Week 6 - CHT Analysis on a Graphics card

CHT ANALYSIS ON A GRAPHICS CARD

AIM: Perform a steady-state conjugate heat transfer analysis on a model of a graphics card. 

OBJECTIVE:

1. Simulate 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.

THEORY:

Conjugate Heat Transfer (CHT) Analysis:

Conjugate Heat Transfer analysis is used for the processes that involve a variety of temperatures concerning solids and fluids, due to thermal interaction between them. A typical example would be the heating or cooling of a solid by the flow of air.

The exchange of heat between two bodies is known as heat transfer and the rate of heat transfer is directly proportional to the temperature difference between them. Heat transfer in solids is usually occurred by conduction whereas in the case of fluids heat transfer occurs by convection. Efficiently combining heat transfers between solids and fluids is the key to design effective coolers, heaters, or heat exchangers. Forced convection is the most common way to achieve higher heat transfers. For some applications, the performance is further improved by combining convection with phase change(for example liquid water to vapor phase change).

Heat transfers in solid and heat transfers in fluids are usually combined in the majority of the applications. This is because fluid flows around solids or between solid walls and usually solids are immersed in the fluids.

An accurate description of parameters such as material properties, flow regimes, geometry configurations, selecting correct models based on the physics to be captured, heat transfer modes near the boundaries is needed for the analysis of temperature fields and heat transfer.

Practical CHT problems are used to determine:

• The maximum and minimum temperature in various parts of the system. eg: combustion chamber, gas turbines, chemical vessels, etc.
• Temperature Distribution is related to the thermal stress over a particular wall or system etc.
• Temperature Response in time-dependent applications such as heating/cooling problem (For example, In the case of cooling a particular system such as engine cooling)
• Overall heat transfer co-efficient. 

This project is divided into 2 parts based on the Mesh ( with a baseline and an upgraded mesh )

In part-1 i'd like to go with the baseline mesh.

1. For base mesh my element size is 7.5 mm and also includes a body sizing for the graphics card which has the size of 1mm also the capture curvature is on to get the results more accurately.

2. Also we have to enable the share topology in the Workbench tool to get rid of the non-conformal mesh.

The geometries in both parts of the project are the same.

GEOMETRY:

Using the given geometry, have to share the topology to reduce the interferences with:

1. Processor

2. Fins

 

3. Base

 

4. Encloser

 

5. Graphic card

MESH:

We start the mesh by naming the parts 

1. Inlet

2. outlet

3. Wall symmetry

4. components

5. Fins

6. Base

7. Processor

My element size is 7.5 mm and also includes a body sizing for the graphics card which has the size of 1mm also the capture curvature is on to get the results more accurately.

Overall we have got the elements count near to 0.2 million which setsup perfectly for a baseline mesh simulation.

SIMULATIONS

SETUP
Solver Type	Pressure-Based
Velocity Formulation	Absolute
Time	Steady
Models	Viscous model: K-omega SST Energy model: On
Material	Fluid type: Air  Solid-type: Aluminium                : silicon                : polystyrene
Cell zones	Fluid type: Air  Solid: fins: aluminium            pcb: polystyrene            processor: silicon
Boundaries	Inlet : inlet velocity: 1m/s         : temperature = 300K Outlet: outlet pressure = 0
Reference values	Area = 0.000032 m^2 Length = 0.008m velocity = 1m/s

Usually, the power consumed by a processor is converted into heat energy. To calculate the heat generated on the processor which is = Power consumed divided by the total volume of the processor

"The power consumption of today's graphics cards has increased a lot. The top models demand between 110 and 270 watts from the power supply; in fact, a powerful graphics card under full load requires as much power as the rest of the components of a PC system combined"

Taking this fact we will assume the power consumed to be 1 (being half of the upgraded graphic card)

Heat generated = 1/64*10^9 =  10^9/64 = 15625000 W/m^3.

Results

1. scaled residuals

2. HTC PLOT

3. maximum temperature

4.velocity contour

5.temperature contour

6.heat conducted on fins from processor

The required values obtained from this simulation is:-

Maximum Temperature = 333.03195 K

Heat Transfer coefficient = -6.236499 W/m^2 K

PART-2

 In part 2 there are total 3 cases with different velocities range from 1m/s to 5m/s to generate different heat transfer coefficient and also different maximum temperature.

Also mesh has been refined for this part where total elements count is 344586 followed by number of nodes which is 63985.

Case-1 - Inlet velocity is 1m/s

Setup

Solver Type	Pressure-Based
Velocity Formulation	Absolute
Time	Steady
Models	Viscous model: K-omega SST Energy model: On
Material	Fluid type: Air  Solid-type: Aluminium                : silicon                : polystyrene
Cell zones	Fluid type: Air  Solid: fins: aluminium           processor: silicon           pcb: polystyrene
Boundaries	Inlet : inlet velocity: 1m/s         : temperature = 300K Outlet: outlet pressure = 0
Reference values	Area = 0.000032m^2 Length = 0.008m velocity = 1m/s
Initialization	Hybrid

Results

1. scaled residuals

2. maximum temperature plot

3. htc plot

4. velocity contour

5. Temperature contour

6.heat conducted on fins from processor

Values obtained are:-

Maximum temperature = 332.66403 K

Heat transfer coefficient = -6.2473026 W/m^2 K

Case-2 - Inlet velocity is 3m/s.

Results:-

1. scaled residuals

2. t_max plot

3. htc plot

4. velocity contour

5. temperature contour

6. heat conducted on fins from processor

Maximum Temperature and Heat Transfer coefficient obtained in this simulation is as follows

T_max = 316.21664 K

HTC = -12,12436 W/m^2 K

Case 3 - Inlet Velocity = 5m/s

Results

1. Scaled residuals

2. T_max plot

3. HTC 

4. velocity contour

5. temperature contour

6. heat conducted on fins from processor

Maximum Temperature and Heat Transfer coefficient obtained in this simulation is as follows

T_max = 311.96414 K

HTC = -16.790102 W/m^2 K

CONCLUSION:

CASES	VELOCITY(m/s)	TOTAL TEMPERATURE OF GRAPHICS CARD(K)	TOTAL HEAT TRANSFER COEFF ON GRAPHICS CARD (W/m^2K)
CASE 1	1	332.66403	-6.2473
CASE 2	3	316.21664	-12.124
CASE 3	5	311.96414	-16.790
BASELINE MESH (part 2)	1	333.03195	-6.236499

• Higher velocity in a particular region means a higher Reynolds number. That’s why the heat transfer coefficient in that area is highest, which means the maximum amount of heat transfer is taking place from fluid to the solid in that region. This also results in a higher temperature of solid in that location.
• Heat transfer coefficient is a function of properties of the fluid at film temperature, temperature difference, temperature coefficient, , hence, mesh resolution near walls usually has the biggest impact on numerical results.
• For the turbulent models, we have to take care of the assumptions and the boundary conditions we put in the program. The mesh is a very important parameter to get nice results so we have to refine it in the important place of our drawing.
• Some factors that should be considered which the most significant is the turbulent boundary layer. If we are using any kind of low Reynolds turbulence model considering enhanced wall treatment (as your K-w SST model), so we need to make sure that we stay in the laminar sub-layer region where velocity profiles are assumed to be laminar and viscous stress dominates the wall shear which means
• Mesh/ grid dependency test is very important to get confidence over results.