Week 6 - CHT Analysis on a Graphics card

                                       Conjugate Heat Transfer (CHT) Analysis on a Graphic Card

Aim

1. To perform a steady-state CHT analysis on a graphic card, materials of our choice can be used and must define correct fluid and solid zones.

2. To run the simulation for the best mesh possible with a combination of coarse and refined mesh in different regions.

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:

Coefficient of heat transfer

By transferring thermal energy at their interfaces, the Conjugate Heat Transfer (CHT) analysis type enables the simulation of heat transfer between solid and fluid domains. The simulation of heat exchangers, the cooling of electronic devices, and all-purpose cooling and heating systems are a few examples of typical uses of this analysis type.

Examples:

Internal combustion engine, heat sink, boiler, reactor, heat pipe, turbocharger, heat exchanger, and tank are all terms related to HVAC.

 1. Conduction- It is the diffusion of heat due to temperature gradients in solids. It is described by Fourier's law which defines the conductive heat flux q proportional to temperature gradient.

2. Convection:- The flow can either be caused by extra influences, forced convection, or natural.

There are 3 ways that fluid motion affect heat transfer

• Convective Contribution- The dominance of conduction and convection changes depending on the characteristics of the fluid and its region of flow.
•  Viscous effects: The viscosity generates heat as a result of fluid flow. Due to its insignificance, this impact is frequently disregarded; nevertheless, in high-velocity situations, it should be taken into consideration.
•  Effects of density: Depending on the temperature distribution, the density will vary at various points throughout the domain as it becomes correlated with temperature. The compression that results from this density fluctuation might result in the production of heat.

In order to replace the empirical relationship between the proportionality of heat flux to temperature difference with the heat transfer coefficient, which is the only tool available for theoretical heat convection since Newton's time , the modern Cht model was created after the computers became widely usage.

Applications.

CHT methods which began with straightforward examples in the 1960s have developed into more potent tools for modeling and investigating natural phenomena and engineering systems in a variety of fields including aerospace and nuclear reactors, thermal goods treatment and foods processing ,complex medical procedures and relatively simple units to multistage ,nonlinear processes. A thorough analysis of more than 100 conjugate modelling examples chosen from a list of 200 early and contemporary publications revealing that conjugate methods are now widely used in a variety of applications.

Geometry setup

• Use the SpaceClaim to edit the geometry and click on share prep to view any edges in interference so that conformal mesh can be achieved between the components.
• Share topology has to be enabled between the components so that necessary information can be shared between the different zones of mesh. 

Mesh Setup

1. Firstly the base mesh has to be generated, before that the naming of the components to be done to make it easier while setting up the fluent.

2. Base, processor, and fins are basic named selections.

3. After the naming is done, click to generate mesh to create a base mesh, and the body sizing is applied for three components.

4. After the creation of body sizing , base mesh generated having more than a lakh elements which helps in finding accurate solutions.

5. Once it's done we update the details in the given page.

Fluent Setup

Physics setting up

using pressure-based steady-state solver

velocity formulation- absolute

Materials 

for fluid- Air

density 1.225 kg/m^3

Viscosity = 1.7894e-05 kg/ms

For Fins: copper

Density = 8978 kg/m^3

Specific heat = 381 j/kg-k

Thermal conductivity = 387.6 w/m-k

For Processor : Silica

Density = 2328 kg/m^3

Specific heat = 710 j/kg-k

Thermal conductivity = 150 w/m-k

 For Processor: Silica

Density = 2328 kg/m^3

Specific heat = 710 j/kg-k

Thermal conductivity = 150 w/m-k

 For Base: steel 

Density = 8050 kg/m^3

Specific heat = 502 j/kg-k

Thermal conductivity = 16.27 w/m-k

Setting up the source terms

In this case we consider the processor is the source terms and its generating heat .

to set up the source term we go to zones -cell zones- and click on the processor for editing which is in the solid.

then select the material to gold nad click the source terms to enable it and set the source to 1

Calculation for the Energy source

Let the processor consume 83 W of power to work.

