The picture below shows a venturi injector.

venturi injector

When the water flow enters the narrower cross section its velocity increases and its pressure decreases. Subsequently, fluid is sucked (or injected) into the venturi. The injected fluid is accelerated as it moves towards less pressure. This answer says that this acceleration is because internal kinetic energy gets converted to external kinetic energy - or random particle movement is converted to organized flow.

This is plausible insofar as particles experience less collisions with other particles when they move towards less pressure - they can move with less resistence in that direction. In the end, random particle motion is converted to directed particle motion without energy expense, which contradicts the 2nd law of thermodynamics.

What's your take on this?

  • $\begingroup$ A venturi is a momentum transfer device. It violates the laws of thermodynamics just as much as a fan does. That is, not at all. $\endgroup$
    – Jon Custer
    Jul 10 '15 at 17:48
  • $\begingroup$ @JonCuster: Could you please elaborate on this a little more? $\endgroup$
    – Chris
    Jul 11 '15 at 6:08
  • 2
    $\begingroup$ Work is performed to establish the water (or air) flow from left to right in the Venturi. This means that each particle has a net momentum to the right. water (or air) from the bottom that moves into that flow undergoes collisions with the stream. The net result of these collisions is to give those particles momentum towards the right. This entrains those particles into the main flow, pumping them away from the bottom entrance. Note that the momentum distribution upon exit is more disordered than at the entrance. Still, the pumping is transfer of momentum. $\endgroup$
    – Jon Custer
    Jul 11 '15 at 15:33
  • $\begingroup$ @JonCuster That's a good answer $\endgroup$ Nov 29 '16 at 6:16

In order to make this work, water needs to flow through the venturi, which takes energy and increases entropy. This energy by far surpasses the energy of conversion of random particle motion to orgonized motion. Thus entropy increase due to making water flow is larger than the corresponding entropy decrease due to making random particles flow in organized manner via a venturi. So second law of thermodynamics holds ie entropy of system as a whole increases.


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