What is meant by friction loss?

[sameeralord]

Hello everyone,

Quick question. When there is a pipe and you constrict it there is higher pressure loss. Is this pressure lost permanent? What is this pressure loss, is it loss as heat, are liquid molecules converted to heat, are they lost? What happens to liquid molecules if the hydrostatic pressure drops to zero do they stop or do they still move because of the kinetic energy? Thanks!

 

[Bob S]

The pressure drop is due to the viscosity of the liquid as it flows through constrictions in the pipe. The power loss (energy loss) is especially high when the flow becomes turbulent (Reynolds number above ~1000). The turbulence heats the water, and the pipe. The power loss becomes zero whenever the pressure drop reaches the hydrostatic value. There are on-line pressure drop calculators on the web. Here is one:

http://www.efunda.com/formulae/fluids/calc_pipe_friction.cfm

Bob S

 

[FredGarvin]

Quick question. When there is a pipe and you constrict it there is higher pressure loss. Is this pressure lost permanent?

There is always going to be a certain amount of permanent friction loss in a constriction. However, depending on the type of constriction, some of the pressure can be recovered. You see this all the time in valves and venturis. There will be a very large drop at the vena contracta, but the pressure will recover to some intermediate value somewhere downstream. Again, the amount of recovery depends on the type of constriction.

What is this pressure loss, is it loss as heat, are liquid molecules converted to heat, are they lost? What happens to liquid molecules if the hydrostatic pressure drops to zero do they stop or do they still move because of the kinetic energy?

The energy usually gets converted to heat, sound and vibrations. If the pressure drops too much in a liquid, the liquid will vaporize and you run into issues like cavitation and boiling.

 

[sameeralord]

Thank you everyone!

 

[PJC01]

Friction loss is the term used to describe the loss of pressure that occurs when a fluid (such as a liquid or gas) flows through a pipe or conduit. This loss is caused by the resistance of the walls of the pipe and the friction between the fluid and the walls. As you mentioned, when the pipe is constricted, the pressure loss is increased because the fluid is forced to flow through a smaller space, resulting in more friction and resistance.

This pressure loss is not permanent, as it is only a temporary decrease in pressure while the fluid is flowing through the pipe. Once the fluid exits the constricted area, the pressure will return to its original level. The pressure loss is not converted to heat, but rather it is dissipated as mechanical energy due to the friction between the fluid and the pipe walls. This energy is then absorbed by the surrounding environment.

As for the liquid molecules, they will continue to move even when the hydrostatic pressure drops to zero because of their kinetic energy. However, as the pressure decreases, the speed of the molecules will also decrease, resulting in a slower flow rate. Once the pressure is completely released, the molecules will cease to move.

I hope this helps clarify the concept of friction loss. Feel free to ask any further questions. Happy researching!