Conjugate Heat Transfer CHT Analysis on a graphics card

Objectives:

  • To find out the maximum temperature attained by the processor.
  • To Prove that the simulation has achieved convergence with appropriate images and plots.
  • To Find out the heat transfer coefficient at appropriate areas of the model.
  • To Identify potential hotspots on the model.

CONJUGATE HEAT TRANSFER

  • CONJUGATE HEAT TRANSFER(CHT) refers to situations in which multiple modes of heat transfer occur simultaneously.
  • Simultaneous multiple heat transfer modes occur in almost all real-world problems, So CHT is quite important as majority of heat transfer applications involve more than one mode of heat transfer, whereas in some cases certain heat transfer modes can be neglected.
  • Conjugate heat transfer (CHT) allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them.
  • It requires a multiregion mesh to have a clear definition of the interfaces in the computational domain
  • Conjugate heat transfer(CHT) analysis is employed in desiging and simulating of heat transfer appliances like heat exchangers, cooling of electronic equipment, Thermal insulators, Heat sink, Exhaust port manifold and general-purpose cooling and heating systems.

Description: 

  •  Graphic card is a component of computer/Laptop which produces   the image which we see on a computer, it converts the data to a   signal which a monitor can understand.  
  • In this process the graphic card becomes hot, for cooling it air is blown over the card, by some external agency.
  • The different parts of the graphic card are:

           1. Processor

           2. Fins

           3. Base or Board 

           1. Processor: It allows to accept the information from the CPU,                     during its function, some amount of heat is generated and air                   which is flowing over it, takes the heat through fins and base.

          2. Fins: Fins are extended surfaces used to dissipate the heat by                  increasing the surface area. 

          3. Base or Board: It is a platform for the processor, fins and some                small ICS, and these combinely become a graphic card which is                intergrated with mother board.

  •  In our case the graphic card model is placed in an Enclosure, through which the air is passed and graphic card gets cooled. 

Assumptions:

      1. The thermal Design power of the processor in  the Graphics card is            10 W. 

      2. Air flows at speed of 8 m/s, and at a temperature of 300 K 

      3. Materials used

             Base - steel

             processor - copper

             Fins - Aluminium

Heat generated in the processor:

             Dimensions of the processor = 8 mm X 8 mm X 1 mm

             volume = 8 * 8 * 1 = 64`mm^3`

                       = 64 * 10^(-9) = 0.000000064 `m^3`

              Heat generated = Thermal Design Power(TDP)/volume

                              = 10/(0.000000064) = 156250000 `W/m^3`

 

CASE 1(BASELINE MESH):  

 Geometry:

 

The Enclosure, Base, processor and fin base must be in share topology

Mesh:

Element size = 10 mm

No. of Nodes = 16208

No of elements = 86103

              

                        

Mesh Quality:

               

SIMULATION SETUP DETAILS:

I. SETUP PHYSICS

SOLVER

     Typer --> pressure based

     velocity Formulation --> Absolute

     Time --> Steady   

MODELS

Energy --> Tick the check box

 Viscous --> k-epsilon 

        k-epsilon Model --> standard

        Near-Wall Treatment --> standard Wall Functions   

MATERIALS

Create/Edit Materials

     Material type --> fluid

     Fluent Fluid Materials : air       

     Material Name : air 

        Properties

                Density(kg/m^3) : 1.225 

                Cp(specific Heat)(j/kg-K) : 1006.43

                Thermal conductivity (w/m K): 0.0242

                Viscocity (kg/m-s): 1.7894e-05

     Click on Fluent Database, choose Material type as solid, select the             appropriate material and press copy.  

     Material type --> solid

     Fluent solid Materials: aluminium(al)

      Properties

                Density(kg/m^3) : 2719 

                Cp(specific Heat)(j/kg-K) : 871

                Thermal conductivity (w/m K): 202.4

       Fluent solid Materials: copper(cu)       

          Properties

                Density(kg/m^3) : 8978

                Cp(specific Heat)(j/kg-K) : 381

                Thermal conductivity (w/m K): 387.6

       Fluent solid Materials: steel          

           Properties

                Density(kg/m^3) : 8030

                Cp(specific Heat)(j/kg-K) : 502.48

                Thermal conductivity (w/m K): 16.27

ZONES 

       Cell Zones    

               zone-->  base 

               zone Name --> base

               Material Name --> steel

               press ok

             

               zone-->  fin base 

               zone Name --> fin base

               Material Name --> aluminium

               press ok

 

               zone-->  processor 

               zone Name --> processor

               Material Name --> copper

               press ok

 

      Boundaries

          Boundary Conditions

              inlet:

                   Momentum

                           velocity (m/s): 8 

                   Thermal

                           Temperature (k) : 300  

           

              wall enclosure enclosure processor: 

                    Thermal

                          Thermal conditions--> coupled

                          Wall Thickness (m) - 0.001

                            Heat generation (W/m3) - 156250000

             outlet:            

                    Momentum

                           Gauge pressure(pascal): 0                                                     

II SOLUTION

INITIALIZATION

Method --> Hybrid

Initialize at t = 0 

III RESULTS

SURFACE

create --> plane

         Name - cut plane -y

         options

         Point and Normal

         Points

         x0 (m)   -  0 

         y0 (m)   -  0 

         z0 (m)   -  0 

        Normal         

         ix (m)   -  0 

         iy (m)   -  1 

         iz (m)   -  0

   save    

GRAPHICS

 Contour --> New 

     options --Filled

     contours of -->temperature

     surfaces :

           plane -surface 

                cut-plane-y

           Wall

    Save/Display

 

Contour --> New 

     options --Filled

     contours of -->velocity

     surfaces :

           plane -surface 

                cut-plane-y

           Wall

    Save/Display

IV SOLUTION

ACTIVITIES 

Create --> solutions Animation

        Animation object --> temperature contour

Create --> solutions Animation

        Animation object --> velocity contour

INITIALIZATION

Method --> Hybrid

Initialize at t = 0 

RUN CALCULATON

Timescale Factor  = 1

No. of Iterations = 500

 scaled Residuals:

The solution is converged at 106 iterations, but it has been simulated for 900 iterations. 

