# Nusselt number For ANSYS

## Main Question or Discussion Point

Hi
i need to calculate nusselt number for stimulation of air flow in a pipe with 5mm diameter and the pressure is 5atm.
it should be in the form:
Nu = N1 + N2 Re^N3*Pr^N4
and i want to calculate N1,N2,N3,N4.
but i dont know how?
can anyone Help?

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minger
For simple air flow through a circular conduit and given proper boundary conditions, you should be able to find analytic...well empirical relations.

hi minger
but all i have is nusselt, prandtl and reynolds numbers...
so i have 3 known factors but 4 unknown: N1,N2,N3 and N4.
it has not a unique solo...

minger
If you have all of thsoe parameters, why do you care what the constants are?

Also, here is a solution
N1 = Nu
N2 = 0.0
N3 = 0.0

The point I'm trying to make is that there will be an infinte number of solutions given one data point.

ANSYS has no input for a Nusselt number. You need to calculate the film coefficient from the Nusselt Number and then use the calculated film coefficient for your boundary condition on the pipe wall.

ok
but how i can calculate film coefficient from nusselt?
i am using fluid116...
and has N1...N4 input.
can i change its options to change required inputs?
tanx....

Is this 2D flow or 3D? (I haven't used the old FLOTRAN elements in years) Pipe flow is axisymmetric so you should do a simple 2D analysis. (3D will not show you anymore information than a 2D analysis would) You calculate the film coefficient from the relationship Nu=hD/k. Where k is the fluid thermal conductivity, h is the film coefficient, and D is the inside diameter of the pipe.

its 3D analize because it has a wavy shape (it's not straight) so i cant do it axis symmetric..
i create 3D pipe geometry in ansys workbench and i import it in ansys.
but i cant mesh it with element "FLUID116" ...
with witch element i can mesh instead??
tanx

Not sure what element you can use in workbench. (I hate workbench, it should be outlawed) I use CFX for all of my fluid simulations. I am having a hard time understanding how a cylindrical corrogated pipe is not axisymmetric. Do you have a picture of the geometry?

the pipe is like this..
i dont work in work bench i just create its geometry in workbench.
but i do other analizes in ANSYS .

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I see. What are you trying to calculate? Pressure drop? Velocity? ... Also, the model is symmetric at the midplane so you could save some time and use symmetry. If I know what you are after I can help you out better.

the problem is :
the fluid with 5atm pressure and -50C flows through the pipe , the pipe is heated by heaters on its side and has constant temp 200C.
i want to find the required length to have fluid with 100C.

minger
Did you check your textbooks as I mentioned earlier? Look for something called the Colburn equation. It is written exactly as you need and is written for computing the local Nusselt number for a fully developed (hydrodynamically and thermally) turbulent flow in a smooth circular tube.

In fact, in my textbook the following example problem is very closely related to what you're trying to do. They give a length and find exit temperature, you need vice versa.

To be honest, do this with a pencil and pad of paper, you'll be much closer than doing a numerical simulation. In order to accurately model the heat transfer from the moving fluid, you'll need to accurately resolve the thermal boundary layer. You're much better off doing a hand-calc.

hi minger
can you tell me what is your textbook?
tanx