Week 6 - CHT Analysis on a Graphics card

CHT Analysis on a Graphics card

Objectives:-

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.

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Steps in Simulation:

Open Ansys workbench, drag & drop fluid flow (Fluent) onto project schematic. below are steps we need to follow to solve a particular engineering system from CFD standpoint.

1. Geometry: Spaceclaim, it is used for import the geometry & other CAD related functions like geometry clean-up etc.
2. Mesh: used to generate mesh & other mesh control option there, also assign name which helps during BCs setup.
3. Setup: used to define physics of problem, like BCs, material & type of solving method, iterations ect.
4. Solution: where runtime results visualization done.
5. Results: CFD-Post, where post processing of results will be done.

 Geometry Setup-

Import graphics card geometry into spaceclaim.

As shown in above figure, we have PCB, Fins, Processor & enclosure in which graphics card is placed. Processor is the main heat generating component which transfers its heat to the to the PCB & Fins.

Mesh Generation-

Baseline Mesh: Name selection: Inlet, Outlet & Symmetry

Refined Mesh:

Mesh size= 5 mm

Assign body sizing:

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

we know processor generates the heat & it transfer to the fins hence we are refining the processor to get much accurate heat transfer coefficient, temp. values.

Fluent Setup:

Setting up physics:  Select k-epsilon standard turbulent 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= 1.7894e-05 kg/m.s

For aluminium: Density= 2719 kg/m^3

Cell zones:

Assign zones as fluid type for enclosure & solid type for PCB, Fins & processor.

Edit processor & assign energy value as 1e8 w/m^3.

Setting Boundary Condition:

For Inlet:

Velocity= 1,3,5 m/s

Temperature= 300 K

Outlet:

Pressure= 0 Pascal.

Assigning different Flow quantities:

• Create temp. contour
• Standard initialize & run the simulation

CFD Result:

• Check heat transfer coefficient wall.
• Create plane along which temperature, heat transfer coefficient & velocity will be shown.

Results & Plots:

Residuals Plots

Baseline mesh                                                                                                                                                           Refined mesh

  

In Baseline mesh solution is converged at 62 iteration.

Temp. plot at midplane of baseline & refined mesh- For velocity 1 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                   

For velocity 3 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                   

For velocity 5 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                     

Velocity plot for baseline & refined mesh- For velocity 1 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                           

For velocity 3 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                           

For velocity 5 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                           

Wall heat transfer coefficient plot for baseline & refined mesh- For velocity 1 m/s

Baseline mesh                                                                                                                                                           Refined mesh

               

For velocity 3 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                         

For velocity 5 m/s

Baseline mesh                                                                                                                                                           Refined mesh

                 

Potential hotspots on model:

Wall heat transfer coefficient on Processor:

Temp. of Fin:

Temp. of PCB:

Temp. distribution on Graphic card:

Heat generated by the processor get transfer to the Fins & PCB which are air-cooled.

Comparison b/w Baseline & refined Mesh:

For Inlet velocity 1 m/s
Parameters	Baseline Mesh	Refined Mesh
Velocity (m/s)	1.14	1.26
Temperature (K)	380.99	374.39
HTC (w/(m^2)k)	169.13	495.48
cells count	84878	384239
For Inlet velocity 3 m/s
Velocity (m/s)	3.43	4.10
Temperature (K)	349.46	338.46
HTC (w/(m^2)k)	169.13	495.48
cells count	84878	384239
For Inlet velocity 5 m/s
Velocity (m/s)	5.76	6.89
Temperature (K)	339.69	329.14
HTC (w/(m^2)k)	169.13	495.48
cells count	84878	384239

Conclusion:

1. In refined mesh, as velocity increases, the heat transfer rate also increases & hence lowers the temp. at the surface of graphics card. But in baseline mesh the values are extremely random.
2. As cell counts of refined mesh are more, it has greater accuracy & computation time also increased.