How to Calculate Nusselt Number for Air Flow in a Pipe with ANSYS?

In summary, the conversation discusses the calculation of the Nusselt number for air flow in a pipe with 5mm diameter and 5atm pressure. The equation for Nusselt number is given and the speaker wants to calculate the constants N1, N2, N3, and N4. The other person suggests finding the film coefficient and using CFX for fluid simulations. They also mention using the Colburn equation to solve the problem and recommend doing a hand calculation instead of a numerical simulation for better accuracy. The speaker asks for the title of the textbook and it is "Introduction to Heat Transfer" by Incropera and DeWitt.
  • #1
amirghaderi
21
0
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 don't know how?
can anyone Help?
 
Engineering news on Phys.org
  • #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
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 infinite number of solutions given one data point.
 
  • #5
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
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 can't do it axis symmetric..
i create 3D pipe geometry in ansys workbench and i import it in ansys.
but i can't mesh it with element "FLUID116" ...
with witch element i can mesh instead??
tanx
 
  • #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 don't work in work bench i just create its geometry in workbench.
but i do other analizes in ANSYS .
 

Attachments

  • pipe.jpg
    pipe.jpg
    35.8 KB · Views: 745
  • #11
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
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
 

What is the Nusselt number in ANSYS?

The Nusselt number is a dimensionless parameter used in fluid mechanics to quantify the convective heat transfer between a fluid and a solid surface in ANSYS. It describes the ratio of the convective heat transfer to the conductive heat transfer across the boundary layer.

How is the Nusselt number calculated in ANSYS?

In ANSYS, the Nusselt number can be calculated using the formula Nu = hL/k, where h is the convective heat transfer coefficient, L is the characteristic length, and k is the thermal conductivity of the fluid. This formula is based on the Nusselt number definition as the ratio of convective heat transfer to conductive heat transfer.

What is the significance of the Nusselt number in ANSYS simulations?

The Nusselt number is an important parameter in ANSYS simulations as it helps in predicting the convective heat transfer between a fluid and a solid surface. It is used in the design of heat exchangers, cooling systems, and other thermal management applications.

How does the Nusselt number affect the heat transfer in ANSYS?

The Nusselt number directly affects the heat transfer in ANSYS simulations. A higher Nusselt number indicates better convective heat transfer, leading to a more efficient transfer of heat between the fluid and the solid surface. A lower Nusselt number indicates poor convective heat transfer, resulting in a slower heat transfer process.

What are the limitations of the Nusselt number in ANSYS?

The Nusselt number is limited to situations where the convective heat transfer is the dominant mode of heat transfer. In cases where other modes, such as conductive or radiative heat transfer, are significant, the Nusselt number may not accurately represent the overall heat transfer. Additionally, the Nusselt number is dependent on the fluid properties and the geometry of the system, so it may not be directly applicable to different systems.

Similar threads

Nusselt number for a developing turbulent flow in the entrance region
    • Mechanical Engineering
Replies
2
Views
2K
Laminar flow in a tube, heat transfer coefficient-sanity check
    • Mechanical Engineering
Replies
4
Views
1K
Please tell me the difference in the size of the air volume
    • Mechanical Engineering
Replies
3
Views
233
Advice on pressure drop calculation
    • Mechanical Engineering
Replies
2
Views
870
How to determine the pressure drop in pipe diameter sizing
    • Mechanical Engineering
Replies
5
Views
1K
Engineering Average heat transfer coefficient (forced convection)
    • Engineering and Comp Sci Homework Help
Replies
6
Views
2K
Bernoulli equation and parallel pipe branch
    • Mechanical Engineering
Replies
31
Views
2K
How can I extract the Nusselt number over time in Ansys for a cavity model?
    • Mechanical Engineering
Replies
4
Views
2K
Designing an Air Float Conveyor: Calculating Pressure and Flow for Moving Objects
    • Mechanical Engineering
Replies
3
Views
341
Can Reynolds Number < 70 Cause Turbulence in Elbow Fittings?
    • Mechanical Engineering
Replies
20
Views
4K

Hot Threads

Recent Insights

Back
Top