In regards to the Darcy-Weisbach equation and Moody Diagram - why does laminar flow (Reynolds number less than 2,000) correlate to a higher friction factor than turbulent flow?
Should turbulent flow correlate to a higher friction factor and therefore a higher pressure loss per foot of pipe?
I'm designing a system using a newly listed antifreeze with a viscosity of over 300 cP at -25 deg F. This causes a very low Reynolds number (around 250) since viscosity is in the denominator of the Reynolds equation. With a low Reynolds number, the Friction Factor is high (64 / Re) which makes sense because I'm calculating for antifreeze, but I don't understand how laminar flow correlates to a higher Friction Factor.
Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
4/21/2021 08:52:33 am
Although the friction factor is generally higher for laminar flow than it is for turbulent flow, the actual friction losses that the fluid experiences are generally lower. The friction factor is effectively a made-up term used to simplify a more complex equation (based on a lot of empirical data) to yield a simpler Darcy-Weisbach equation.
4/21/2021 08:56:50 am
Reynolds # is directly proportionate to velocity. There are 2 types of friction occurring, static and kinetic. As flow begins it must overcome static friction. It requires greater pressure to overcome inertia of rest. Once moving the friction lessens until the flow develops to the transition region, after which the behavior of the flow changes. If you look at the velocity profile over the cross-section proven by Blasius and known as the no slip condition, you will see that even in a fully developed flow as you get infinitesimal closer to the inside wall of the conduit the velocity approaches zero. That means, even in high turbulent flow the fluid at the pipe wall has a velocity close enough to consider it zero. It's helpful to think of the static friction as if the fluid at rest is a semi-solid with the outer portion having friction with the pipe wall and the inner portion having friction with itself.
Leave a Reply.
Subscribe and learn something new each day:
Top Feb 2023 Contributors
Get 100 Days of Free Sample Questions right to you!
PE PREP SERIES