Week 6 - CHT Analysis on a Graphics card

Aim: CHT Analysis on a Graphics card using ANSYS FLUENT

Objectives:

1. Run the simulation 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:

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 interfaces between them. Typical applications of this analysis type exist as, but are not limited to, the simulation of heat exchangers, cooling of electronic equipment, and general-purpose cooling and heating systems.

Examples:

HVAC, Heat Exchanger, Tank, Internal Combustion Engine, Heat Sink, Boiler, Reactor, Heat Pipe, Turbocharger.

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

2. 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 convection.

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

1. Convective contribution: Depending on the properties of the fluid and its region of flow, the domination of conduction and convection varies.

2. Viscous effects: 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 effects: When the density becomes related to the temperature, the domain will have different densities at different locations 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 proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model, based on a strictly mathematically stated problem, describes the heat transfer between a body and a fluid flowing over or inside it as a result of the interaction of two objects. The physical processes and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along the body–flow interface, eliminating the need for a heat transfer coefficient. Moreover, it may be calculated using these data.

Applications:

Starting from simple examples in the 1960s, the conjugate heat transfer methods have become a more powerful tool for modelling and investigating natural phenomena and engineering systems in different areas ranging from aerospace and nuclear reactors to thermal goods treatment and food processing, from complex procedures in medicine to atmosphere/ocean thermal interaction in meteorology, and from relatively simple units to multistage, nonlinear processes. A detailed review of more than 100 examples of conjugate modelling selected from a list of 200 early and modern publications shows that conjugate methods are now used extensively in a wide range of applications. Common applications and use cases of this thermal simulation type include electronics cooling, heat exchangers, industrial machinery, some AEC cases, and more. In order to demonstrate best practices for this application type, an electronics cooling application will be used as an example.

Geometry setup:

1. Use Spaceclaim to edit the geometry of the downloaded model of the graphics card which is in the enclosure.

2. Click on share prep to view any edges in interference 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 is easier to find them while setting up fluent.

2. PCB, graphics card and fins are basic name selections which will help us in the simulation.

3. Click generate mesh to create a base mesh. Create face sizing for proper meshing.

4. After creating the mesh, exit Mesh setup to simulate in fluent.

Fluent setup:

Setting up the physics :

Setting up the physics for CHT analysis on a graphics card in ANSYS fluent.

• For this case we are using pressure-based steady state solver
• Velocity formulation - Absolute

Model setup

• Energy
• Viscous - k-omega

Materials :

We are using different materials for all the components-

	Air	Aluminum (Fins)	PCB (Pcb)	Silicon (Processor)
Density (kg/m3)	1.225	2719	55	2330
Specific heat (J/kgK)	1006.43	871	1210	703
Thermal conductivity (W/mK)	0.0242	202.4	0.027	153

Setting up the source terms :

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

Calculation for the Energy source:

Let the processor consume 1 W of power to work.

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

So, Energy produced by the processor is 15625000 w/m^3.

Case I:

Inlet velocity = 1 m/s

Base Mesh size=10 mm

Face sizing= 1mm

Mesh count=184929

Residual plot:

Maximum temperature plot

Avg temperature plot

Heat transfer co-efficient plot

Temperature contour plot

Potential hotspots

Contour plot of wall HTC:

Case II:

Part I:

Inlet velocity = 1 m/s

Base Mesh size=9 mm

Face sizing= 0.7mm

Mesh count=365352

Residual plot:

Maximum temperature plot

Heat transfer co-efficient plot

Temperature contour plot

Potential hotspot

Contour plot of wall HTC:

Part II:

Inlet velocity = 3 m/s

Residual plot:

Maximum temperature plot

Heat transfer co-efficient plot

Temperature contour plot

Potential hotspot

Contour plot of wall HTC:

Part III:

Inlet velocity = 5 m/s

Residual plot:

Maximum temperature plot

Heat transfer co-efficient plot

Temperature contour plot

Potential hotspots

Contour plot of wall HTC:

Conclusion:

	Max Tempr (K)	Avg Tempr (K)	HTC
1m/s (Base mesh)	327.15	327.04	-7.11
1 m/s (Refined mesh)	327.24	327.10	-7.43
3 m/s (Refined mesh)	314.06	313.96	-15.04
5 m/s (Refined mesh)	310.31	310.22	-19.77

A converged solution for the given sets of input was observed on the basis of repetitive plots in the Residual plot. The processor acted as a source of heat input and thus one of the hotspots for such study. With an increase in velocity, we saw a considerable drop in temperature and a substantial 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 hotspots 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 throughout the component. We see a few high-temperature zones on fin away from the 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 that the solution has 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 co-efficient but the coarse mesh heat transfer for fins and other components in the graphics card has the same values. Hence refinement of mesh is done for fins and base of the fin and base of the graphic card in order to calculate the wall heat transfer coefficient 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 airflow. Conduction occurs where there is a contact region between the solids in this case it is between processor and fins and processor and base. Convection occurs when the heat generated in the components of the graphic card is removed by the inlet airflow. The maximum temperature in the components is inversely proportional to the inlet airflow velocity. With the increase in the airflow velocity, the heat removal process by convection is faster. And also recirculation of air (vortex) at appropriate places will increase the heat transfer efficiencies hence greater amount of heat is removed by convection.