Tempertature distribution on the Graphic card in Fluent:

Tempertature distribution on the Graphic card in CFD post:

Temperature distribution on the base

Temperature distribution on the processor:

Temperature distribution on the fin base:

Velocity distribution on the Graphic card:

Wall Heat transfer coefficent on cut plane:

Maximum temperature obtained at the top of the processor = 338.9 K

Maximum velocity attained by air = 9.719 m/s

The hot spots on the graphics card are obtained at the interfaces of processor-base and processor-finbase. 

The maximum wall heat transfer coefficient is obtained between the wall and air = 1174 `W/m^2`

CASE 2 (FINE MESH):  

 Geometry: 

The Enclosure, Base, processor and fin base must be in share topology 

Mesh:

Element size = 4 mm

processor Element size = 0.25 mm

Base = 1 mm

Fin base Element size = 1 mm

smaller parts on base, Element size = 0.25 mm

No. of Nodes = 87857

No of elements = 461255

 

         

               

SIMULATION SETUP DETAILS:

I. SETUP PHYSICS

SOLVER

     Typer --> pressure based

     velocity Formulation --> Absolute

     Time --> Steady   

MODELS

Energy --> Tick the check box

 Viscous --> k-epsilon 

        k-epsilon Model --> standard

        Near-Wall Treatment --> standard Wall Functions   

MATERIALS

Create/Edit Materials

     Material type --> fluid

     Fluent Fluid Materials : air       

     Material Name : air 

        Properties

                Density(kg/m^3) : 1.225 

                Cp(specific Heat)(j/kg-K) : 1006.43

                Thermal conductivity (w/m K): 0.0242

                Viscocity (kg/m-s): 1.7894e-05

     Click on Fluent Database, choose Material type as solid, select the             appropriate material and press copy.  

     Material type --> solid

     Fluent solid Materials: aluminium(al)

      Properties

                Density(kg/m^3) : 2719 

                Cp(specific Heat)(j/kg-K) : 871

                Thermal conductivity (w/m K): 202.4

       Fluent solid Materials: copper(cu)       

          Properties

                Density(kg/m^3) : 8978

                Cp(specific Heat)(j/kg-K) : 381

                Thermal conductivity (w/m K): 387.6

       Fluent solid Materials: steel          

           Properties

                Density(kg/m^3) : 8030

                Cp(specific Heat)(j/kg-K) : 502.48

                Thermal conductivity (w/m K): 16.27

ZONES 

       Cell Zones    

               zone-->  base 

               zone Name --> base

               Material Name --> steel

               press ok

             

               zone-->  fin base 

               zone Name --> fin base

               Material Name --> aluminium

               press ok

 

               zone-->  processor 

               zone Name --> processor

               Material Name --> copper

               press ok

 

      Boundaries

          Boundary Conditions

              inlet:

                   Momentum

                           velocity (m/s): 8 

                   Thermal

                           Temperature (k) : 300  

           

              wall enclosure enclosure processor: 

                    Thermal

                          Thermal conditions--> coupled

                          Wall Thickness (m) - 0.001

                            Heat generation (W/m3) - 156250000

             outlet:            

                    Momentum

                           Gauge pressure(pascal): 0                                                     

II SOLUTION

INITIALIZATION

Method --> Hybrid

Initialize at t = 0 

III RESULTS

SURFACE

create --> plane

         Name - cut plane -y

         options

         Point and Normal

         Points

         x0 (m)   -  0 

         y0 (m)   -  0 

         z0 (m)   -  0 

        Normal         

         ix (m)   -  0 

         iy (m)   -  1 

         iz (m)   -  0

   save    

GRAPHICS

 Contour --> New 

     options --Filled

     contours of -->temperature

     surfaces :

           plane -surface 

                cut-plane-y

           Wall

    Save/Display

 

Contour --> New 

     options --Filled

     contours of -->velocity

     surfaces :

           plane -surface 

                cut-plane-y

           Wall

    Save/Display

IV SOLUTION

ACTIVITIES 

Create --> solutions Animation

        Animation object --> temperature contour

Create --> solutions Animation

        Animation object --> velocity contour

INITIALIZATION

Method --> Hybrid

Initialize at t = 0 

RUN CALCULATON

Timescale Factor  = 1

No. of Iterations = 500

 scaled Residuals:

 The convergence occurs before 500 iterations, but it has been simulated for 900 iterations. 

Tempertature distribution on the Graphic card in Fluent:

Tempertature distribution on the Graphic card in CFD post:

Temperature distribution on the base

Temperature distribution on the processor

Temperature distribution on the fin base

Velocity distribution on the Graphic card:

Wall Heat transfer coefficent on cut plane:

Maximum temperature obtained at the top of the processor = 330.2 K

Maximum velocity attained by air = 10.20 m/s

The hot spots on the graphics card are obtained at the interfaces of processor-base and processor-finbase. 

The maximum wall heat transfer coefficient is obtained between the wall and air = 1897 `W/m^2`

Conclusion:

  • From both the results, it is clear that the results obtained from the refined mesh is smoother.
  • Refined Mesh results are smoother and close to the true value.
  • The maximum velocity and maximum heat transfer coefficient has been increased, which clearly says that the air extracts maximum amount of heat from the graphic card and the maximum temperature on the graphic card drops.

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The End