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

So, Energy produced by the processor is 1296875000 w/m^3.

Inlet

Air is used as a fluid

Velocity = 1 m/s

Temperature = 300 K

Outlet

Pressure = Gauge pressure (0 pa)

Results:

1. Base Mesh:

Inlet velocity= 1m/s

Mesh:

Main enclosure: 3.9mm

Fins size: 2mm

Processor: 1.5mm

Base: 2mm

Enclose and graphic card

Processor & Fins

BASE

MESH QUALITY

 2. Plots: 

1. Wall HTC and Max temperature of the processor:

2. Refined mesh:

1. Mesh:

Main enclosure: 3.35mm

Fins size: 0.6mm

Processor: 0.2mm

Base: 0.8mm

Enclosure and graphic card:

Processor and Fins:

 Base

Mesh quality

a. Inlet velocity= 1m/s

 Plots:

1. Wall HTC and Max temperature of the processor:

2. Wall HTC at fins and base:

3. Potential hotspots at fins and base:

4. Wall HTC and max temperature of the graphic card:

5. Residuals and average temperature plot:

Maximum temperature: 1892K

Wall HTC: 264.8 W/m^2 K

Average of temperature: 1521.7 K

b. Inlet velocity: 2.5m/s

Plots:

1. Wall HTC and Max temperature of the processor:

2. Wall HTC at fins and base:

3. Potential hotspots at fins and base:

4. Wall HTC and max temperature of the graphic card:

5. Residuals and average temperature plot:

Maximum temperature of processor: 1192 K

Wall HTC of processor: 1341 W/m^2 K

Average of temperature: 921.97 K

c. Inlet velocity: 5m/s

Plots:

1. Wall HTC and Max temperature of the processor:

2. Wall HTC at fins and base:

3. Potential hotspots at fins and base:

4. Wall HTC and max temperature of the graphic card:

5. Residuals and average temperature plot

Maximum temperature of processor: 902 K

Wall HTC of processor: 1341 W/m^2 K

Average of temperature: 675.6 K

•  Conclusion:

	Baseline - 1m/s	Refined -1m/s	Refined - 2.5m/s	Refined - 5m/s
Max temperature Processor (K)	1887	1892	1192	902
Wall Heat transfer coefficient processor (W/m2K)	264.8	1128	1341	1341

Repetitive plots in the residual plots revealed a converged solution for the provided input sets. The processor served as a heat source making it one of the study's focal points.

We can observe a significant decrease in temperature with increasing velocity and the significant increase in HTC demonstrating that High-velocity air can be a useful method of heat dissipation in graphic cards.

 We can see hotspots in the fin and base close to the surface that is in touch with CPU from the temperature distribution. the component's temperature is dispersed appropriately based on the material's thermal conductivity. A few high-temperature areas on fins that are not near the processor show their great susceptibility to heat. 

Additionally ,there is a significant difference between baseline and refined mesh,leading us to the conclusion that more refined mesh offers a superior solution and can be used to optimise the design for these applications.The change in the gradients is quite slight and this leads us to belive that the solution has converged based on the residual plot.

The high-temperature locations can be discovered at the fins ,processor and base area close to the processor, according to the temperature plot of the coarse mesh.The coarse mesh heat transfer for the fins and other components in the graphic cards has the same values even the wall HTC is a wall phenomena and fins dissipate a lot oh heat resulting in a greater HTC.

The centre of the fins the base and processor walls are where the high temperatures are located.With many iterations ,the temperature is dispersed in the direction in the input airflow to various areas of the graphics card. Conduction happens when two solids are in a contact region; in this situation, that contact region is between the processor and the base and the processor and the fins. Convection happens when the inlet airflow removes the heat produced in the graphic card's components. The inlet airflow velocity has an inverse relationship with the maximum temperature in the components. Convection is a faster method of removing heat when airflow velocity is increased. Recirculation of air (vortex) at strategic locations will also improve heat transfer efficiency