Week 6 - CHT Analysis on a Graphics card

1.Aim:

• To perform the Conjugate heat transfer analysis on a graphics card.

2.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.

3.Introduction:

Simulate fluid flow and assume that the walls of the container were at a constant temperature or even adiabatic. In reality, however, the heat transfer between solids and surrounding fluids couples the temperature field in both materials, and thus your CFD software must be able to account for this phenomenon. CONVERGE features a state-of-the-art conjugate heat transfer (CHT) model for simultaneously predicting heat transfer in both the fluid and solid portions of the domain, which is critical for many applications.

3.1 Conjugate Heat Transfer

• Conjugate heat transfer problem is a coupled, fluid-structure, heat transfer problem where conduction heat transfer in a solid wall interacts with fluid flow and the convection heat transfer in fluid flow interacts at the solid boundary. Fluid network modeling with conjugate heat transfer has many applications.
• The Conjugate Heat Transfer analysis type allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interfaces between them.
• The finite volume formulation requires governing equations to be expressed in conservative form. The rate of change of a conserved property in a given control volume is expressed as the vector sum of transported property from neighboring control volumes together with source or sink terms. The unknown variables in the flow circuit of pressure, temperatures of fluid and solid and flowrate.

3.2 Application

• Wall temperature is a function of flow, a coupled analysis of temperature of solid and fluid is necessary. In cryogenic applications, such as medical and space technology, modeling of conjugate heat transfer is of great importance. In space technology applications, correct prediction of boil-off rate in propellant tanks and chilldown of Hitrogen, Nitrogen and etc.

3.3 Heat transfering methods

• Heat transfer in a solid - conduction: Diffusion of heat due to temperature gradient 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, propotional to the temperature gradient.
• Heat transfer in 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.

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

1. Convection contribution: Depending on the properties of the fluid and its region of flow, the domination 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.

3.4 Steps involves in Ansys Fluent software

1. Geometry: Construct a two or three dimensional representation of the object to be modelled and tested using the work plane co-ordinate system within Ansys.
2. Mesh: At this point understands the makeup of the part. Now define how the modelled system should be broken down into finite pieces.
3. Setup: This case setup will be done using Ansys Fluent.
4. Solution: Ansys Fluent where run time result visualization is done.
5. Results: Post processing of results will be done using CFD post.

4.Procdure:

4.1 Geomentry

1.Open new project, by clicking on Fluid flow(Fluent) in Ansys workbench.

2.Now click on geometry to import Graphics card. It will redirect to Spaceclaim, Ansys geometry model.

Fig: Graphics card with enclosure

Fig: Body Geometry

Fig: Fins Geometry

Fig: Processor

3. Now select the share topology

Fig: Share Topology

4.For a Good quality mesh interference, select workbench > share the entire body.

5.Exit from Spaceclaim and click on mesh to open mesh menu.

4.2 Mesh

6.Select wall face and press right click of mouse, it will open a menu. Click on option create named selection and write suitable name for face. Repeat for other faces and bodies.

7. Now click on mesh option and choose generate mesh.

8. To create a finer mesh. named three different body sizing

• Body sizing 1(Body) = 0.0004 m
• Body sizing 2(Fins) = 0.00035 m
• Body sizing 3(Processor) = 0.0001 m
• Quality of Mesh is 0.0045 m.
• Number of Nodes = 147699
• Number of Element = 256870

Fig: Mesh

Fig: Zooming Mesh

Fig: Body Mesh

Fig: Fin Mesh

Fig: Processor Mesh

Fig: Element matrics

9.Exit from Mesh and click on Setup to open Setup menu

4.3 Setup and Solution

Fig: Graphics Card with Enclosure

10.Exit mesh and open setup. First, Check mesh, it is throwing no error then only we can proceed further. We are going to do pressure based steady state simulation, So choose these options on task page.

Fig: Task page

11. Go to physics menu and select viscous model. It will open a menu, choose realizable K-omega model and close it.

Fig: Viscous model parameters

12.Click on material and choose air as working fluid. check working fluid by clicking on cell zone conditions option, if it Fluid is air & Solid are aluminum, Silica and wood then proceed.

Fig: Fluid Mateials

Fig: Solid materials

13.Define the fluid and solid parts in the geomentry.

Fig: Cell Zone

14.Now go to boundaries option to set boundary conditions. Select inlet x and click on option edit. It will open a menu, Where it is thermal radiation, momentum etc. Put the value of velocity as 1m/s, 2.5m/s and 5 m/s and go to thermal parameter to set temperature as 300k.

Fig: Boundary condition

15. In this problem, Processor emitted the heat, so Give the energy source for convection area.

• Calculated by using this formula Energy source = 1W/voulme (Approximately 1 Watt emitted source)

Fig: Energy Source

16.Now click on intialization to calculate approximate values of velocity and pressure.

17.Change the number of iteration 300 and click on calculate.

18.Exit setup and double click on result to open CFD post. Check boxes to see walls and other parts.

5.Results:

Case 1:

Velocity = 1 m/s

Residual

Temperature plot

Section plane with Mesh

Full internal body

Fins

Convection Area of Processor

Processor

Body

Case 2: Velocity = 2.5 m/s

Residual

Temperature

Section Plane with Mesh

Full internal body

Processor

Convection area of processor

Body

Case:3 Velocity = 5m/s

Residual

Temperature plot

Section plane with Mesh

Full Internal body

Fins

Processor

Convection area of processor

Body

6.Conclusion:

• The CHT analysis is done for graphics card
• Effect of various mesh parameters on simulation results is stuided.
• Use of intermediate meshing method is studied.