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Difference in 3D simulation of duct to rotated 2D simulation?!?

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Hi everyone!

I created a simulation for a circular duct which is heated by a constant heat flux. Does it make a difference to simulate a 3 dimensional geometry or a 2 dimensional geometry (rectangle: radius*length; rotated around axis)? Are there different results?

I have the following problem right now:
I need to verify that my simulation reaches Nusselt number Nu=4.364 (equals a fully developped thermal flow profile) in the outlet of the duct. To calculate Nu first I need to determine the mixed temperature. There are two methods:

1. Integral from axis to wall over the local Temperature divided by radius OR
2. Volume integral over the local Temperature divided by the basis (radius^2*pi)

Due to the fact that the temperature closer to the wall has a greater influence caused by the higher perimeter than the temperature near the axis, method 2 gives a higher mixed temperature.

BUT the Nusselt number of method 2 is always too high. Only method 1 gives the correct answer although it does not weigh the influence of each local temperature considering the location.

PS: Both methods were tested with a fictional temparture profile of T(r)=r^2. Method 2 is 99.9999% of the analytical solution. Method 1 only about 67%.

I simply don't get it, that a tested method to determine the mixed temperature leads to a wrong result...

2 Replies Last Post 4 mar 2015, 18:20 GMT-5
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Hello Fabian Holz

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Posted: 10 years ago 4 mar 2015, 05:47 GMT-5
Hello, Fabian Holz

Could you mind telling me in your case,
how to impose a constant heat flux BC?
i am doing the some job as you to find the Nusselt number close to 4.364, but when i impose a heat flux BC with
genearl inward heat flux equals to a constant value (150 W/^2), i find the heat flux varies from one end of the circurlar tube to the other end, and only average heat flux over the wall is constant (150 W/m^2).
SO, Could you help me with this problem?
Thanks a lot!
Hello, Fabian Holz Could you mind telling me in your case, how to impose a constant heat flux BC? i am doing the some job as you to find the Nusselt number close to 4.364, but when i impose a heat flux BC with genearl inward heat flux equals to a constant value (150 W/^2), i find the heat flux varies from one end of the circurlar tube to the other end, and only average heat flux over the wall is constant (150 W/m^2). SO, Could you help me with this problem? Thanks a lot!

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Posted: 10 years ago 4 mar 2015, 18:20 GMT-5
I sent you a pm. Regards, Fabian
I sent you a pm. Regards, Fabian

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