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The Prony equation (named after Gaspard de Prony) is a historically important equation in hydraulics, used to calculate the head loss due to friction within a given run of pipe. It is an empirical equation developed by Frenchman Gaspard de Prony in the 19th century:
Moody's team used the available data (including that of Nikuradse) to show that fluid flow in rough pipes could be described by four dimensionless quantities: Reynolds number, pressure loss coefficient, diameter ratio of the pipe and the relative roughness of the pipe.
The data for these points lie to the left extreme of the abscissa and are not within the frame of the graph. When R ∗ < 5, the data lie on the line B(R ∗) = R ∗; flow is in the smooth pipe regime. When R ∗ > 100, the data asymptotically approach a horizontal line; they are independent of Re, f D, and ε / D .
After both minor losses and friction losses have been calculated, these values can be summed to find the total head loss. Equation for total head loss, , can be simplified and rewritten as: = [() + (,)] [5] = Frictional head loss = Downstream velocity = Gravity of Earth
The new flow rate, = + is the sum of the old flow rate and some change in flow rate such that the change in head over the loop is zero. The sum of the change in head over the new loop will then be Σ r ( Q 0 + Δ Q ) n = 0 {\displaystyle \Sigma r(Q_{0}+\Delta Q)^{n}=0} .
Given a starting node, we work our way around the loop in a clockwise fashion, as illustrated by Loop 1. We add up the head losses according to the Darcy–Weisbach equation for each pipe if Q is in the same direction as our loop like Q1, and subtract the head loss if the flow is in the reverse direction, like Q4.
The following table gives flow rate Q such that friction loss per unit length Δp / L (SI kg / m 2 / s 2) is 0.082, 0.245, and 0.816, respectively, for a variety of nominal duct sizes. The three values chosen for friction loss correspond to, in US units inch water column per 100 feet, 0.01, .03, and 0.1.
This page was last edited on 2 November 2022, at 19:05 (UTC).; Text is available under the Creative Commons Attribution-ShareAlike 4.0 License; additional terms may apply.