Week 6 - CHT Analysis on a Graphics card

Aim: To analyse the conjugate heat transfer on a graphics card.

Problem Statement:

Perform a steady-state conjugate heat transfer analysis on a model on a graphics card . you can use appropriate materials of your choice for the simulation . Make sure to properly define the correct solid and fluid zones. Refer the video for further clarification and the model is provided below the video.

Run the simulation for the best possible mesh with the combination of coarse and refined mesh in different regions. Explain the reason for particular mesh setting.

Objective:

1. Run the simulation by varying the velocity from 1m/sec to 5 m/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 hotpots on the model

Theory Explanation:

Conjugate Heat Transfer :

The Conjugate heat transfer (CHT) analysis type allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interface between them.Typical applications of the analysis type exist as , but are not limited to the simulations of heat ex-changers, Cooling of Electronic Equipment , and General -purpose cooling and heating system.

Examples:Heat ex changers, Tank , IC Engine, Heat sink, Boiler, Reactor, Heat Pipe, Turbo charger.

Heat transfer in a solid- conduction : Diffusion of heat due to temperature gradients in a solid. A measure of the amount of conduction for a given gradient is heat conductivity. Conduction is described by the Fourier’s law defining the conductivity heat flux ,q, proportional to the temperature gradient

Heat transfer in a fluid- Convection:When heat is carried away by moving fluid . the flow can either be caused by external influences, forced convection or by buoyancy forces, natural forces.

Due to fluid motion, There are three contributions to the heat transfer:

1. Convective contribution: Depending up on the properties of the fluid and its region of flow , the domination of conduction and convection varies.
2. Viscous Effect: Due to the fluid flow, the viscosity creates heat ,This effect is often neglected due to its negligible value but should be taken into account for high-velocity cases.
3. Density Effect:when the density becomes related to temperature , the domain will have different densities at different location based on the temperature distribution . This variation in density can cause the compression which in-turn generates the heat.

The contemporary conjugate convective heat transfer model was developed after computers came into wide use in order to substitute the empirical relation of proportionally of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat coefficient since the times of newton. This model based on a strictly mathematical based problem , describe the heat transfer between a body and a fluid flowing over or inside it has a result of intersection of two object. The physical process and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for the solutions at the interface provide the distributions and heat flux along the body -flow interface , eliminating the need for a heat transfer coefficient . moreover it may be calculated using the data.

Applications:

Starting from simple examples in 1960’s the conjugate heat transfer method have become a more powerful tool for modelling and investigating natural phenomenon and engineering systems in different areas ranging from aerospace and nuclear reactor to thermal good treatment and food processing , from complex procedures in medicine to atmosphere / ocean thermal interaction in meteorology , and from relative simple units to multistage , non linear process . A detailed review of more than 100 examples of conjugate modelling selected from the list of 200’s early and modern publications shows that conjugate methods are now used extensively in a wide range of applications.That also been confirmed by numerous results published after this book appearance (2009) that may see from example from web of science.This application in specific areas of conjugate heat transfer at periodic boundary conditions and in ex-changer ducts are considered in two recent books.

Geometry Setup:

1. Use space claim to edit the geometry of the downloaded model of the graphics card which is in the enclosure.
2. Click on share preparation to view any edges in interface so that conformal mesh can be achieved between different components.
3. Share topology has to be enabled between the different components so that necessary information can be shared between different zones of the mesh.

Mesh setup:

1. Before generating a base mesh , let’s name the different components of the graphic card , so it ios easier to find them while setting up fluent.
2. Base processor and fins are basic name selection which will help us in the simulation.
3. Click generate mesh to create a base mesh , body sizing can be applied for the three solid components.
4. After creating body sizing for three components , generate mesh as that it is considered as a base mesh having more than 254791 elements which help us in finding accurate solution
5. After creating mesh exit mesh setup to simulate the fluent

Base Mesh:

Main Enclosure:3.9 mm

Fins size:2 mm

Processor:1.5mm

Base:2mm

Fluent setup:

Setting up Physics:

Setting up physics for CHT analysis on a graphics card in an ansys fluent

For this case we are using pressure -based study state solver

Velocity formulation-absolute

Model setup

Energy

Viscous-k-omega

Materials:

We are using different materials for all the components

For Fluid: Air

Density:1.225 kg/m^3

Viscosity:1.7894e-05 kg/ms

For Fins: Copper

Density:8978 kg/m^3

Specific heat:381 j/kg-k

Thermal conductivity:387.6 w/m-k

For processor: Silica

Density:2328 kg/m^3

Specific heat:710 j/kg-k

Thermal conductivity:150w/m-k

For Base: Polyethylene

Density:940 kg/m^3

Specific heat:1330 j/kg-k

Thermal conductivity:0.52 w/m-k

Setting up the source terms:

For this case we consider that, our processor is a source terms and it generating heat.

