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Heat sink in natural convection

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Old   September 27, 2011, 12:22
Default Heat sink in natural convection
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Hi,

I have only started with Star-CCM+ (a month) and I would appreciate advise/tips on the model I am trying to make. I am modelling a heat sink with a 100W of energy on it's base plate dissipating it through the fins. I want to model the heatsink with ambient temperature at 55 celcius and no wind.

I am modelling steady state with no radiation at the moment but would like to add it in the future.

I would like to know what boundary conditions would be appropriate? Should I model it as walled regions (in a box)? or a velocity inlet with zero velocity?

Which solver should I use? coupled or segregated?

What volume mesh should I use? and should I use a prism layer mesher?

What equation of state would be appropriate? Should I use an ideal gas model or real gas model?

Is a laminar model ok? or should it be modelled using a turbulence model?

To ensure 100W is dissipated over the base of the heat sink (0.016m^2 area) should the base plate have a heat flux of 6250?

Thank you for your help

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Old   September 28, 2011, 03:01
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Brenton King
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-For the boundaries try a pressure outlet or free stream to allow free flow of air.

-segregated solver is usually faster to solve than coupled.

-depending on your geometry you might try a trimmer mesh. It handles square/rectangle shapes well but has some trouble with rounded shapes. The prism layer is good for capturing boundary layer behavior. It can't hurt to use it but it will increase you cell count. You might want to read up about wall y+ values and see whether you want high or low y+.

-I think ideal gas would be fine

-I would stick with the turbulent flow but it depends on what velocities/turbulent bahavior you expect to get.

-If i recall correctly the heat flux is entered as a W/m^2 value so if you have 0.016m^2 surface then 6250W will give you 100W on the surface.

hope this helps
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Old   September 28, 2011, 05:37
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BKaus,

Thank you for your reply.

My inlet velocities (and Mach number for free stream) would be 0. When I input freestream as a 0 mach number I get a floating point error.

I expect to get at best 0.2m/s as a velocity, the temperature of the heat sink should be around 140 Celcius.

I ask about what equation of state as I found this in a different thread

Quote:
Originally Posted by ping View Post
buoyant flow is naturally quite unstable and you might never achieve a steady solution. You need to set some monitoring points and dump some scenes to see what is changing every few iterations. Then try the unsteady solver with a reasonable dt - start with .01s maybe.
Also depending on you density range, you probably don't need ideal gas - try gravity + Boussinesq
and

Quote:
Originally Posted by Vinicius View Post
Is not that you canīt use the coupled formulation, but it has some peculiarities. For example, the convergence happens in a determined number of iterations, independently of the mesh size, so, probably in your case you didnīt run iterations enough.
To accelerate the solution you can increase the courant number, but it might cause instabilities. The segregated solver is much faster and in your case that you canīt increase the number of cells, it is the most appropriated one.

The y+ values indicates which wall function will be used to calculate the boundary layer. There are two stables regions, below 3 and between 20-300. To change this values, you have to change the thickness of the first prism layer. To do that, you can specify the value of this first prism or change the number of prisms and their stretching factor.

I need to be able to do a boundary condition like this

Open — a free boundary of constant pressure through which air can flow and
Symmetry — a frictionless, impermeable and adiabatic planar surface through which neither air nor heat can flow. This is the symmetry boundary condition.


Does the symmetry boundary condition make everything symmetrical? Say for the heated fin tutorial, would a symmetrical boundary condition on the solid fin (the AL: Symmetry Plane boundary condition) also model the air as symmetry too?

Thanks again!

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Old   September 28, 2011, 19:27
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For the inlet/outlet boundary if you want a free flowing boundary (let's air in/out with constant pressure) then I would use a pressure outlet boundary. This let's air in and out with no resistance/forcing.

Without seeing your geometry I'm not sure why you need a symmetry boundary. However, you will need to put a separate boundary on the air region and fin region to make them both symmetrical.

As for the bouyant flow question. What you said is true you might want to try using unsteady flow with a Boussinesq model. For this you will need to find the expansion coefficient of the air which can easily be found on google. Try this if you are having troubles but ideal gas should also work with unsteady flow.

The wall y+ value can be measured as a scalar in CCM+ and you can view this to determine whether you are in high (20-300) or low (< 3) wall y+. its best to stay in these brackets of values or else your calculation may be in transitional flow which would impact your accuracy. If you aren't in these values you need to adjust your mesh.

good luck!
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Old   September 30, 2011, 06:15
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Quote:
Originally Posted by BKaus View Post
For the inlet/outlet boundary if you want a free flowing boundary (let's air in/out with constant pressure) then I would use a pressure outlet boundary. This let's air in and out with no resistance/forcing.

Without seeing your geometry I'm not sure why you need a symmetry boundary. However, you will need to put a separate boundary on the air region and fin region to make them both symmetrical.

As for the bouyant flow question. What you said is true you might want to try using unsteady flow with a Boussinesq model. For this you will need to find the expansion coefficient of the air which can easily be found on google. Try this if you are having troubles but ideal gas should also work with unsteady flow.

The wall y+ value can be measured as a scalar in CCM+ and you can view this to determine whether you are in high (20-300) or low (< 3) wall y+. its best to stay in these brackets of values or else your calculation may be in transitional flow which would impact your accuracy. If you aren't in these values you need to adjust your mesh.

good luck!
BKaus, thank you again for your help!

The pressure outlet is working well! I had previously tried to use a laminar model and my results were wrong but the turbulence seems to be working well.

How can i measure wall y+? How do I implement the Boussinesq model?

For the radiation from the heat sink to the air, Would I have to use surface to surface or participating media? and grey thermal or multiband?

Thank you again for your help!

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Old   September 30, 2011, 11:20
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Quote:
Originally Posted by BKaus View Post
As for the bouyant flow question. What you said is true you might want to try using unsteady flow with a Boussinesq model. For this you will need to find the expansion coefficient of the air which can easily be found on google. Try this if you are having troubles but ideal gas should also work with unsteady flow.

The wall y+ value can be measured as a scalar in CCM+ and you can view this to determine whether you are in high (20-300) or low (< 3) wall y+. its best to stay in these brackets of values or else your calculation may be in transitional flow which would impact your accuracy. If you aren't in these values you need to adjust your mesh.
From the paper found here, these are the dimensions of the heat sink.

L=38mm
W=160mm
S=99mm
d=3mm
b=7.73176964954mm

The base thickness of the plate is 8mm.

The backplate has 100Watts entering for my convection only model I used 72Watts (4460W/m^2) as this is what I got from theory.

Single heatsink
http://img850.imageshack.us/img850/452/heatsinkt.jpg

I have doubled the heatsink and rotated it so the two heatsinks are opposite each other. Each base plate has 72Watts coming in. I have modelled the heatsink in a box of air with the air boundaries being pressure outlets at 328.15K (55 C).

The heatsink doubled looks like this
http://img718.imageshack.us/img718/9863/heatsink2c.jpg

When I use the Ideal gas for my solution I get a boundary layer which leaves the model with the following vector plot.

http://img820.imageshack.us/img820/4889/idealgas.jpg


For the Boussinesq Model I get the following

http://img137.imageshack.us/img137/3...inesqmodel.jpg

I think there is a boundary layer in the Ideal gas model causing this difference in velocity plot, although I'm not sure.

Any help/suggestions?

Thanks again!

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