Venturi effect: How does a second stage mapping to a Venturi tube?

[nohappy]

Quote from wiki:

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe.

But I don't understand that which part of the second stage is mapping to the "constricted section"? The orifice? The mouth piece part? Because the way I see it is that the air is coming from a constricted second (LP hose) to a larger space (second stage), and that seems to be the opposite to the concept.

 

[Bob DBF]

When air is leaving the constriction, the second stage regulating valve, it comes out a high velocity and will create a low pressure area by moving air in the regulator with the flow. This high velocity air is directed in such a manner as to create the low pressure under the diaphragm to help open the valve and make it easier to breathe.

 

[nohappy]

Thank you @Bob DBF , that's one of the explanation I've seen on the internet. However, my question is that description doesn't look like "Venturi tube" at all.

According to your paragraph, can I say that pressure P2 in the above image is actually lower because air velosity V2 is high when it comes out from the constricted area point 2? (But that's in contrast to how Venturi tube is described)
Also, if you say that "high velocity will create a low pressure area", it sounds like it doesn't matter to any change of the volume. It sounds like the high speed air create low pressure area anyway. Does that mean even if LP hose is not connected to a second stage, it still generates a low pressure around the orifice when it's freeflowing? If yes, how is that related to Venturi tube (Or Venturi effect)? Or

 

[Jcp2]

The tube is a concept for fast even smooth flow of air. If the air bounces around the second stage before reaching the mouthpiece, there is little fast flow, and no Venturi assist. If the air is directed straight to the mouthpiece by a fin or vane, it will flow smoothly and quickly. If it flows smoothly and quickly, the pressure inside the second stage will be lower, and the diaphragm will be pressed in by the water pressure for Venturi assist. In essence, there is a mini “purge” of the regulator that you will perceive as an easier breath, since you won’t have to suck as hard to breath in. A predive and dive switch on a second stage simply moves the fin or vane so that the air bounces around (predive) or is directed smoothly to the mouthpiece (dive).

 

[Bob DBF]

Eductor - Principle, Operation and Maintenance on Ships

Look at the diagram of the eductor, by directing the flow of the nozzle through the eductor it creates a vacuum that pulls the water through. By directing the high velocity air in the reg in the proper direction it will make the low pressure under the diaphragm.

The useful part of the Venturi effect is not just the ability to calculate flow, but how a nozzle can be used.

 

[nohappy]

Thank you for the article, @Bob DBF Actually, I've been reading these articles about eductor or ejector (including yours) before posting the question. But it seems to me that non of them has clearly explained how does a eductor resemble to a venturi tube or explained that phenomenon by physics.

The thing I still don't understand is that, is the air flow (or fluid) pressure drop at point 3? If yes, is that result has anything to do with the outer tube or not? i.e. Is the suction result from the air pressure drop from point 3? Or is the suction result from the pressure drop at point 2 since that cross section area is smaller to the cross section of the tube at point 3?
IOW, the pressure drop is happened at poitn 3 or point 2? If it's at point 2, I could understand that from Venturi effect (or Venturi tube). If it's at point 3, I am not able to relate that to Venturi tube.

Again, how do you refute me if I said the following statement according to Venturi effect or Bernoulli's principle - "When air is coming out from LP hose to second stage, the cross section area became larger thus the velocity decreases. If the air velocity decreased, the pressure should be higher."

Thank you for your reply, @Jcp2 Your concept is what I comprehend so far. But what I need is an explanation of "how a fast air create low pressure is the same thing as Venturi effect". Because it seems to me that Venturi effect is saying that a relative faster fluid has relative lower pressure on its path. That's a difference concept to me Because what you were saying is like when a faster air enter a space (like a second stage) that has slower air (or air is not moving), it creates low pressure. How is that resemble to Venturi or how do you explain that by physics?

 

[Zef]

Thank you for the article, @Bob DBF Actually, I've been reading these articles about eductor or ejector (including yours) before posting the question. But it seems to me that non of them has clearly explained how does a eductor resemble to a venturi tube or explained that phenomenon by physics.

The thing I still don't understand is that, is the air flow (or fluid) pressure drop at point 3? If yes, is that result has anything to do with the outer tube or not? i.e. Is the suction result from the air pressure drop from point 3? Or is the suction result from the pressure drop at point 2 since that cross section area is smaller to the cross section of the tube at point 3?
IOW, the pressure drop is happened at poitn 3 or point 2? If it's at point 2, I could understand that from Venturi effect (or Venturi tube). If it's at point 3, I am not able to relate that to Venturi tube.
View attachment 571320
Again, how do you refute me if I said the following statement according to Venturi effect or Bernoulli's principle - "When air is coming out from LP hose to second stage, the cross section area became larger thus the velocity decreases. If the air velocity decreased, the pressure should be higher."

Thank you for your reply, @Jcp2 Your concept is what I comprehend so far. But what I need is an explanation of "how a fast air create low pressure is the same thing as Venturi effect". Because it seems to me that Venturi effect is saying that a relative faster fluid has relative lower pressure on its path. That's a difference concept to me Because what you were saying is like when a faster air enter a space (like a second stage) that has slower air (or air is not moving), it creates low pressure. How is that resemble to Venturi or how do you explain that by physics?

