Boyle's law vs the Venturi effect

[homeylova223]

So this is something I been thinking about. In venturi principle when a fluid or gas is moving across a tube when it is constricted the fluid has increased velocity. Because it in constriction the pressure goes down.

But in Boyle law if volume is decreased the pressure goes up.

So in the venturi tube when the tube gets smaller less volume should not the pressure go up

 

[homeylova223]

Because when a fluid moves through a venturi tube when the tube gets more narrow the pressure goes down and the fluid moves faster. But if the tube get more narrow would not there be more pressure because of boyle law. Boyle law means less volume more pressure.

 

[cjl]

When fluid goes through a venturi, the volume doesn't decrease. The same parcel of fluid occupies a longer portion of the tube, and (at least at speeds below a few hundred miles per hour in air) the volume remains basically constant. The velocity goes up, but the volume does not go down.

 

[homeylova223]

Why does the fluid occupy a longer part of the tube when it narrower. Then when it is less narrow does the fluid occupy more height??

 

[cjl]

Because the pressure drops, so the fluid has to occupy the same (or actually slightly more, but this is negligible at lower speeds) volume. There's nothing constraining it to compress it, and yes, when the pipe is wider, the fluid occupies more width (because the pipe is wider) but less length.

 

[russ_watters]

Why does the fluid occupy a longer part of the tube when it narrower. Then when it is less narrow does the fluid occupy more height??

Exactly.

In a basic venturi the fluid is incompressible so by definition it must be constant volume.

The volume spreading out horizontally in the construction is the increase in velocity and the pressure drop is the result of static pressure energy being converted to kinetic energy.

 

[jbriggs444]

So this is something I been thinking about. In venturi principle when a fluid or gas is moving across a tube when it is constricted the fluid has increased velocity. Because it in constriction the pressure goes down.

But in Boyle law if volume is decreased the pressure goes up.

If the flow velocity is enough faster, you can sustain a high mass flow rate with a low density fluid (low pressure = low density) in a tube with a small cross sectional area.

You are constricting the fluid in the direction of the tube diameter. But the fluid is stretching out even more in the direction of the tube length.

 

[Lnewqban]

Because when a fluid moves through a venturi tube when the tube gets more narrow the pressure goes down and the fluid moves faster. But if the tube get more narrow would not there be more pressure because of boyle law. Boyle law means less volume more pressure.

The main difference to note is that Boyle's Law describes how pressure of a gas is inversely proportional to the volume it occupies when expanded and compressed.
That means that the gas is a compressible fluid.
It is a process not much different from a steel spring being compressed and stretched, trading length for stored force.

The Venturi effect is observable for liquids (non-compressible) and for gases (behaving as non-compressible at subsonic speeds).
For ideal conditions, the energy of the fluid going through a Venturi remains the same.
As the geometry forces it to speed up (increased kinetic energy), its pressure (potential energy) must decrease, in order to comply with Bernoulli's equation, keeping its internal energy in balance.
It is a process not much different from a ball rolling down a ramp, trading altitude for velocity.

 

[cjl]

The Venturi effect is observable for liquids (non-compressible) and for gases (behaving as non-compressible at subsonic speeds).

To elaborate a bit, to treat a gas as incompressible, the maximum velocity at the venturi throat should be less than mach 0.3, not just subsonic. However, even if the velocity exceeds mach 0.3 (but still stays below mach 1), a venturi will still act qualitatively the same way, trading pressure for velocity as the tube narrows. The mathematical relations will be a bit different though, due to compressibility.

(Of course, if you continue to increase speed until you hit mach 1 at the narrowest point, now you have a converging-diverging nozzle, which will act quite differently)

 

[homeylova223]

I think it makes sense. Say you put have a tube and you put a fluid and it moves from left to right at the tube. At the left of the tube it is greater height less width and has a high pressure and lower velocity then it goes to the constriction and the pressures goes down but the velocity goes up.

So at the begging it is high pressure and low velocity the fluid has more height but less width, in the constriction point the fluid has more width less height and more velocity but less pressure.

But pressure is Force/Area. And the are of the narrow part of the venturi tube and the more tall part is the same Area.But I think I get it.

 

[russ_watters]

But pressure is Force/Area. And the are of the narrow part of the venturi tube and the more tall part is the same Area.But I think I get it.

This doesn't make sense. By area we are referring to the cross sectional area. Obviously the narrower part has a smaller area.

The pressure drop mechanism isn't the same as the velocity change mechanism. The pressure drop is best viewed through conservation of energy and Newton's laws of motion. In order to speed up, the fluid has to have a force applied. That force is from the static pressure. It's like the pressure in all directions is converted to a pressure only in the forward direction. It's pressure energy converted temporarily to kinetic energy.