Conjugate Heat Transfer Analysis on a graphics card.

Aim:

• To Perform a steady-state conjugate heat transfer analysis on a model of a graphics card.
• To Run the simulation for best possible mesh with combination of coarse and refined mesh in different regions.

Objective:

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.

CHT Analysis -

1. CHT stands for Conjugate Heat Transfer.
2. This type of analysis allows the simulation of heat transfer between solids and fluids boundaries by exchanging thermal energy at the interfaces between them.
3. CHT analysis has an advantage over the FE thermal analysis in calculating the wall heat transfer coefficient. It is calculated directly rather than solving empirical equations.
4. CHT analysis is used to correctly predict the surface temperature and heat exchanging capabilities of engineering devices.

Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas, in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers. Forced convection is the most common way to achieve a high heat transfer rate. In some applications, the performances are further improved by combining convection with phase change (for example liquid water to vapor phase change).

The CHT approach has an advantage over FE thermal Analysis in that wall heat transfer coefficients and their local variations on surfaces are directly calculated within the model rather than based on simplified empirical calculations. The CHT approach, therefore, has benefits for those applications where heat transfer is either non-uniform or difficult to calculate empirically.

Typical design problems involve the determination of:

• Overall heat transfer coefficient, e.g. for a car radiator.
• The highest (or lowest) temperature in a system, e.g. in a gas turbine, chemical reaction vessels, food ovens.
• Temperature distribution (related to thermal stress), e.g. in the walls of a spacecraft.
• Temperature response in time-dependent heating/cooling problems, e.g. engine cooling, or how fast does a car heat up in the sun and how is it affected by the shape of the windshield.

Benefits :

1. Effective Heat transfer
2. Energy savings
3. Interactions between solid and fluid bodies.

Applications:

1. Heat exchangers
2. electronic equipment like graphic cards, motherboards, etc
3. General heating and cooling systems.
4. turbochargers
5. Exhaust ports.

Inlet :- Velocity of air=5m/s

Temperature = 300K

Graphic card:

The main component of graphic cards are:

• Processor
• Fins
• Base

Processor

A GPU(graphics processing unit) is designed specifically for performing the complex mathematical and geometric calculations that are necessary for graphics rendering. Some of the fastest GPUs have more transistors than the average CPU. A GPU produces a lot of heat, so it is usually located under a heat sink or a fan.

Fins

Fins act as a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device temperature at optimal levels.

Aluminum alloys are the most common heat sink material. This is because of aluminum cost lesser than copper. However, copper is used where higher levels of thermal conductivity are needed. Some hear sink use a combination of aluminum fins with copper base.

Base

The base is defined as a part on which various component is attached. The base is the part where processor, fins and other connections are connected to it.

Geometry:

The geometry is setup in Ansys Space claim. The model for the same is shown below. This model is enclosed in an enclosure.

Using the share topology feature in the space claim, all the components viz the PCB, the processor and the fins, and the enclosure are shared together so as to get the conformal mesh. The conformality of mesh is very important to see the heat transfer effects at the surface of the solid boundary.

Case setup :-

Solver time = steady state

Solver type = pressure based

Viscous model = k-epsilon and energy equation is solved

Adding materials to the components of graphic card

Base = copper

fins = aluminium

processor = gold

CASE :1: Base line mesh

Residuals:

Temperature distribution in processor:

Temperature distribution in base:

Temperature distribution in fins:

Velocity distribution :

Above plot the airflow over the graphic card, inlet air velocity is 5m/s and temperature at 300k but we see an increase in outlet temperature because the air helps to diffuse the temperature over the graphic card where temperature developed by the heat source. So that the temperature of air increases.

Wall heat transfer coefficient distribution:

The heat transfer taking place from solid to fluid. Where the fluid passes over the graphic card it takes heat from the solid. The given picture shows that, heat transfer coefficient on the wall. As we can see that the wall heat transfer coefficient is higher at places where velocity is higher. As the mesh is not fine enough, it isn't able to capture accurately.

Case 2(1): Refined mesh:

Inlet velocity:1m/s

temperature: 300k

Residuals:

Temperature contour:

Velocity Distribution:

Case 2(2): Refined mesh:

Inlet velocity:3m/s

temperature: 300k

Residuals:

Temperature contour:

Velocity Distribution:

Case 2(3): Refined mesh:

Inlet velocity:5m/s

temperature: 300k

Residuals:

Temperature distribution in processor:

Temperature distribution in Base:

Temperature distribution in fins:

Velocity distribution :

Above plot the airflow over the graphic card, inlet air velocity is 4m/s and temperature at 300k but we see an increase in outlet temperature because the air helps to diffuse the temperature over the graphic card where temperature developed by the heat source. So that the temperature of air increases.

Wall heat transfer coefficient distribution:

The heat transfer taking place from solid to fluid. Where the fluid passes over the graphic card it takes heat from the solid. The given picture shows that, heat transfer coefficient on the wall. As we can see that the wall heat transfer coefficient is higher at places where velocity is higher.

CONCLUSION:

maximum temperature and heat transfer coefficient attained by the processor.

	Number of elements	Max Temp (K)
Baseline	84390	390.341
Refined(5m/s)	483489	399.518

As the number of elements increased the more accurate results of temperature distribution on the components were obtained and thereby enhanced values of maximum temperature

From the temperature contours, we can say see that the temperature on the fins is approximately equal to to the temperature of the processor. This is an ideal behaviour for the graphic card since fins takes most of the heat generated by the processor. Usually the surface area of the fins is larger which improves the cooling of the graphic card.

The potential hotspots on the model are processor, fins and the contact region between processor and base.