Hydraulics and the darcy weisbach formula

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Discussion Overview

The discussion revolves around determining the diameter of a steel pipe using the Darcy-Weisbach equation and the Moody diagram, specifically in the context of a hydraulics assignment involving fluid flow and pressure loss calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant outlines a specific assignment requiring the determination of pipe diameter without using the friction factor directly.
  • Another participant suggests using an energy balance approach via the Bernoulli equation to relate the driving head to friction pressure loss, indicating that an initial guess for flow rate is necessary for iteration.
  • A third participant proposes a step-by-step iterative method involving estimating flow rate, calculating Reynolds number to find friction factor, and refining the flow rate until convergence is achieved.
  • A fourth participant references an external document for additional equations relevant to the problem.

Areas of Agreement / Disagreement

Participants generally agree on the need for an iterative approach to solve the problem, but there are variations in the specific methods suggested for estimating flow rate and calculating the friction factor.

Contextual Notes

The discussion does not clarify the assumptions behind the initial flow rate guess or the specific conditions under which the Darcy-Weisbach equation is applied, nor does it resolve the mathematical steps involved in the iteration process.

daiv
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I am a hydraulics student and have a question, hoping someone here could help me out. In my specific assignment i am required to determine D of a pipe without the friction factor using moody diagram and the darcy-weisbach equation.

here is the exact question:
A steel pipe (e=0.065mm) 4200m long is to convey oil (kinematic viscosity=5.2x10^-5 m/s^2) at 300 L/s from a resevior with surface elev. 247m to one with surface elev 156m. Theoretically, what pipe diameter size is required?

All the prof says is Iteration is required to find pipe diameter.


Any thoughts? anybody?

Thanks!
 
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Write an energy balance (Bernoulli equation). The driving head (the difference in elevation from one end to the other) creates a flow rate (velocity) that results in friction pressure loss that balances the driving head. So, you have to guess a flowrate and iterate. Does that make sense?
 
I would probably do the same as gmax137 suggested.

My step by step approach would be1) Use Bernoulli to get an estimate of Q (ie Q if there was no friction in the system)
2)from the estimate of Q, find your Re and hence get a value for f
3)calculate a new Q and continue to iterate between (2) and (3) until your answer converges.

Elbarto
 
See page 14 (equations 15 and 16) on the attached.
Q_Goest said:
link.
(click on the link to my post to find the attachment "Pipe-Flo Pro.pdf"
 
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