Week 6 - CHT Analysis on a Graphics card

Conjugate heat transfer (CHT):-

Conjugate Heat Transfer (CHT) refers to the coupling of conduction in solids with the conductive and radiative heat transfer in the surrounding fluids. It is used in processes which involves variation of temperature within solids and fluids, due to thermal interaction between solids and fluids.

CHT analysis on graphics card:-

Graphics card or a video card is an expansion card which generates a feed of output images to a display device such as a computer monitor. In this project a steady state conjugate heat transfer analysis is performed on a graphics card using Ansys Fluent. In this simulation the type of cooling system used is asumed as air with a velocity of 5m/s. And the processor is assumed as the only source of heat. The heat generated by the processor is calculated as:-

Graphics card power (P) = 75W

Volume of processor (v) = 8 x 8 x 1

=64mm2=6.4E−8m3=64mm2=6.4E-8m3

∴∴Heat Generated =PV=7564E−9=1171875000Wm3=1.1719E9Wm3=PV=7564E-9=1171875000Wm3=1.1719E9Wm3

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.

The different components were named as follows:-

The inlet is the face from which the air flows into the enclosure and exit through the outlet

Different materials were assigned for different components, viz:-

Aluminum for fins:

Silicon for Processor:

Acrylonitrile butadiene styrene (ABS) for PCB

Air as the fluid.

A steady state, pressure based, absolute Fluent solver was used to solve this project along with k-eplison model for solving turbulent equations. Energy equations were also used to solve the temperature variations in the fluid and solid domains .

The initial boundary conditions used were:-

Inlet :- Velocity of air=5m/s

Temperature = 300K

Heat generated by the processor = 1.1719E9 W/m^3 (from above calculations)

This was solved for two cases i.e 2 variation of mesh, (baseline mesh and refined mesh)

CASE 1:- Baseline Mesh (General Mesh)

In this case the mesh used is kept general with the total elements of 82178. The images below shows the mesh and its conformality with all the components, which is a result of using share topology while setting up the model in the spaceclaim.

The components viz. PCB, processor and the fins were given body sizing as follows

Results:-

The residual Plot:- From the residual plot as show below, it can be said that the solution is converged around 125 iterations.

Temperature Plot:

Temperature contour: The temperature contour on the graphics card (shown in the image below) gives us the hotspot on the model.

The maximum Temperature attained by the processor is 1400K.

Velocity Plot:-

Wall Heat Transfer Coefficient:-

The maximum heat transfer coefficient at the fins is 20 W/m2 K and at the processor is 120 W/m2 K (at the side face)

CASE 2:-

In this case the mesh is refined with overall mesh size as 1mm. The total number of elements are 1926305.

The components viz. PCB, processor and the fins were given body sizing as follows

Results:-

The residual Plot:- From the residual plot as show below, it can be said that the solution is converged around 300 iterations.

Temperature Plot:

Temperature contour: The temperature contour on the graphics card (shown in the image below) gives us the hotspot on the model.

The maximum Temperature attained by the processor is 1100K.

Velocity Plot:-

Wall Heat Transfer Coefficient:- The maximum heat transfer coefficient is 500 W/m2 K.

The maximum heat transfer coefficient at the fins is 50 W/m2 K and at the processor is 500 W/m2 K (at the side face)

Reasons to refine the mesh:-

• As the number of elements increases the accurate results of temprature distribution were obtained.
• It also helps to get more defined plot of the temperature and Wall heat transfer coefficient contour i.e we can see the different regions more clearly by virtue of which it can be easily distinguish and studied

The processor is the main source of generation of heat and the fins are provided to increase the rate of heat transfer with the air by convection, hence these two regions were mainly focused while remeshing the model.

Comparison between both the cases, viz Case 1 (General mesh) and Case 2 (Refined mesh)

Case No. Max Temperature (K) attained by the processor Max WHTC (W/m2 K) at fins Max WHTC (W/m2 K) at processor

Case 1 1400 20 120

Case 2 1100 50 500