6 replies to this topic

[Kevinyun]

Hi everyone

 

I am currently trying to solve for pressure drop through a trapezoidal tube. I do not know how to calculate this and any help would be greatly appreciated. h = 2.5mm, a= 20mm and b=25mm and length = 275mm. Flowrate = 100kg/min, viscosity is 0.35Pa.s, density = 1g/cm3 (rough estimate of density) but it is slightly thicker then water

 

I have found articles online but not ones that have been useful.

 

Regards

 

Kevin

[breizh]

Kevin,

 

 

http://www.engineeri...eter-d_458.html

 

Go to this link and you will get your answer .

 

Same equation than for pipe , you need to calculate the hydraulic diameter Dh .

 

Dh =4*cross section /(wetted perimeter) = 4.321mm

 

 

Hope this helps you out.

Breizh

Edited by breizh, 30 April 2013 - 03:25 AM.

[Kevinyun]

Hi Breizh.

 

It doesn't have an equation for a trapezoid. It has equations for a cylindrical pipe and rectangular duct.

 

Will it still be accurate if I used the rectangular duct equation?

[Shivshankar]

Kevinyun,

 

How can you use rectangle instead of trapezoid ? 
 
http://www.engineeri...nels-d_965.html
 
Q= V*A
 
 Flow rate- Q
 Cross sectional area - A (Use Hydraulic diameter (Dh))
 Velocity - V
 
Flow rate = Driving force (i.e Pressure difference)/ Resistance
 
Hope this helps.
 
Regards
Shivshankar

Edited by Shivshankar, 29 April 2013 - 02:45 AM.

[Bobby Strain]

You use the hydraulic diameter to calculate Reynolds number and as the diameter for fL/D. Calculate the velocity using the flow area.

 

Bobby

[Kevinyun]

Hi Shiv 

 

What do you mean by resistance? i.e. Q=Delta P/resistance?

 

Regards

 

Kevin

[Shivshankar]

Kevinyun ,
 
I hope you have taken basic course in fluid mechanics.

 
If you take some efforts and show us your calculations then, it will be more easy for you and also attach your genuine question assignment as we don't know what is in the question. 

 

Since, viscosity is given, you solve this by two approaches using Darcy-Weisbach equation or by using simple bernouli's equation.
 
Step 1.........Calculate Hydraulic diameter
 
D_H = 4 * A / P
 
Where
 
A - Area of trapezoid
 
P - Perimeter of trapezoid
 
Step 2...........  Calculate Reynolds Number
 
ReDH = U * D_H / ν
 
Use Q= U * A to calculate velocity
 
Q - Flow
 
A - Area
 
U - Velocity
 
D_H  - Hydraulic diameter
 
ν - Kinematic viscosity
 
* Check if your reynolds number is Laminar or Turbulent depends on it proceed further.
 
Step 3.......... Calculate the pressure drop (you can use darcy or fanning equation for closed conduit and for open conduit you use manning eq. )

 

http://en.wikipedia....isbach_equation

 

http://en.wikipedia....friction_factor
 
 ΔP = f * 4* L / D_{H *} (0.5 * ρ *  U^2)
 
Where
 
ΔP - Pressure drop
 
f - friction factor
 
D_H - Hydraulic diameter
 
ρ - Density
 
U - Velocity
 
Step 4.........Calculate friction factor (here you have to take efforts for finding friction factor)

 

I have not came across equation of friction factor for trapezoid. If you know then you can solve easily. May be your professor have given clue about it.
 
 

 
 

Edited by Shivshankar, 14 May 2013 - 02:34 AM.