In the diagram you present, the pressure drop is not at point 3...it is just ahead of point 3 where the opening of the "suction" area meets what I will call the "discharge" chamber of the eductor (the space ahead of the nozzle). The flow of liquid past the suction/discharge junction creates a reduction in pressure at that point. The reduced pressure gradient results in the now higher pressure of the fluid in the suction area advancing to the lower pressure of the discharge area. The liquid emitted from the nozzle sustains the flow of liquid from the suction area of the eductor by maintaining the low pressure gradient at the junction of the suction/discharge areas relative to the pressure of the suction area area of the eductor.

-Z

 

[Zef]

Thank you for your reply, @Jcp2 Your concept is what I comprehend so far. But what I need is an explanation of "how a fast air create low pressure is the same thing as Venturi effect". Because it seems to me that Venturi effect is saying that a relative faster fluid has relative lower pressure on its path. That's a difference concept to me Because what you were saying is like when a faster air enter a space (like a second stage) that has slower air (or air is not moving), it creates low pressure. How is that resemble to Venturi or how do you explain that by physics?

Here is my stab at answering your question:

The flow of liquid or gas results in reduction of pressure on either side of the stream of flow. An example of this can be seen when two vessels (ships or boats) are traveling in close proximity to each other in the same direction. The flow of water between the vessels (in this case the flow is caused by the forward movement of both vessels) results in a reduction of pressure between the vessels relative to the pressure on the other sides (the outside) of the 2 vessels. This reduction of pressure between the vessels, known as the venturi effect, causes the vessels to move closer together due to the higher pressure on the outsides of the 2 vessels compared to the pressure on the sides facing each other. As the vessels become closer, the effect is more pronounced because the reduced space acts as a constriction speeding up the flow of fluid between the 2 vessels. In order for the 2 vessels to not collide they need to counter steer towards the high pressure gradient (away from each other).

In a 2nd stage regulator, the flow that enters the body of the 2nd stage and then exits the mouth piece causes a reduction of pressure inside the 2nd stage...the only difference between the 2nd stage and the 2 vessels in the above example, is that the 2nd stage body will not (significantly) relative to the side of the 2nd stage where the diaphragm is located. The diaphragm is pressed inward due to the higher pressure on the outside of the 2nd stage relative to the lower pressure caused by the flow of air from inside the 2nd stage. The venturi lever assists things further by reducing the turbulent movement of air inside the 2nd stage making it flow more efficiently out the mouthpiece orifice....this efficiency results in a slightly faster flow of air out of the 2nd stage resulting in a small drop in pressure inside the 2nd stage relative to the pressure outside of the 2nd stage. This venturi effect therefore assists the movement of the diaphragm resulting in pressure on the demand lever which assists in opening and maintaining the flow of air into the 2nd stage.

-Z

 

[VsubT]

Thank you @Bob DBF , that's one of the explanation I've seen on the internet. However, my question is that description doesn't look like "Venturi tube" at all.
View attachment 571307
According to your paragraph, can I say that pressure P2 in the above image is actually lower because air velosity V2 is high when it comes out from the constricted area point 2? (But that's in contrast to how Venturi tube is described)
Also, if you say that "high velocity will create a low pressure area", it sounds like it doesn't matter to any change of the volume. It sounds like the high speed air create low pressure area anyway. Does that mean even if LP hose is not connected to a second stage, it still generates a low pressure around the orifice when it's freeflowing? If yes, how is that related to Venturi tube (Or Venturi effect)? Or

The referenced diagram is poorly labeled, because it makes P2 and V2 appear to be in the larger cross-section downstream. Those should actually be back farther left at section A2 to make more sense.

The major constriction is the valve itself, where the air is controlled to flow into the chamber of the second stage from the hose. A secondary constriction is sometimes introduced purposefully to allow venturi assist of breathing effort via lower relative pressure on the second stage diaphragm.

The important thing to keep in mind is relative cross-section at any given point in the flow path. Because a second-stage regulator is more complex than a simple tube, this takes a bit more mapping / visualization to follow. For constant flow, smaller cross-section equals lower pressure and higher velocity.

There's a simple diagram on this page that may be helpful to visualize flow inside a second stage. There's a more complex one here that shows how the adjustment comes into play.

Lance

 

[herman]

Let me try this with a simpler idea.
In your eductor drawing ( this is how regulators are designed) the gas coming out of the nozzle is moving at high speed. As the jet of air shoots down tube 2, the high speed air molecules leaving nozzle 3 "grab/shove/push" other static molecules around them and pull them down the tube with the high speed ones. This leaves the area void of air molecules (vacuum) which causes others from the area behind the jet in areas 1 and 5 to move to fill the void left by the air molecules that were moved away. As long as the jet continues, it will pull more molecules away that have to be replaced, nature hates a vacuum, if the area 1/5 above were sealed off, those areas would become a vacuum as many of the molecules from the area had been swept away with none to refill the space.
Might not be a physics book answer but maybe it will make it more clear.