Nusselt number For ANSYS

  • #1

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?
 

Answers and Replies

  • #2
minger
Science Advisor
1,495
2
For simple air flow through a circular conduit and given proper boundary conditions, you should be able to find analytic...well empirical relations.
 
  • #3
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...
 
  • #4
minger
Science Advisor
1,495
2
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.
 
  • #5
783
9
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.
 
  • #6
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....
 
  • #7
783
9
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.
 
  • #8
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
 
  • #9
783
9
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?
 
  • #10
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 .
 

Attachments

  • #11
783
9
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.
 
  • #12
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.
 
  • #13
minger
Science Advisor
1,495
2
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.
 
  • #14
hi minger
can you tell me what is your textbook?
tanx
 

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