Week 6 - CHT Analysis on a Graphics card
AIM
Perform a steady-state conjugate heat transfer analysis on a model of a graphics card.
OBJECTIVE
1) To learn the procedure for CHT analysis in ANSYS Fluent
2) To understand various meshing concepts and methods
3) To infer various physical parameters from the simulation
INTRODUCTION
A CHT (Conjugate Heat Transfer) analysis is useful where there are multiple modes of heat transfer in our computational domain, like conduction and convection. Having such a great number of applications for this type of simulation makes it important to be studied in depth.
Assumptions:
1) The airflow profile from the inlet is considered to be of uniform value, instead of a profile of different values.
2) Airflow resistance due to the cabinet's back mesh is considered to be 0.
3) The heat-producing chip is small in size compared to real-life graphic cards, so the heat generation capacity is reduced accordingly.
4) The physical constraints to the flow due to side walls are considered to be zero. (The temperature gradient along side walls is found to be negligible, so they are replaced with symmetry boundary conditions)
Input calculations:
1) Inlet velocity calculations:
According to Ref[1], the inlet velocity of the fan reached a maximum of 2 m/s in an experimental study. So, the inlet velocity here is kept to be 2 m/s.
2) Heat generation:
A basic 2Gb graphic card will consume 75 W of energy and 99.9% of it will be given out in the heat (Ref[2]). It will have a GPU of 40 x 40 x 2 mm. Accordingly,
75/(0.04 x 0.04 x 0.002) = 23437500 W/m^3 = 2.3437e7 W/m^3
The GPU chip used here is 8 x 8 x 1 mm, but we can keep the heat generation capacity as same, so the above value is used for calculations.
Materials & their properties: (Ref [3],[4])
1) Fins --> Aluminium
Standard aluminum is available in the Fluent database.
Density = 2719 Kg/m^3
Heat capacity = 871 J/KgK
Thermal conductivity = 202.4 W/mK
2) Processer --> Silicon
Since the silicon is not available in Fluent's solid database, I had to define it
Density = 2000 Kg/m^3
Heat capacity = 710 J/KgK
Thermal conductivity = 150 W/mK
3) PCB --> FR4 (glass-reinforced epoxy laminate material)
Since the FR4 is not available in Fluent's solid database, I had to define it
Density = 1850 Kg/m^3
Heat capacity = 950 J/KgK
Thermal conductivity = 0.29 W/mK
4) Fluid --> Air
Standard air is available in the Fluent database.
Density = 1.225 Kg/m^3
Heat capacity = 1006.43 J/KgK
Thermal conductivity = 0.0242 W/mK
Viscosity = 1.7894e-05 Kg/m-s
GEOMETRY:
it consists of
1. processor
2. PCB
3. fins
MESH
SETUP:
Inlet velocity = 2 m/s
Heat generation rate = 2.3437e7 W/m^3
Side walls --> symmetry
Number of iterations = 500
the solution converged around 325 iterations
RESULTS:
1. temperature contour: (global)
2. temperature distribution (local)
3. temperature distribution
4. temperature streamline
5. temperature volume rendering
6. Processor temperature
The temperature is highest in the region a little back from centre. This is due to more cooling from the front fins.
7. velocity contour
8. temperature - streamline-recirculation region
It is seen that the components behind the fin plate receive a high temperature(38 Deg C) air with low velocity (<0.3 m/s). If any component generates heat over that region, it would not cool down faster. If a component doesn't generate heat, the air would heat that component. So, in either way the heat will stay in that region, causing problems to the components present there. So, the processor must be placed at last in the flow direction.
9. heat flux
1) The heat flux seems to be more from the front end. This is due to the higher temperature differences in solid and fluid. As the air moves forward, it takes less heat, so the region appears blue.
2) The heat flux distribution over a fin is interesting. The corners and centres have a higher contact with air, so they give up more heat.
10. Heat flux - processor
11. wall heat transfer coefficient
CASE:2
1) Mesh:
Global mesh setting of element size = 5 mm
1) PCB body sizing
element size = 1 mm
capture curvature - yes, 18 deg
2) Proc body sizing
element size = 0.5 mm
capture curvature - no
3) Fins body sizing
element size = 0.5 mm
capture curvature - no
Global number of elements = 422289
mesh:
results
1. residual
2. temperature plots:
3. velocity plots:
heat flux:
case: 3
1. GEOMETRY:
In order to refine the mesh and keep the element numbers to a minimum, an enclosure is made surrounding the graphics card. An important step while doing this is selecting all three bodies(fins, processor, PCB) and then applying the enclosure feature in ANSYS Fluent. The outer enclosure is deleted and then created again by selecting the inner enclosure. The above-given image gives information on the dimensions needed to make the enclosures. The dimensions are used based on the lengths of volume surrounding the graphics card where there is a gradient in temperature.
2) Meshing:
Global controls:
Element size = 5 mm
Capture proximity = yes
Proximity size = 0.1 mm
1) Multizone - outer enclosure
2) Body sizing - inner enclosure
Element size = 4 mm
Capture curvature - yes
3) Body sizing = fin body
Element size = 0.4 mm
Behaviour - hard
Capture curvature - no
Capture proximity - no
4) Body sizing - processor body
Element size = 0.4 mm
Behaviour - hard
Capture curvature - no
Capture proximity - no
5) Body sizing - PCB
Element size = 2 mm
Capture curvature - yes
Capture proximity - no
6) Pinch - on that edge
tolerance = 0.1 mm
Element number = 1852356
RESULTS
1. residuals
2. temperature plots:
3. velocity plots:
4. wall heat flux
4. wall heat transfer coefficient
FINAL COMPARISON:
| S.NO. | Max velocity(m/s) | Max temp | max wall heat transfer coefficient | average wall transfer coefficient | number of elements |
| 1 | 2.875 | 53.175 | 331.652 | 133.981 | 201920 |
| 2 | 2.522 | 52.01 | 468.258 | 256.822 | 422289 |
| 3 | 2.368 | 52.292 | 714.442 | 351.972 | 1852356 |
conclusions:
1) The CHT analysis is done for graphic card
2) Effect of various mesh parameters on simulation results is studied.
3) Use of the intermediate meshing method is studied.
referneces:
Air velocity for 25 mm fan = 2 m/s
Material selection