Conjugate Heat Transfer Analysis on a graphics card.

Aim - To perform a steady-state conjugate heat transfer analysis on a model of a graphics card and determine the maximum temperature attained by the processor and heat transfer coefficients at appropriate areas as well as potential hotspots on the model.

Conjugate Heat Transfer analysis is useful where there are multiple modes of heat transfer in our computational domain, like conduction and convection. It provides a clear insight into the velocity distribution and mechanism of heat transfer of coolant.

Assumptions :

1. The power output of the processor is assumed as 20 W.

2. The processor is the only heat source in the Graphics Card.

3. The volume of the processor is (8*8*1) mm.

4. The processor is in idle condition.

Heat generation:

A basic 2Gb graphic card will consume 25 W of energy and 80% of it (20W) will be given out in the heat (Ref[2]).

Accordingly,

20/(0.008 x 0.008 x 0.001) = 312500000 W/m^3 = 3.125e8 W/m^3

The materials and the properties selected for each component is shown below: (Ref[2])

Component	Material	density ( kg/m3)	Cp (j/kg-k)	k (w/m-k)
Processor	Silicon	2000	710	150
Fins	Aluminum	2719	871	202.4
PCB	FR-4	1850	950	0.29

Running the simulation with a combination of coarse and refined mesh in different regions. By varying the velocity from 1m/sec to 5m/sec

Coarse Mesh -

Geometry

The model is placed in an enclosure.

This is done as the flow is simulated around the model. Shared topology is applied on the entire setup to create a conformal mesh where the surfaces share a common face.

Meshing -

Global mesh setting of element size = 10 mm

1) PCB body sizing
element size = 2 mm

2) Processor and small Components body sizing
element size = 0.5 mm

3) fins body sizing
element size = 1 mm

Nodes-29293

Elements-163910

Setup:

Steady-state analysis

Solver - pressure based

Turbulence model - k-e model

Inlet velocity = 2.5 m/s

Heat generation rate = 3.125e9 W/m^3

The solution converged at 125 iterations.

Total Temperature

The max temperature attained by the graphic card processor is 626.967 K.

The highest wall heat transfer coefficient is 803.428 Wm^-2 K^-1.

The velocity behind the fins is near to zero. there is the stagnation of air near the capacitors and memory chips.

there are air recirculating zones behind the fins where the temperature is near around 450 K is leading to an increase in the temperature near the capacitors.

therefore it becomes a hotspot region after the processor.

The average wall heat transfer coefficient is 458.098 Wm^-2 K^-1 at the processor wall which is in contact with the air.

Fine mesh -

Global mesh setting of element size = 8 mm

1) PCB body sizing
element size = 1 mm

2) Processor and small Components body sizing
element size = 0.5 mm

3) fins body sizing
element size = 0.5 mm

Nodes-75938

Elements- 434323

Setup:

Steady-state analysis

Solver - pressure based

Turbulence model - k-e model

Inlet velocity = 2.5 m/s

Heat generation rate = 3.125e9 W/m^3

Result

The solution converged at 150 iterations.

total temperature contour

Temperature Contour, Highest temperature attained is 620.840 K.

The highest wall heat transfer coefficient is 1193.843 Wm^-2 K^-1.

The velocity of air behind the fins is very low.

The temperature of air behind the fins is 621.340K

From the vector and streamline plot recirculating zone are clearly visible

The average wall heat transfer coefficient is 397.469 Wm^-2 K^-1 at the processor wall which is in contact with the air.

INLET VELOCITY 1m/s

Setup:

Steady-state analysis

Solver - pressure based

Turbulence model - k-e model

Inlet velocity = 1 m/s

Heat generation rate = 3.125e9 W/m^3

Result

The solution converged at 180 iterations.

Total Temperature

Temperature Contour, the highest temp attained is 869.332 near the processor.

The highest wall heat transfer coefficient is 1193.840 Wm^-2 K^-1.

The velocity of air behind the fins is very low.

The temperature of air behind the fins is around 600K

The average wall heat transfer coefficient is 397.715 Wm^-2 K^-1 at the processor wall which is in contact with the air.

INLET VELOCITY 5 m/s

Setup:

Steady-state analysis

Solver - pressure based

Turbulence model - k-e model

Inlet velocity = 5 m/s

Heat generation rate = 3.125e9 W/m^3

Result

The solution converged at 200 iterations.

Total Temperature contour

the highest temp attained is 510.90 K near the processor.

The highest wall heat transfer coefficient is 1194.840 Wm^-2 K^-1.

here also, The velocity of air behind the fins is very low.

The temperature of air behind the fins is 492.37 K

The average wall heat transfer coefficient is 398.224 Wm^-2 K^-1 at the processor wall which is in contact with the air.

Conclusion -

• The solution has reached to convergence we can compare it with residual plot and HTC plot, we can also add a temperature plot to check whether the solution becomes steady.
• While checking the temp contour and WHTC contour the area near the processor, Fins, and capacitor are in potential hotspot regions.
• The region behind the fins (where the capacitor and other small components are present) is getting heated up because less amount of air is reaching towards that area and the air is also recirculating in that region and the temperature of the air is also high in that area.
• With the increase in the air velocity, there is a significant drop in temp (near about 100K) but the region behind the fins still getting heated up due to stagnation of air, either we have to make slots in fins to pass the air in that region or we can add another source of wind from the back of the graphic card.