Week 6 - CHT Analysis on a Graphics card

Aim:- To Perform a steady-state conjugate heat transfer analysis on a model of a graphics card. 

Introduction:- Conjugate heat transfer is a most common heat transfer phenomenon in which heat is transfer between solid and fluid both. It includes both conduction and convection heat transfer. Common application of this type of simulation includes cooling of electronics component and transformers, Heat exachange, cooling tower and bolider of a power plant, condensor of air conditioning system ect.

Objective of the Project is to Carry out CHT analysis at different velocities To determine the hot spots and Heat transfer co-efficient at different velocities.

Fins:- It is a conduction-convection system employed to dissipute heat from a surface at faster rate to surroundings fluid. These are nothing but extended surface so that it convect more heat to surrounding. The matrial of fins should be choosen in shuch a way it conduct heat at faster rate.

 The most common application of fins are 

1. Two wheeler vehicle engine

2. Electrical component like transformer and motor

3. Reciprocating compressor

4. Electronics component (computer Processor,graphic card, SMPS, etc.)

Modeling Approach

The graphic card is the main component which is responsible for the better quality of the visuals displayed. In this experiment we study the heat generated by the graphic card with effect of different inlet velocities. 

 Here in this image you can see image of an actual graphic card and CAD Model of graphic card. And here you can see fins ar actteched on the top surface of graphic card so heat conduct from grapic card to fins via conduction, in actual practices a bonding marteral is pasted between grapic card and fin so that no thermal resistance is between the processor and fins and this matrial knowns as Thermal paste.

Materail Use 

1. PCB :- A printed circuit board is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. In theory, the simplest printed circuit board has three layers. A conductive layer, generally made out of copper, is encased in two layers of strong, insulating dielectric materials like fiberglass. That outer layer is called the mechanical layer. There are three main different types of PCB materials.

 The first is metal. Metals like copper, aluminum, and iron are generally used as the conductive layer in PCBs. Copper is the most popular. That is why most PCBs are “copper clad.” 

The second type of material used in PCB manufacturing is PTFE or Teflon. It is a strong, lightweight, and somewhat flexible material. PTFE is preferred in applications with very tight tolerances because it doesn’t expand much on exposure to high temperatures. Another point in favor of this material is that it is flame resistant.

FR-4 is a glass-reinforced epoxy laminate sheet. It is a composite made of woven fiberglass cloth and a flame-resistant epoxy resin binder. FR-4 is the most commonly used material in printed circuit boards. Most of us have seen the green FR-4 board to which the electrical components are mounted, though it comes in other colors.FR-4 is the NEMA grade designation for the material. FR itself stands for flame resistance. Most FR-4 boards have bromine in the epoxy laminate to help extinguish flames.

2. Processor :- It is a Integrated Circuits (ICs) or chip which is usally made of slicion because is a good semiconductor widely use in al electronics componects.

3. Fins :- Fins should be made of high thermal conductivity material and most economic high thermal conductvity material is Aluminium.

Assumption:- Here our assumption for this study is the runing fluid is air and material for PCB is polystrene and for processor is si and fins is Al.

Second assumption is heat generation from processor only which is 1 wats.

Now open model in space claim and measure volume of processor which come 64mm3. so heat genrate per unit volume is

q=1/64e-9 =15625000 watt per m3.

Procedure:-

1.  Pre procdure:- Open ansys spaceclaim import model and make an encloser  and active share topology as shown in figure 

Now close spaceclaim and open the mesher now first hide encloser geometry by pressing F9 and now select fin geometry and pres N and Name it to fins.

Similary hide fins you can see processor of grapic card name it processor then hide this processor and box select all and name PCB.

 Now click F9+shift for show all and name wall inlet and outlet in incloser

Meshing:- Hide encloser again and click on mesh give element size value 10mm now for model we use sizing click on sizing select body selection and select all bodies give value 1mm or 0.5 mm for fine mesh conduction. you can see elements in corse mesh 199340 and in fine mesh 328256. 

Here in sizing we active capture curvature or we can make aniother body sizing for fin body or processor body.

Course mesh in section view

Fine Mesh

2.Setup:-Now close mesher and double cilck setup

Material Assinement:-Now come to cell zone select encloser domain and click edit and select fluid materal as air

Now go to physics click matrial open fuient material database and select Aluminim solid and copy it two times by name silicion and PCB 

Material	Density(kg/m3)	Specific heat (J/kgk)	Conductivity (w/mk)
Aluminium	2719	871	202.4
Polystrene	55	1210	0.027

Now apply these material in cell zonnes

Material for Fins 

Material for PCB

 Material for processor

Active Heat generation:- In cell zone condition edit processor change material as siliocn as shoen in figure then go to source terms and edit energry sourse and give value 15625000 which we claculated in theory.

