Week 6 - CHT Analysis on a Graphics card

AIM

To simulate a steady state flow over a given graphic card model by varying the inlet velocity of air as 1 m/s, 3 m/s and 5 m/s and perform the conjugate heat transfer analysis on the given graphic card model.

OBJECTIVES

1. Carry out the simulation by varying the velocity from 1 m/sec to 5 m/sec for at least 3 velocities and discuss the results.
2. Run the simulation for best possible mesh with combination of coarse and refined mesh in different regions. Explain the reason for choosing the particular mesh settings.
3. Find out the maximum temperature and heat transfer coefficient attained by the processor, by using the appropriate materials of your choice for the simulation.
4. Prove that the simulation has achieved convergence with appropriate images and plots.
5. Identify potential hotspots on the model.

INTRODUCTION

We are going to simulate the flow over a given graphic card model and perform the CHT analysis on the graphic card. In real world application, the processor in the graphic card generates heat which is reduced by the flow of air over the processor by use of the cooling fan present in some computers. Also, In order to increase the heat transfer rate of the processor with the surrounding air flowing over it, fins are placed above the processor. Fins are nothing but a extended surface which increase the heat transfer rate by means of convection. So, we are going to provide the heat source to the processor from which the heat is dissipated to the air flowing around it and also perform the CHT analysis for different cases by varying the inlet velocity of air as 1m/s, 3m/s and 5m/s and compare the results.

THEORY

Conjugate heat transfer :

Conjugate heat transfer is a type of heat transfer analysis between solids and fluids. This type of heat transfer includes both convection (between fluids) and conductive (between solids) heat transfer, as well as both forced and natural convection. Common applications and use cases of this thermal simulation type include electronics cooling, heat exchangers, industrial machinery, heaters, some AEC cases, and more.

Conjugate heat transfer is defined as the heat transfer between two domains by exchange of thermal energy. For a system the thermal energy available is defined by its temperature and the movement of thermal energy is defined by its heat flux through the outer walls. Heat transfer in solid happens through conduction and through walls by convection and in liquid phase through convection.

CHT provides the temperature prediction and the hotspot regions at the solid-fluid interface and we can also predict the heat transfer accurately for example- Conduction through solids, convection through fluid and thermal radiation. It also provides the velocity and pressure distribution of fluid moving inside the solid. We can also use CHT in design optimization for improvement for heat transfer and cooling capacity.

Advantages of CHT analysis

1. The Conjugate Heat Transfer setup replaces the empirical relation of proportionality of heat flux to temperature difference with heat transfer coefficient. The wall heat transfer coefficients & their local variations are directly computed within the model and do not involve the solving of complex empirical relations. Thus the simulations are carried out much faster.
2. As long as the boundary conditions are accurately defined, the solutions are highly accurate due to the simplicity of the CHT model.
3. There is no limit for the number of surfaces and hence can be applied for a large number of regions in a given assembly.
4. Since the CHT model is a combination of the heat transfers in both a solid & fluid model, it can be applied for all combinations of interactions (i.e. Solid - Fluid interaction, Solid - Solid interaction and Fluid - Fluid interaction)

Application of CHT Analysis

CHT analysis is used in various applications to understand the heat transfer between two domains (solid and water or for more than two sub-domain). Some example:-

1. Heat transfer analysis in the cooled turbine blade Wang.
2. Measurement of the heat transfer coefficient in a Channel with Pin Fins.
3. Analysis of a Thermocouple Sensor in a Low-Temperature Nitrogen Gas Ambient.
4. Analysis of Natural Convection and Radiation Heat Transfer from Various Shaped Thin Fin-Arrays Placed on a Horizontal Plate.
5. Conjugate heat transfer and coupled field analysis is aerospace engineering.
6. Conjugate-Heat-Transfer Immersed Boundary Method for Turbine Cooling.
7. Thermal Treatment of Materials.

Graphics card

video card (also called a graphics card, display card, graphics adapter, or display adapter) is an expansion card which generates a feed of output images to a display devices (such as a computer monitor).Most video cards are not limited to a 6 inch simple output. Their integrated graphics processor can perform additional processing, removing this task from the central processor of the computer.

Conjugate heat transfer

MODE OF HEAT TRANSFER

Conduction

Conduction is a mode of heat transfer which generally occurs in solid due to temperature difference associated with molecular lattice vibrational energy transfer and also by free electron transfer. The reason behind all electrically good conductors are also in general good conductors of heat is that the presence of abundant number of free electrons. Example: ALL METALS

Fourier Law Of Conduction

The law states that the rate of heat transfer by conduction along a given direction is directly proportional to the temperature gradient along that direction and is also directly proportional to the area of heat transfer lying perpendicular to the direction of heat transfer.

Convection

Convection is a mode of heat transfer which generally occurs between a solid body and the surrounding fluid due to temperature difference associated with macroscopic bulk motion of fluid transporting thermal energy.( in the form of enthalpy).

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.

Newtons Law Of Cooling

The law states that the rate of heat transfer by convection between a solid body and the surrounding fluid is directly proportional to the temperature difference between them and is also directly proportional to the area of contact between them.

