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#21 | |
New Member
Martijn
Join Date: Jan 2022
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Quote:
Aluminum Commercial Sheet: 0.09 Aluminum Heavily Oxidized: 0.2 - 0.31 Aluminum Highly Polished: 0.039 - 0.057 Aluminum Anodized: 0.77 Aluminum Rough: 0.07 I have no idea what I should use but the radiation heat transfer is much lower with these kind of values. I think the 'Aluminium Commercial Sheet' is a good approximation for the emissivity. |
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#22 |
Senior Member
Lucky
Join Date: Apr 2011
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Yeah this is exactly why you run heat leakage experiments. It's really not cool having an uncertainty of 100% in a quantity that you've assumed until now to be 0.
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#23 |
New Member
Martijn
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I get why its not 'cool', but I don't have the experience, time or equipment to eliminate this. If I assume a worst case scenario of an emissivity of 0.1 and delta T of 100 C, the radiation leakage will only be 1.2 W. Everyone has to start somewhere, I get that my experiment is not extremely accurate but that's why I'm here.
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#24 | |
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Gerry Kan
Join Date: May 2016
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Having said that, 0.1 seems like a reasonable value, since it is not so finely polished, which still gives you 0.3 W (from the 0.6 you see now). Gerry. |
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#25 | |
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Martijn
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#26 |
Senior Member
Gerry Kan
Join Date: May 2016
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Martijn:
Just out of curiosity, could you show the plots from your previous message with the normalized temperature ![]() ![]() ![]() This way you have a more meaningful basis of comparison between the experimental and CFD data. Of course, you should use the same ![]() Gerry. |
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#27 |
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Martijn
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I have another question; STAR-CCM+ assumes a Turbulent Prandtl Number of 0.9, in my opinion it should be around 0.7-0.71 for dry air at 20C right? |
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#28 |
Senior Member
Lucky
Join Date: Apr 2011
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0.7 is the molecular Prandtl number, which is a material property (of air).
The turbulent Prandtl number is a flow property and is for modeling of the turbulent heat flux. This number varies from flow to flow but you shouldn't ever touch this unless you really know what you are doing. It's the heat equivalent of turbulent viscosity. |
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#29 | |
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Martijn
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#30 |
Senior Member
Lucky
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The turbulent Prandtl number is not a mislabel. It is indeed a turbulent Prandtl number. But it is here being listed as a material property, even though it is not a material property (in the thermodynamic sense). It shouldn't be there, but it is.
And this is one of my grievances of Star. This is what happens when you hire programmers to write a CFD code. They're really good at object oriented programming (emphasis on the object part) and doing things that makes sense to them and no one else. |
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#31 | |
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Martijn
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#32 |
Senior Member
Lucky
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I don't want to add too much that it distracts from the original topic. But you will eventually find that turbulent Schmidt numbers, etc. are all also listed under material properties. It is a global Star philosophy to put them there and it's not a bug.
As for why this is allowed to happen? Well it probably doesn't help that the governing equations in laminar and (U)RANS are identical and we can use an effective viscosity (and effective conductivity, and so on) for turbulent flows and the rest of the code is the same. This convenience lasts only as long as we stick to eddy viscosity models. But if you weren't using an eddy viscosity model then you wouldn't have a turbulent viscosity, turbulent Prandtl number, turbulent Schmidt number, etc. to worry about anyway. |
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#33 |
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Gerry Kan
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Hi Martijn:
Sorry for not getting back to you sooner. I was stuck in a writer's block for 3 days and now it's over! Thank you for plotting this in normalized form. You can now really the level of discrepancy between observation and simulation results, which is about 20% in the steady state. For heat transfer problems I would say it is not unreasonable. Of course, the various contributing factors can be the starting point of your discussion. I am interested to see this. For the record, the turbulent Prandtl number is usually 1 or close to 1 to reflect the effects of mechanical mixing due to turbulence in the flow. I doubt you need to change this value because you don't know what additional calculations are done under the hood of Star-CCM+, in addition to what we were taught from text books. Gerry. |
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#34 | |
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Martijn
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heat transfer cooling, validation |
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