Model selection

Now setting up the physics for CHT analysis on a graphics card in ANSYS fluent.

• For this case we are using pressure-based steady state solver
• Velocity formulation - Absolute

Model setup

• Energy
• Viscous - k-omega

Physics setup:- Active the energy calculation

Analysis setup:- steady state anlysis

Boundary conditions

Inlet velocity 1 ,3 ,5 m/s so makes it parametric and temperature is 300k

Outlet

Result plot

HTC (heat tensfer coefficent at processor surface comes contact )

Maximum temp

Now initialize simulatio and run model for 200 iteration

Now after complete analysis close fluient and and go to project parameters for parametric study now right click DP0 and duplicate 2 times  and give valociry value 3 and 5 for DP1 and DP2 then updates all design points.

Result plots:- Now duplicate our project for fine mesh study for fine mesh condition we run analyis for only 1m/s and updates all design points

and plot results are

1) Residual plots

for 1 m/s coarse mesh

for 3m/s coarse mesh

for 5m/s coarse mesh

For fine mesh 1m/s

With these above residual plot you can see 1m/s model with coarse mesh converge at 68 iteration which is least in above casses.

so as velocity increase Number of iteration incrase or time for simulation also increases.

And for case 4 i.e. 1m/s with fine mesh converge at 87 iteration which also grater then coarse mesh study for same velocity so we can say that with increse in number of element number of itertion for convergance also increse.

2) Heat transfer coeffient 

for 1m/s coarse mesh

for 3m/s coarse mesh

for 5m/s coarse mesh

for 1m/s fine mesh

Now go to parametric study 

Here column E and G both showing values of heat transfer coeffients for Design points DB0,DB1 and DB2 only deferance is in columnt E the heat transfer coeffient for coarse mesh study and column G for fine mesh

with comnparing these graphs and parametric study we can say as we inrease velocity our heat transfer coeffieent increases. because heat transfer coefficient is directly proportional to the velocity of the liquid.

3) Temperature plot

for 1m/s coarse mesh

for 3m/s coarse mesh

for 5m/s coarse mesh

for 1m/s fine mesh

So here we can see temperatre is decrease with increse in velocity.

Contour plots

for 1m/s coarse mesh

for 3m/s coarse mesh

for 5m/s coarse mesh

for 1m/s Fine mesh

Now in above Images you can see hot spots at fints tips and surfaces. in last image you can see hot spot in processor and fint interface i.e here temperature is high and its sliglty decresing till tip of fin as shown is 3 m/s and 5 m/s mode.This is because in higher velocity model HCT is also high so heat convection rate also high so we can easly see better temperature distribustion in fins so we get hot spots ony near to processor but in 1m/s model we cansee hot spot in complete fins because here HTC is less.

Now in CFD post we can see it more properly with tmperature contours for 1m/s

Here you can see hot spots at center of fins because here it is incontact with processor

for 3m/s

For 5m/s Tempareure contour on a plane

Here you can see in comlete enclose temperature is constant 300k it is more near to fins ansd precessor  and also some area below PCB

this can be more understand by velocity contours

1m/s

3m/s

5m/s

Now here in above velocity contour you can see are less near to interface fin and air and you can also see some region of zero velocity above the fins this is the region of hot spots because here heat convection rate is less.This region of zero velocity can be expaline by respective stream line plot in front and top views 

In stream lines plots you can see flow particals strikes with front send of fis it makes vortex which is also region of zero velocity but if you se top view you can see palner steams goes to transfer direction also after striking front end of fins so that side region cools more faster rate and because of this we get hot spot at center region.

Conclusion

1. As increase in velocity heat tranfer rate also increase increse and the temperature at interface is decreases.
2. As increase in velocity number of iteration also increase so more computatinal time and memory is required for simulation.
3. If we increase velocity till 10m/s HTC increse but after 10 m/s velocity increase HTC is less as compare to velocity below  10m/s so we can say as increase in velocity heat tranfer rate also increase increse but after some sufficent value it not increse much more so our decrese in temperature is also not much more.
4. We can see Hot spot near to the center because of Procersor posiotion is here and here HTC is also less because less partical of fluid contact here because of diverging stream lines at front end of fins