# Validation case query

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 February 7, 2012, 08:48 Validation case query #1 Member   Join Date: Aug 2011 Posts: 53 Rep Power: 7 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

 February 7, 2012, 17:28 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 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.

 February 8, 2012, 00:10 #3 Member   Join Date: Aug 2011 Posts: 53 Rep Power: 7 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?

February 8, 2012, 00:33
#4
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 13,449
Rep Power: 104
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.

 February 8, 2012, 01:07 #5 Member   Join Date: Aug 2011 Posts: 53 Rep Power: 7 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.

 February 8, 2012, 05:56 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 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?

February 8, 2012, 09:03
#7
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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.

Attached Images
 def file.jpg (22.8 KB, 8 views)

 February 8, 2012, 17:48 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 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.

 February 8, 2012, 22:52 #9 Member   Join Date: Aug 2011 Posts: 53 Rep Power: 7 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

February 8, 2012, 23:34
#10
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 13,449
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Quote:
 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.

 February 10, 2012, 08:58 #11 Member   Join Date: Aug 2011 Posts: 53 Rep Power: 7 yeah Thanks

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