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That Awful Reynolds Number (Part 1 of 2)

By Walt Boyes

E-Zine November 2013

Recently, it occurred to me that there are a whole lot of people who do not understand the Reynolds number, what it means and how to use it.

The Reynolds number is used as a means of determining the flow regime inside the pipe, but it really isn’t that. It is a dimensionless number (meaning it is the same number in any units system) that is the ratio of the “inertia forces” inside the pipe to the “viscous forces” in the pipe. The inertial forces are the fluid momentum, gravity, pressure and all of the conditions that force the fluid through the pipe. The viscous forces are all the effects that tend to introduce drag and slow down the flow in the pipe. The viscosity of the fluid, the drag of the interior surface of the pipe, all are part of the “viscous forces” that slow down the flow. Reynolds number, which is often shown as RD, is calculated:

RD = 3160 x Qgpm x SG / ucP x D for liquids. Gas is similar.

You can see from the equation that viscosity is very important, since it can range from less than 1 (water) to into the millions (molten peanut butter is about 10,000 cP).

In laminar flow, the fluid slows as it passes the walls of the pipe, creating a theoretically parabolic “bullet-nosed” flow profile. In turbulent flow, the pipe wall effects are lower, and the velocity throughout the pipe cross-section is relatively constant. In the transition zone between laminar and turbulent flow, the flow may exhibit properties of both laminar and turbulent flow regimes, or oscillate between them. Flow is in the laminar region when the pipe Reynolds number is less than 2,000. Reynolds numbers greater than 4,000 are generally turbulent flow region indicators. The transition zone lies between Reynolds numbers of 2,000 and 4,000.

Because there is a “diameter” term (D2) buried in “Q,” when the diameter of the pipe increases, higher Reynolds numbers are easier to achieve. This means that turbulent flow is easier to maintain in larger diameter pipes.

Click here to review Part 2

From Flow Control (Nov/Dec 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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