Imagine I have a section of pipe that increases in cross-section at some point. If the gas in the pipe before this section is being displaced with a piston, will the gas increase its pressure and reduce its velocity while going through increasing area, or will it maintain its velocity and pressure at expense of energy delivered by the piston?

  • $\begingroup$ Consider the expression for pressure losses in a pipe: it has an L/D term so diameter does have an effect Check out ConDi nozzles (Convergent / Divergent)... $\endgroup$
    – Solar Mike
    Feb 8 at 5:41
  • $\begingroup$ The source of energy input is irrelevant unless you are actually pressurizing the system because the piston moves faster then materials exit the system. In general, it's safe to say that increased velocity equals decreased pressure, and vice versa. And there is no way in a steady state system for velocity to stay the same for varying pipe diameters. $\endgroup$
    – Tiger Guy
    Feb 9 at 21:49
  • $\begingroup$ i had this realization yesterday. The velocity has to drop in order to maintain the same flow. Therefore to maintain energy the pressure will have to increase to maintain energy. For it to do otherwise would be the opposite of acquiring energy. Sometimes when you look at formulas you lose the intuitive understanding $\endgroup$
    – Francis L.
    Feb 10 at 2:22

See the gas law. The pressure x volume / temperature remain constant. For the speed component see the kinetic theory in the wiki article. https://en.wikipedia.org/wiki/Ideal_gas_law

  • $\begingroup$ you're missing the motion of fluid here. $\endgroup$
    – Tiger Guy
    Feb 9 at 21:50
  • $\begingroup$ The speed of the gas is a consequence of the pressure difference. So no speed without pressure difference. Assuming that no energy is lost, the pxv / t value remains constant. If the piston is moved back again, for example by an external energy source, the original tooth setting can be reached again. $\endgroup$ Feb 10 at 5:41

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