones:

Assign zones as Fluid type for Enclosure and Solid type for PCB, Fins and Processor.

Edit Processor and assign Energy value as 1e08 w/m^3.

Setting Boundary Condition:

For Inlet:

Velocity =1,3, 5m/s

Temperature = 300kelvin

Outlet:

Pressure= 0 Pascal. (Ambient Pressure)

Assigning different Flow quantities:

• Create Temperature contour
• Standard Initialize and run the Simulation

CFD Result:

• Check heat Transfer Coefficient at the wall.
• Create plane along which temperature, Heat transfer coefficient and velocity will be shown.

Results and Plots:

Residuals Plots

Baseline Mesh Refined mesh

In Baseline Mesh the simulation has achieved convergence at 375 iterations. In Refined Mesh the simulation has achieved convergence at 750 iterations.

Temperature Plot at the midplane of Baseline and Refined Mesh:

For inlet velocity of 1m/s:

Baseline Mesh Refined mesh

For inlet velocity of 3m/s:

For inlet velocity of 5m/s:

Velocity Plot of Baseline and Refined Mesh:

For inlet velocity of 1m/s:

Baseline Mesh Refined mesh

For inlet velocity of 3m/s:

Baseline Mesh Refined mesh

For inlet velocity of 5m/s:

Baseline Mesh Refined mesh

Velocity Plot at midplane of Baseline and Refined Mesh:

For inlet velocity of 1m/s:

Baseline Mesh Refined mesh

For inlet velocity of 3m/s:

Baseline Mesh Refined mesh

For inlet velocity of 5m/s:

Baseline Mesh Refined mesh

Potential hotspots on the model:

Temperature of Processor Wall Heat Transfer Coefficient on Processor

Temperature of Fin Temperature on PCB

Temperature distribution on Graphics card:

Here the Heat generated by the Processor get transfer to the Fins and PCB which are air-cooled.

Comparison between Baseline and refined Mesh:

For Inlet velocity 1m/s

Parameters	Baseline Mesh	Refined Mesh
Velocity (m/s)	1.1526	10.105
Temperaure (Kelvin)	868.75	374.692
Heat Transfer coefficient (w/(m^2)k)	1215.25	589.925
Cells count	86208	3,88,383

For Inlet velocity 3m/s

Parameters	Baseline Mesh	Refined Mesh
Velocity (m/s)	3.54	15.56
Temperaure (Kelvin)	1114.25	338.21
Heat Transfer coefficient (w/(m^2)k)	1210.38	594.13
Cells count	86208	3,88,383

For Inlet velocity 5m/s

Parameters	Baseline Mesh	Refined Mesh
Velocity (m/s)	5.86	18.144
Temperaure (Kelvin)	824.45	329.56
Heat Transfer coefficient (w/(m^2)k)	1210	601.04
Cells count	86208	3,88,383

Conclusion:

• In Refined Mesh, when the Velocity increases, the Heat transfer rate also increases and hence lowers the temperature at the surface of the Graphics card. But in Baseline mesh the values are extremely random and hence the results are highly inaccurate.
• As cell counts of refined Mesh are more, therefore it has greater accuracy, however, Computation time is also increased.