Week 6 - CHT Analysis on a Graphics card
Aim: CHT analysis on the graphic card.
Objectives:
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 fan reached to a maximum of 2 m/s in experimental study. So, the inlet velocity here is kept to be 2 m/s.
2) Heat generation:
An basic 2Gb graphic card will consume 75 W of energy and 99.9% of it will be given out in heat (Ref[2]). It will have the 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 aluminium is available in 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 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
This report has 3 cases of meshing:
(The geometry and setup is discussed only in case 1 as it remains same for other cases)
2) Mesh settings:
Global mesh setting of element size = 10 mm
1) PCB body sizing
element size = 1 mm
capture curvature - yes, 18 deg
2) Processor body sizing
element size = 1 mm
capture curvature - no
3) fins body sizing
element size = 1 mm
capture curvature - no
The global number of elements = 201920
3) Setup:
Steady state analysis
Solver - pressure based
Turbulence model --> realizable k-e model
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.
4) Results:
(It might appear in some images that contours are plotted on interior, but pay attention if its a shadow or a real interior volume. There seemed to some problem with naming of interiors and walls but the end results are plotted where they should be.)
1) Temperature contour - Global
The max temperature reached by system is 53.175 Deg C. The PCB doesn't conduct the heat to a great extent so it remains around 33 Deg C.
Also, there appears to be a local hotspot in centre of the fin base.It can be seen that fins at the front are cooler than the ones behind.
This happens because the cold air first hits the front fins, it gets heated up and goes to next fin. Hence, the next fins can't give it enough heat as difference in temperature is lower in between solid and fluid.The outer fins are more cooler than inner fins.
This is due to velocity of air passing from sides is more than that of centre.
Heat Flux Plot
The heat flux seems to be more from front end. This is due to higher temperature difference in solid and fluid. As the air moves forward, it takes less heat, so the region appears blue.
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.
Heat Transfer coefficient plot
The heat flux is shown around the walls of processor. It is more from the top because the fins take more heat than the pcb plate.
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.
The temperature is highest at the region little back from centre. This is due to more cooling from the front fins.A potential hotspot is also present below processor, on the PCB. So one must avoid placing components on that place.
The flow has a dead (wake) region behind the fins. It should be seen correctly through a vector plot.There are 3 recirculation regions fromed and their velocity is less than 0.3 m/s. So, it is important to look at flow over this region using streamlines.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.
Final Comparison:
The trend can be observed using values from the table:
Conclusion:
1) The CHT analysis is done for graphic card
2) Effect of various mesh parameters on simulation result is studied.
3) Use of intermediate meshing method is studied.