flow and velocity contradiction???

[tarek12]

Hi
when we talk about pressure , velocity and flow of a liquide we have this theory:
flow of a liquide is the same at all points of a cylinder
and if pressure increase velocity should be decrease ...thats clear
what i don't understand :
for a same diameter in a pipe but after a pressure valve, pressure will decrease we know that
Q = V * S / Q : volumique flow, S : area ; V : velocity
so if Q , S are the same in different points A and B ( see the image )we have a contradiction because
V should be in the point B is greater than in the point A , ( pressure are lower in the point B )
could u explain that to me?

 

[Latexman]

No image attached.

Are you asking about an incompressible fluid or a compressible fluid? Maybe you should think in terms of the continuity equation, mass flow rate = density x area x velocity.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

The part that is tripping you up is the part that you dismiss with a "that's clear". It is anything but true that when pressure increases, the velocity has to decrease (or increase). If differential pressure increases, then velocity increases, but I can raise upstream pressure a bit less than I raise downstream pressure and lower velocity.

Liquid is essentially incompressible, so for a conduit that does not have any addition or removal of fluid between the inlet and the outlet will have the same mass flow rate everywhere (by the continuity principle), and with an incompressible fluid, the volume flow rate will be the same everywhere, and the velocity at any point will be a function of the [constant] volume flow rate and the cross-sectional area of the conduit at each point.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[tarek12]

Latexman
I attached the image
zdas04...sorry i douldnt get what do u want say here!!

 

[LittleInch]

Still no image.

Difficult to answer without it as we don't know where A and B are....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

tarek12,

Here it is:

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

tarek12,
What part of what I said did you not understand? Let us assume that your "liquide" is water. Water requires several hundred thousand psi to increase its density by 1%. That is incompressible for most engineering applications. So if:

q = (mass flow rate)/density
v = q/(flow area)

If density is constant for all expected pressures, and mass flow rate is constant at every point in your system, then changing pressure does not change velocity.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[tarek12]

understood!!
thak you
zdas04