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!