The Venturi Effect cannot explain how an eductor works

[nohappy]

TL;DR

Bernoulli principle is only describing a fluid along a streamline while eductor's suction effect involves ambient air

I've google all the keywords like "eductor", "ejector", "Venturi pump". All of them are the mechanism or applications that utilize "Venturi Effect". All of them are trying to explain the "suction effect" by saying that restriction area makes the fluid's velocity higher and hence lower pressure which is the definition of Venturi Effect. So far, I can comprehend that. That can be explained by Bernoulli's principle as well. However, it is the following statement that confuses me for couple of weeks - "..., and the lower pressure generate a kind of vacuum thus suck the ambient fluid or air in."

First of all, the definition of Bernoulli's principle only describes a single fluid along a streamline. I get that the fluid will have higher speed and lower pressure at the restriction area than the other point on its own path. But how can you prove that the "lower pressure point on its path" is lower than ambient pressure so that it can suck fluid or air from outside? And even if you can prove that effect by physics, how is that related to Venturi Effect or Bernoulli's principle if you're discussing two different fluid?

Secondly, if the suction is created by low pressure result from high speed, why do I need to create a restriction area that generates Venturi Effect? Why not just generate high speed air at the first place? In other words, I guess you can also make a eductor with a straight pipe? Again, if that assumption is correct, how is an eductor/ejector related to Venturi Effect?

 

[Dullard]

A couple of things:

The low pressure is not 'created' by the high velocity. You have a fluid with a 'total energy' - potential [pressure] and kinetic [speed]. The converging nozzle causes an increase in speed (compared to a spot upstream of the nozzle) - this increases the kinetic energy. As energy is conserved, this has to be 'funded' from potential energy [pressure]. The pressure in the 'throat' of the device is lower than the pressure at the entrance to the nozzle. The fluid then enters a diverging nozzle - velocity is reduced and pressure increases (same mechanism). There are friction losses, but they can be surprisingly (to me, anyway) small for a properly designed device.

I'm familiar with water-motive eductors (air-motive, not so much). Eductors are designed so that there is 'space' for the 'pumped' fluid at the inlet to the diverging nozzle. It (the pumped fluid) enters the low pressure area of the eductor and is 'carried' into the diverging nozzle with the motive fluid. The designs diverge wildly, depending on the target motive/pumped fluids.

For the reasons described in the first paragraph, you can't make an eductor from a 'straight pipe.'

 

[jack action]

how can you prove that the "lower pressure point on its path" is lower than ambient pressure so that it can suck fluid or air from outside?

It is assumed that the pressure somewhere upstream the venturi, is the same as the air outside, i.e ambient pressure. Therefore, the pressure in the venturi throat has to be lower than ambient pressure.

how is that related to Venturi Effect or Bernoulli's principle if you're discussing two different fluid?

Having two different fluids doesn't change anything. If you can define a pressure at some point (say, the venturi throat), for one fluid, then this pressure is the same for another fluid at the same point.

For example, on the top of the ocean, the air pressure over it is 15 psi because there is 15 lb of air per squared inch on top of the water. The pressure on the water is also 15 psi at that point.

if the suction is created by low pressure result from high speed, why do I need to create a restriction area that generates Venturi Effect?

I think the answer from @Dullard is clear when he talks about energy conservation. It is not just about accelerating the fluid, it is about accelerating the fluid without any energy input.