Radiation

Radiation is the mode of heat transfer which does not require any material medium for its propagation and hence occurs by electromagnetic wave propagation travelling with the speed of light.

Radiation mode of heat transfer completely predominates over conduction and convection particularly when the temperature difference is sufficiently large.

Stefan Boltzman Law

The law states that the radiation energy emitted from the surface of a black body per unit time per unit area is directly proportional to the fourth power of the absolute temperature of black body.

FINS

Fins are projection protruding from the hot surface in to ambient fluid atmospheric air and they are meant for increasing the heat transfer rate by increasing surface area of heat transfer. It is a conduction convection system employed to dissipate heat from a surface at a faster rate to the surrounding fluid.

Example:

1. Air cooled internal combustion engine.
2. Reciprocating Air compressor/ positive displacement type of compressor
3. Electric motors and transformers
4. Electronic devices
5. Automobile Radiator

Fins are always used only when the convective heat transfer coefficient are relatively low i,e, with atmospheric air.

CHT Analysis On Graphics Card

All the heat is generated in the processor so for cooling the surface here different fins are present at the top of the processor. Heat is generated at the processor it will conducted throughout the fins the whole purpose of adding the fins here to increase the surface area, hence when air will pass over the fins it will going to get cooled so due to which we can operate a processor at optimum temperature.

Heat Transfer Co-Efficient :

Heat transfer co-efficient defines the rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference. It refers to how well heat is conducted through a series of resistant mediums.

The general formula of the heat transfer coefficient is:

Where,

q = Local heat flux density (W/ m²)

h = Heat transfer coefficient (W/m2⋅K)

ΔT = Temperature difference (K)

Power density

Power density is the amount of heat generation rate per unit volume.

It can be written as,

SOLVING AND MODELLING APPROACH

• Here, we run the simulation for Six different cases. The first three cases is for performing the inlet velocity study with inlet velocity of 1m/s, 3m/s & 5m/s . Then, choose the best cooling velocity for performing the Mesh independence study with mesh sizes of 8mm, 6mm, 4mm &2mm.
• The K-omega SST turbulence model is selected for this simulation, since the CHT analysis is mainly focussed over the graphic card region for which the K- omega SST will give more accurate results for the near wall conditions
• We take the processor as the heat source. Assume the heat generation rate by the processor as 1 Watts,and the volume of the processor can be determined by measuring its dimensions in spaceclaim,

Hence the power density of the processor can be written as,

This value should be given under cell zone conditions of the processor which is taken as the heat source for this simulation.

Materials and its properties for different components of the graphic card :

For Fins, the material chosen is aluminium

Density = 2719 kg/m³

Cp (Specific Heat) = 871 J/(kg K)

Thermal Conductivity = 202.4 W/(m K)

For Processor, the material is taken as silicon

Density = 2330 kg/m³

Cp (Specific Heat) = 703 J/(kg K)

Thermal Conductivity = 153 W/(m K)

For PCB(Printed circuit board), the material is polystyrene

Density = 55 kg/m³

Cp (Specific Heat) = 1210 J/(kg K)

Thermal Conductivity = 0.027 W/(m K)

PRE PROCESSING AND SOLVER SETTING

In our challenge we will create a flow simulation of air around a graphics card. In general I will be explaining only one case and posting the screenshots of the other cases.

Case 1

Mesh size - 8mm

Fluid - air, 1 m/s

• Open spaceclaim from the Ansys fluent flow system in workbench. Import the given graphic card model into spaceclaim and measure the dimensions of the processor using measure tool. This dimensions can be used to find the volume of the processor which can be used in determining the power density of the processor.

• Cross sectional view of the graphic card model which is enclosed within its fluid domain.

• Use share topology option under workbench menu bar. This will help to create the conformal mesh by sharing common node points between any intermediate surfaces.

• Open the mesh window from Ansys fluent flow and give necessary names to different components of the model.

• Now, generate the mesh. This will generate the base line mesh for the model with 8mm element size. Body sizing of 1mm mesh is used for the graphics card.

• After the meshing and face naming are done move on to Fluent Solver. In the fluent launcher select double precision, display mesh after reading and give the appropriate solver processors and GPUs.

• For this simulation, K omega SST turbulence model is preferred since the CHT analysis is done near to the wall surface area of the graphic card model.

• Since the chosen materials for different components of the graphic card were not available in the fluent database, those materials need to be created. This can be done by copying any of the existing material present in fluent database and giving the new material name with its properties to it.

• Now, assigning required cell zone conditions for the components.
• For PCB (Printed circuit boards), board is selected

• For Fins, aluminium is selected.

• For Processor, Silicon is selected. Here, the processor is the heat source from where the heat is being generated. So, select the processor as the source term and give the required value for power density which was calculated before.

• For the inlet boundary condition, select velocity inlet and give the velocity as 3 m/s and also provide the temperature as 300K.

