Sudden expansion in duct

Forum about flow calculators available on and how to find solution for your fluid flow problem.
Post Reply
Joined:Wed Oct 02, 2013 3:35 pm
Sudden expansion in duct

Post by Expo » Wed Oct 02, 2013 3:56 pm

Last edited by Expo on Thu Oct 24, 2013 2:26 pm, edited 1 time in total.

Site Admin
Joined:Mon Feb 08, 2010 7:47 pm

Re: Sudden expansion in duct

Post by admin » Wed Oct 02, 2013 10:00 pm

Here is what I have with pressure drop calculator just for sudden expansion:

1. volume flow rate (q):
q : 0.75 m3/s

3. pipe length (L):
L : 1 mm

4. pipe diameter (D):
D : 120 mm

5. pipe roughness (kr):
kr : 0.1 mm

6. density (ρ):
ρ : 1.293 kg/m3

7. kinematic viscosity (ν):
ν : 17.3 mm2/s

8. dynamic viscosity (μ):
μ : 2.2368898E-005 Pas

9. K factor - minor losses coefficient (K ):
K : 1

10. velocity (V):
V : 66.31455 m/s

11. cross section area (A):
A : 11309.734 mm2

12. friction coefficient (f):
f : 0.019525288

13. Reynolds number (Re):
Re : 459985.38

14. pressure on the pipe start (p1):
p1 : 3 kPa

15. pressure on the pipe end (p2):
p2 : 0.15647608 kPa

16. pressure drop (p1-p2):
p1-p2 : 2.843524 kPa

So pressure drop is about 2.8 kPa.
Can you tell me about the calculator you used and provide me inputs?
Pipe flow calculations - since 2000

Joined:Wed Oct 02, 2013 3:35 pm

Re: Sudden expansion in duct

Post by Expo » Thu Oct 03, 2013 12:41 pm

Thank you for taking the time to reply.

I didn't use any calculator inputs for the expansion of the ducting, other than using the K factor calculator, I was just using the minor loss equation. I have realised that in my previous post I gave a head (loss) of 55m, and then considered this as a water head to get 540kPa. If using air as the media (which I guess is correct) it brings the head loss to around 67Pa (p2 - p1 = h x specific weight).

It's been quite a while since I've done any fluid calculations, so please excuse any stupid assumptions.

Looking at your numbers above, there is a 95% pressure loss as the pipe changes section, which as I understand it means 95% of the energy in the air flow has been dissipated due to friction/turbulence within the flow. This seems like a very large loss, especially as when using the minor loss equation I get a figure of 67Pa (which does seem very small).

To shed a little more light on the application, I require a volumetric flow rate of 0.75m^3/s at a pressure of 2kPa at the end of the ducting. Losses throughout the rest of the ducting and air filter come to about 300Pa, so I was thinking a fan that can provide the required flow at 3kPa should be sufficient.

Post Reply