Week 6 - 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 Im/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.

Case setup geometry.

1. Click on share topology option when we import the geometry.

2. Click on  enclosure option.

Mesh:

We start the mesh by naming the parts

1.inlet

2.outlet

3.symmetry

4.fins

5.processor

6.base

 A basic mesh is generated using the standard values to obtain the initial solution which help us to determine the location where the mesh requirement is required.

SIMULATION

SETUP

Solver Type                               Pressure-Based

Velocity Formulation                   Absolute

Time                                         Steady

                                                viscous model: K-omega SST

Models                                        Energy model: On

                                                  Fluid type: Air

Material                                       Solid-type: Aluminium

                                                                 : gold

                                                                 : copper

Cell zones                                     Fluid type: Air

                                                    Solid: Al, Au, Cu

Boundaries                                   Inlet: inlet velocity: (1/3/5)m/s

                                                          : temperature = 300K

                                                   Outlet: outlet pressure = 0

Reference values                              Standard

Initialization                                     Standard

Results:

Tempearture contour

Residual plot

Temperature contour

Velocity contour

Heat Transfer Coefficient:

2. Velocity =3m/s

Residual plot

Tempearature contour:

velocity contour:

Heat Transfer Coefficient:

3. Velocity=5m/s

Residual plot:

Temperature contour:

Velocity contour:

Heat transfer coefficient:

CONCLUSION:

Cases                  velocity(m/s)     Total temperature        Total heat transfer coefficient

                                                    of graphic card(K)   

Case1                    1                          426                               536   

case 2                    3                          363                               536

case 3                    5                          349                                536

• 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, buoyance force et, 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 you 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.