• Create report definitions for average temperature , maximum temperature and heat transfer coefficient over the processor

• After the physics part is done move to the solution part. Initialise the simulation first. click Initialize to initialize the boundary conditions. Hybrid method is selected before initialization. Click on Autosave to obtain a animation of the flow in the post results.

• Then run the calculation for a given number of iterations till the convergence is obtained. Depending upon the number of elements and model selected the time required for convergence varies. In the CFD module itself we can compute, measure, plot, animate, etc., if needed.
• After the solutions and calculations move to the result module to get different graphs, plots, contours, animations etc.,. Sectional views can be created if required.

RESULTS

Case 1

Mesh size - 8mm

Fluid - air, 1 m/s

Residual

Temperature Contour of graphics card

Temperature contour of cut sectional plane

Velocity contour of cut sectional plane

Velocity vector of cut sectional plane

Average temperature at processor surface

Heat transfer coefficient at processor surface

Maximum temperature at processor surface

Case 2

Mesh size - 8mm

Fluid - air, 3 m/s

Residual

Temperature Contour of graphics card

Temperature contour of cut sectional plane

Velocity contour of cut sectional plane

Velocity vector of cut sectional plane

Average temperature at processor surface

Heat transfer coefficient at processor surface

Maximum temperature at processor surface

Case 3

Mesh size - 8mm

Fluid - air, 5 m/s

Residual

Temperature Contour of graphics card

Temperature contour of cut sectional plane

Velocity contour of cut sectional plane

Velocity vector of cut sectional plane

Average temperature at processor surface

Heat transfer coefficient at processor surface

Maximum temperature at processor surface

Case 4

Mesh size - 6mm

Fluid - air, 5 m/s

Residual

Temperature Contour of graphics card

Temperature contour of cut sectional plane

Velocity contour of cut sectional plane

Velocity vector of cut sectional plane

Average temperature at processor surface

Heat transfer coefficient at processor surface

Maximum temperature at processor surface

Case 5

Mesh size - 4mm

Fluid - air, 5 m/s

Residual

Temperature Contour of graphics card

Temperature contour of cut sectional plane

Velocity contour of cut sectional plane

Velocity vector of cut sectional plane

Average temperature at processor surface

Heat transfer coefficient at processor surface

Maximum temperature at processor surface

Case 6

Mesh size - 2mm

Fluid - air, 5 m/s

Residual

Temperature Contour of graphics card

Temperature contour of cut sectional plane

Velocity contour of cut sectional plane

Velocity vector of cut sectional plane

Average temperature at processor surface

Heat transfer coefficient at processor surface

Maximum temperature at processor surface

Average temperature, maximum temperature and heat transfer coefficient of the processor is found out for each cases and are tabulated as above. Each of the study is discussed separately for better understanding.

VELOCITY STUDY

In this study we are changing the velocity used in the computation. Three different inlet velocity (1,3,5m/s) conditions are used in the study. So there are a total of 3 cases for the comparative study. All the parameters are kept the same and the mesh size is taken as 8mm .

It is noted that as the velocity increases the heat transfer coefficient also increases. Which means more cooling of the battery pack giving us a much lower maximum and average temperature processor. So we can come to a conclusion that the efficiency of cooling increases as the inlet velocity increases and in this case 5m/s has the highest heat transfer coefficient.

MESH INDEPENDENCE STUDY

For the mesh independence study we have taken 8mm as the starting mesh size and decreased the mesh size in a step of two. So there are a total of 4 cases 8,6,4,2mm respectively. It can be noticed that the mesh element count increases drastically as the mesh size decreases. More element means more computational time.

In order to get accurate results we have to reduce the mesh element size as we see from the above graph study cases from 8mm to 2mm. Here the mesh size of 4mm has the desirable results as the temperatures are minimum and the heat transfer coefficient is high. Computational time required for 2mm mesh size was also very high compare to others. Since there is not much difference in the results hence we can go with the optimum mesh size of 4mm.

CONCLUSION

• The processor is considered to be the main heat source of the graphics card, which in operation, produces a lot of heat. We have carried out 3 case studies wherein we have increased the air inlet velocity to study the effects of cooling. Here, we can observe from the cases that, as we increase the inlet flow velocity of air, it thereby leads to a reduce in the temperature of the processor. Thereby, an increase in inlet flow velocity leads to more efficient cooling of the processor. The processor can be maintained at optimal temperature by increasing the inlet velocity of air, thereby reducing the chances of damage.
• As we know, the heat transfer coefficient expresses the amount of heat transferred between a fluid and a solid surface by convection. The larger the heat transfer coefficient, the more heat transfer occurs. Another factor that greatly influences the magnitude of the heat transfer coefficient is the fluid flow velocity over the object surface. Therefore, higher the inlet velocity, the larger the heat transfer coefficient.
• Also we can see that, for the case with 5 m/s inlet velocity, for the different mesh element size, the results are varying between the two cases. Since the refined mesh have the higher number of mesh elements, it gives more accurate results compared to the base line mesh. Although the base line mesh takes less computational time compared to refined mesh case, it doesn't provide accurate results. So, the refined mesh is chosen in order to get the best result for this simulation.