Conjugate Heat Transfer Analysis on a graphics card.

CHT ANALYSIS ON A GRAPHICS CARD

AIM

• To run and simulate conjugate heat transfer of a typical Graphics card by varying velocities and finding out parameters like Temperature ,Heat transfer coefficient of the processor.
• To Identify Potential Hotspots On the Graphics card.

INTRODUCTION

With a large chip, more transistors, and more frames, questions always pivot to the efficiency of the card, and how well it sits with the overall power consumption, thermal limits of the default ‘coolers’, and the local noise of the fans when at load.

Considering one of the high end graphics card Nvidia Rtx 2080 ti the max power output is 359.7W at its maximum settings or by Over clocking

The GeForce RTX 2080 Ti registers ~277W through our stress test and almost 279W in our gaming loop (nearly 20W higher than Nvidia's official TDP rating)

This indicates the total heat generated can be calculated as:

H=359.7/(volume of processor)

=359.7/(0.04m*0.04m*0.002m)

=112,406,250 W/m^3

THEORY

Heat generation from CPU or Graphics card are genarated by Power loss= dynamic power loss+Short circuit power loss+Leakage Power loss + other factors

Dynamic power loss:

Inside a CPU when the logic gates toggle,Energy is flowing as the Capcitors inside them are charged and dischard simultaniously.The Dynamic Power consumed by a CPU is Approximately proportional to the CPU frequency and To the square of the CPU voltage.

Pdynamic=C(load)*(v^2)*f*Nsw

Where, C(load)=the Switched load Capacitance

f=frequency

V=supply voltage

Nsw=Number of Switching bits

Short Circuit Power loss:

Short Circuit power loss between the gates.The magnitude of this power is dependent on the logic gates and is rather complex to model on a macro level.

Leakage power:

Power Consumption due to leakage power emanates at a micro-level in transistors.Small amounts of current are always flows between the transistors.

• Both Dynamic and Short circuit power consumption are dependent on the clock Frequency
• Leakage Current power is dependent on the CPU Supply voltage

LANDUER'S PRINCIPLE: "States that The Erasure of One bit of Information in a computational device is necessarly accomppanied by a generation of heat (E=KT ln2)"

E=Kb *T *ln(2)

where Kb=Boltzmann constant ,1.38*10^(-23) J/K

T=temperature of Heat sink(k)

ln2=0.69315

Coming to Conjugate Heat Tranfer:

The term conjugate heat transfer (CHT) is used to describe processes which involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids. The exchange of thermal energy between the two physical bodies is called study of Heat Transfer, the rate of transferred heat is directly proportional to the temperature difference between the bodies. A typical example is the heating or cooling of a solid object by the flow of air in which it is immersed and some other example includes conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers. Forced convection is the most common way to achieve high heat transfer rate. In some applications, the performances are further improved by combining convection with phase change (for example liquid water to vapor phase change).

Heat transfer in solids and heat transfer in fluids are combined in the majority of applications. This is because fluids flow around solids or between solid walls, and because solids are usually immersed in a fluid.

Modes of heat transfer

• Conduction: diffusion of heat due to temperature gradients. A measure of the amount of conduction for a given gradient is the heat conductivity.

• 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. Convective heat transfer is tightly coupled to the fluid flow solution.

• Radiation: transfer of energy by electromagnetic waves between surfaces with different temperatures, separated by a medium that is at least partially transparent to the (infrared) radiation. Radiation is especially important at high temperatures, e.g. during combustion processes, but can also have a measurable effect at room temperatures.

SOLVING AND MODELLING APPROACH

Geometry:

Bounding Box:

mesh : base line

Materials and its properties:

Component	Material	Density(kg/m^3)	CP/Specific heat(j/kg k)	Thermal conductivity k(w/mk)
Fins	Silicon	200	710	150
processor	Aluminium	2719	871	202.4
PCB	FR-4	1900	1200	0.23

Default Mesh Sizes

• Bounding box=0.01m
• fins,PCB and processor=0.001m

Mesh elements :224576:

Element sizes of

of Bounding box Graphics card

Graphics card:

Fins PCB

processor

Inlet: Symmetry Wall:

and outlet

Solver Settings

• k-omega SST
• Steady State
• Energy equation Turned on
• Heat generation:112,406,250 W/m^3

Case 1: Velocity 2 m/s

Residual plot

Velocity Plot

Temperature plot

Temperature Distribution

Area Weighted Average of velocity magnitude :1.871807 m/s

Area Weighted average of Temperature:306.12852k

Velocity Contour plot

Wall heat transfer coefficient:2289.351 w/m^2 k

Velocity stream line

temperature distribution through out the Graphics Card, Temperature of the processor temp 435.267k

Animation Videos:https://drive.google.com/drive/u/1/folders/1pi24D7EWGyRKN_1yfLiHzVcTj2K9NWUk

Case 2: inlet velocit 4 m/s:

Residuals

Velocity Contour:

Temperature contour:

Area-Weighted average of Velocity=3.7410806 m/s

Area weighted average of Temperature:303.43569k

temperature distribution

Area weighted Wall Adjacent Heat Transfer Coef=3276.2639 W/m^2 k

Temperature plot:

Velocity plot:

Wall Heat Transfer Coefficient Plot:

Streamline plot:

Temperature Plot of Enclosure,The processor temperature was found to be: 385.91k

Animation Videos:https://drive.google.com/drive/u/1/folders/1pi24D7EWGyRKN_1yfLiHzVcTj2K9NWUk

Case 3 Inlet velocity 5m/s

Residual plot

velocity:4.6826713 m/s

Area Weighted average of Temperature :302.83631k

velocity contour plot:

Temperature contour plot:

temperature distribution

Area weighted average of Heat transfer coefficient of wall =1327.8131 w/m^2 k

Velocityplot:

temperature plot:

Wall heat transfer coefficient plot:

Stream line plot:

temperature distributions : Processor temperature was found to be 376.13814k

Animation Videos:https://drive.google.com/drive/u/1/folders/1pi24D7EWGyRKN_1yfLiHzVcTj2K9NWUk

Result and Conclusion

"As a fact: the more Refiner the mesh The better the result"

We can conclude from the bare results of processer under the inlet velocities that higher velocity speeds reduce the temperature of processor.

• Case 3(5m/s): temperature of processor was found to be :376.13814k

• Case 2( 4m/s):temperature of processor was found to be :385.91k

• Case 3(2m/s)::temperature of processor was found to be :435.267k

from these Experimental results we can confirm that higher velocity streams of air from the graphics card blower is necessary to function the processors also to keep in check of Temperature overload happenning in processors.