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Heat transfer simulation of exhaust manifold

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Old   May 16, 2014, 04:23
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Glenn Horrocks
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But is it significant? Is the thermal time lag across it significant? Do you know the boundary condition on the outside of the pipe better than you know it on the inside? These are the questions you need to answer to choose the location of the boundary.
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Old   May 16, 2014, 06:59
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Well it depends on what ones definition of significant is. At high exhaust gas temperatures I have got a gradient from inner to outer wall temperature of about 7% (measured experimentally).
But yeah, since I rather know the bc on the inner side of the wall, I'll apply my bc there.
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Old   May 16, 2014, 07:37
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You are the one who knows how accurate you need to be. If you are looking for a super-accurate result then 7% is a lot. If you are looking for a general idea of what goes on then it is fine.

But your plan sounds fine by me
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Old   June 3, 2014, 17:40
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Just wanted to keep you guys updated:
I've used your approach for my simulation and I am now waiting for the test bench to be available, in order to validate my results.

I have one more question. Since the test bench won't be free for another 2-3 weeks, I'm interested in doing a CHT simulation as well.
I already started modeling the pipe as a solid region.
Ghorrocks, you proposed a heat flux boundary condition on the outer wall pipe using a CEL expression.
I wonder where you would get a reasonable convection coefficient from. Is that something you would find in the literature you mentioned (from DeWitt and Incropera)?

Once again, thanks in advance.

Edit:
Let me put my question differently. If I model the solid region in a CHT simulation and apply the combined radiation and convection boundary condition on the outer wall, do I then use an overall heat transfer coefficient or only the HTC from the solid material to the surrounding air (so the "outer" HTC)?

Last edited by Phili; June 5, 2014 at 14:17.
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Old   June 10, 2014, 05:49
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Any advice?
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Old   June 10, 2014, 06:43
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To do the combined convective and radiative heat transfer boundary you use the specified heat flux boundary, and set the heat flux to the sum of convective and radiative components.
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Old   June 10, 2014, 07:03
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Thanks for your reply ghorrocks.
Yeah, so far I understood the method. I just wonder now, whether the HTC I have to use is a combined HTC (so HTC at the inner wall and HTC at the outer wall) or just the HTC for the outer wall (defining the heat transfer from the exhaust pipe material to the ambient air).

I had a look at Fundamentals of Heat and Mass Transfer by Incropera and DeWitt and noticed, that there's always a separation of the heat transfer coefficient for the inner and outer walls, with the inner HTC being significantly bigger.
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Old   June 10, 2014, 08:25
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The HTC coefficient is a function of the gas properties and the gas flow and a weak function of the temperature difference. The fluid velocities inside an exhaust pipe are going to be much higher than outside, so it follows that the HTC is high on the inside and low on the outside.
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Old   June 10, 2014, 08:38
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Got it. So am I assuming correctly, that since I'm applying the heat flux boundary on the outer pipe wall, I'm just using the lower HTC value for the outside wall.
Moreover the heat flux and the HTC at the fluid-solid interface (at the inside wall) is being calculated by the solver on its own.

So under the assumption that the ambient air is still and my material being cast iron, I found HTCs of around 6-10 W/m^2*K.
Does that seem to be a reasonable value?
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Old   June 12, 2014, 03:37
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Quote:
Originally Posted by Phili View Post
Got it. So am I assuming correctly, that since I'm applying the heat flux boundary on the outer pipe wall, I'm just using the lower HTC value for the outside wall.
Moreover the heat flux and the HTC at the fluid-solid interface (at the inside wall) is being calculated by the solver on its own.

So under the assumption that the ambient air is still and my material being cast iron, I found HTCs of around 6-10 W/m^2*K.
Does that seem to be a reasonable value?
Let me add something to my question:

Is there a common way to calculate/find out heat transfer coefficients using the CHT approach ?
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Old   June 12, 2014, 03:40
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Your post #29 is on the right track. And yes, h=6 or 10 for a pipe in ambient air with no forced flow would seem about right.

You can see the HTC it works out for the fluid-solid interface in the post processing.
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