So, for setting up the source term go to zones-cell zones-and click on the processor for editing which is in the solid.

After that select the material to gold and click on the source term to enabled it and set the energy source to 1.

Calculate the Energy Source:

Let the processor consume 83 w of power to work

Dimension of the processor:8*8*1mm^3

So energy produced by the processor is 1296875000 w/m^3

Inlet:

Air is used as the fluid.

Velocity=1m/s

Temperature=300k

Outlet:

Pressure=Gauge pressure(0 pa)

Results:

1. Base mesh

Inlet Velocity=1m/s

Plots:

1.Wall HTC and Max temp at the processor

Wall HTC and

2.max temperature of the processor.

Wall HTC at

3.Fins and Base.

Potential

4.Hotpots at fins and base

Wall HTC and

5.max temperature of the graphic card

6.Residual and average temperature plot

Results:

2.Refine mesh

1.Mesh

Main Enclosure:0.005 m

Fins size:0.0005 m

Processor:0.0003 m

Base:0.0006 m

Number of Elements:493884

Number of nodes:86315

Enclosure and Graphic Card:

Processor and Fin Mesh:

Mesh Quality:

A.)Inlet Velocity=1m/s

Plots:

1.Residual and average temperature plot

Wall HTC and

2. Wall HTC and max temperature of the processor.

3. Wall HTC at Fins and Base.

4.Potential Hotpots at fins and base

5. Wall HTC and max temperature of the graphic card

Maximum Temperature:1892 k

Wall HTC :264.8 w/m^2k

B.)Inlet Velocity = 3 m/s

Plots:

1.Residual and average temperature plot

2.Wall HTC and max temperature of the processor.

3.Wall HTC at Fins and Base.

4.Potential Hotpots at fins and base

5.Wall HTC and max temperature of the graphic card

Maximum Temperature:1088 k

Wall HTC :1012 w/m^2k

C.)Inlet Velocity=5m/s

Plots:

1.Residual and average temperature plot

2.Wall HTC and max temperature of the processor.

3.Wall HTC at Fins and Base.

4.Potential Hotpots at fins and base

5.Wall HTC and max temperature of the graphic card

Maximum Temperature:780 k

Wall HTC :1134w/m^2k

Conclusion:

Name	Refined -1m/s	Refined - 3m/s	Refined -5m/s
Max temp processor (k)	1892	1088	780
Wall Heat transfer Co efficient processor (w/m^2K)	264.2	1012	1134

A converged solution for the given sets of input was observed on the basis of repetitive plots or Residual plot. The processor acted as a source of heat input and thus one of the hot-spots for such study.

With an increase in velocity , We saw a considerable drop in temperature and rise in heat transfer coefficient which shows that high velocity air can be an effective mode of heat dissipation in graphics card.

From the temperature distribution , We can see hot-spots in the fin and base near the surface that is in contact with the processor , Based on the thermal conductivity of the material this temperature is distributed accordingly among the component ,We can see a few high-temp zones on Fin away from processor depicting their high susceptibility to heat due to their geometry.

Also the difference in baseline and refined mesh is considerable and thus we can conclude that more refined mesh provides a better solution and can be used in optimizing the design for such applications . From the residual plot, the change in the gradients is very small with which we can conclude the solution has been converged

From the temperature plot of the coarse mesh,The high- temperature points are found at fins , Processor, and Base area near the processor. As the wall heat transfer coefficient is a wall phenomenon and fins dissipate a large amount of heat which results in higher heat transfer coefficient but the coarse mesh heat transfer for fins and other components in the graphics card has the same values. Hence refinement is done for fins, Base of the fins and base of the graphics card in order to calculate the heat transfer co-efficient at the fins and base.

The high-temperature regions are found at the center of the fins , Base , and the processor walls, With further iterations , the temperature is distributed to other regions in the graphics card in the direction of the inlet air flow. The maximum temperature in the components is inversely proportional to the inlet air flow velocity.With in increase in the air flow velocity , the heat removal process by convection is faster.And also recirculate of air ( vortex) at appropriate places will increase the heat transfer efficiency . Hence greater amount of heat is removed by convection.