Pressure Drop

tonyjk

Hello... I can't find the difference between the pressure drop in a pipe due to frictionnal loss and the Pressure difference that cause the flow like in Poiseuille Flow.. Thanks

Travis_King

They are completely different ideas...what specifically don't you understand about either?

Edit: Oh are you talking about like the losses associated with flows like Poiseuille Flow?

Travis_King

If you are asking how this solution to pressure drop is associated with similar ideas such as the Darcy-Weisbach equation, the answer is that the analytical (experimentally derived) Darcy-Weisbach equation is employable under broader circumstances.

The assumptions taken for Poiseuille Flow equations are (from wikipedia) "...that the fluid is viscous and incompressible; the flow is laminar through a pipe of constant circular cross-section that is substantially longer than its diameter; and there is no acceleration of fluid in the pipe". In theory this is a nice equation to look at to understand where the mechanical energy is being lost to, but in practical applications of pressure drop analysis it is rarely, if ever, employed.

Basically: Poiseuille equation is theoretical, solutions like Hazen-Williams and Darcy-Weisbach are analytical.

tonyjk

we say that gradient pressure cause flow but in pipe flow the pressure different is due to friction loss

Travis_King

To see how people generally use these equations (Poiseuille is mentioned in there), see here

A solution to Poiseuille's equation is used to approximate the D-W friction factor for Laminar, fully developed flow in long pipes.

tonyjk

Thank you i understand it

Chestermiller

In polymer processing applications, typically involving high viscosity polymer melts (say 1000 Poise), the Poiseuille pressure drop equation is used extensively. This includes the entire man-made fiber industry, polymer granule production industry, and plastics manufacture industry. In addition, it applies to flow of ordinary fluids through capillaries.

Travis_King

Thanks for that, I wasn't aware it was so widely used in that industry. I've never personally had any experience with such high viscosity fluids.