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Old   February 7, 2012, 08:48
Default Validation case query
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Hi

I am trying to validate a flow through straight fin heat sink.

My reference paper is" Analytical Forced convection modeling of plate fin heat sink, by P Teertstra, M M Yavanovich and J R Culham and T Lemczyk"

I have taken the experimental results from these paper where the author specifies a input heat in watt and velocity, It was assumed that the base plate temperature is constant for any heat input.

I have just taken half fin and half air flow path with symmetry conditions.

I am using air at 25 deg C, and specified the heat flux at the bottom surface = (heat input/area of base plate)

I have to get the average base plate(surface) temperature to compute the Nu defined in the paper.

As i understand the wall variable has to be hybrid and i have taken it. but trend is quite different.

pl do help asap

Regards
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Old   February 7, 2012, 17:28
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What is your question? Is it that your results are inaccurate?

If so this might help:
http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

And in your case I think you will find the symmetry condition is wrong. The flow is likely to be asymmetric, even though the geometry is symmetric.
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Old   February 8, 2012, 00:10
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Thanks ghorrocks.

Just want to know few more things,

is the air at 25deg c is OK for this heat transfer simulation or i have to use air ideal gas?

ignoring the conduction losses, can we assume the total heat in watt to be equally dissipated throughout the area by using a surface with

heat flux = (total heat/area of base) ?

what are the implications of domain imbalances of H-energy and T-energy of around 0.4% to 0.5%

can anyone help?
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Old   February 8, 2012, 00:33
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Quote:
is the air at 25deg c is OK
That depends on what you are modelling and what results you want to see.

Quote:
ignoring the conduction losses, can we assume the total heat in watt to be equally dissipated
That depends on what you are modelling and what results you want to see.

Quote:
what are the implications of domain imbalances of H-energy and T-energy of around 0.4% to 0.5%
That depends on what you are modelling and what results you want to see.

Quote:
can anyone help?
Yes, we can help, but I will need some idea of what you are trying to do.
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Old   February 8, 2012, 01:07
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Hi,

I am trying to validate my run with an experiment result- forced convection over shrouded plate fin heat sink ,

Experiment input
heat is given and is assumed to be spread uniform throughout the base plate.
velocity between fins(flow passage) is given.

Experiment output
temperature of base plate, which the author claims to be uniform.
Nusselt number(defined by author) from the above temperature and heat input.
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Old   February 8, 2012, 05:56
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What temperatures do you expect to see? What Re is the air flow velocity? Or is it driven by natural convection? What does it look like (post an image)? What surrounds the region of interest?
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Old   February 8, 2012, 09:03
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Hi

My case has Re = 1044 ; U =3.754m/s, Dh = 4.17mm,

There is no natural convection, it is forced convection.
I expect a temperature at the bottom surface of air flow passage to be of the order of 10.

I attach the image herewith.
this has a half fin with a symmetry surface and a half flow passage with a symmetry surface. a fluid solid interface is created in between.

heat flux is specified all throughout the bottom. air velocity is specified at inlet.

other surfaces are adiabatic
Attached Images
File Type: jpg def file.jpg (22.8 KB, 8 views)
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Old   February 8, 2012, 17:48
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OK, thanks. I assume you mean 10C by your estimated temperature difference - this should mean a constant properties, incompressible buoyant fluid approach should be good enough unless you are looking for super accuracy.

Also I assume at this Re the flow is laminar. Is this the case?

Upstream and downstream conditions might be important. Have you consdered them?

And as I previously said, you probably need to model the full geometry, no symmetry plane. The flow is probably not symmetric.
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Old   February 8, 2012, 22:52
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Thanks ghorrocks

Yes, the flow is laminar for this case.

What do you mean by upstream and downstream conditions?

and, everything being symmetric about a plane why should the flow be unsymmetric?

Regards
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Old   February 8, 2012, 23:34
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everything being symmetric about a plane why should the flow be unsymmetric?
Sounds like you are new to CFD and fluid dynamics. Ever heard of a Von Karman vortex street? Look it up on Google. This is the most famous example of a symmetric geometry generating an asymmetric flow.
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Old   February 10, 2012, 08:58
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yeah

Thanks
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