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One question about dynamic pressure

  1. Jul 15, 2017 #1
    As per wikipedia, dynamic pressure is nothing but the expression of kinetic energy of unit volume of a fluid in motion. That's applicable to gases too. Question is, when a fluid is passed through a convergent and/or convergent-divergent nozzle, then its speed towards a specific direction increases. Does that mean that its dynamic pressure also increases in that direction too?
    And, does this dynamic pressure can be converted into static pressure by simply passing through a diffuser?
     
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  3. Jul 15, 2017 #2

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    Yes.
    Yes.
    Static pressure has no preferred direction and dynamic does. They sum to total pressure, which remains constant if no energy is added or taken away.
     
  4. Jul 15, 2017 #3
    Well, then as per your answer, we can inject a low pressure gas into higher pressure with less energy consumption by using a convergent and/or convergent-divergent nozzle. Right? As the nozzle means higher velocity at the entry, that means the conversion into pressure is also higher. That's as per the answer you have given.
     
  5. Jul 15, 2017 #4

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    So you are adding energy to inject?
    Do you mean total pressure?
    In the narrow part of a venturi the dynamic pressure is higher, the static pressure is lower, and the total pressure is unchanged.
    Are you saying that it is easier to force more air into a high pressure container through a small opening than through a large opening? That sounds correct, but I am not sure that it is a good application of Bernoulli's principle. I think that you would still have to add enough energy to raise the total pressure of the outside air above that of the air in the container.
     
    Last edited: Jul 15, 2017
  6. Jul 15, 2017 #5
    Certainly. It's impossible to inject air/gas without energy input. What I want to mean is that whether the nozzle can do some kind of multiplying effect or not. No energy means multiplying something with zero.
    Actually, when the diffuser will convert the dynamic pressure into static pressure, then what will be left at the end is static pressure alone. I want to mean the pressure after the diffuser converting the dynamic pressure into static pressure.
    Not only the pressure, but also the temperature is lowered and this increased velocity come at the expense of both the pressure and temperature.
    I specifically want to use a convergent and/or convergent-divergent nozzle for injecting lower pressure air/gas to higher pressure because the nozzle will convert internal enthalpy (both pressure and temperature) into motion i.e. dynamic pressure.
     
  7. Jul 15, 2017 #6

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    I don't believe that will work. If anything I said implied that, then I didn't mean it that way.
     
  8. Jul 15, 2017 #7
    Kindly just read this thread. It's a fact that convergent and/or convergent-divergent nozzles can convert internal heat into motion.
     
  9. Jul 15, 2017 #8

    Nidum

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    If that were actually true you could build a jet engine with no moving parts .
     
  10. Jul 15, 2017 #9

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    I think we have all witnessed that air escaping through a nozzle from a high pressure tank is cold. But that is air flowing from high pressure to lower pressure. That is how it will always flow. Your question was whether a venturi would help you to get air to flow from low pressure to higher pressure. The answer is no. You will have to raise the pressure of the air source higher than the pressure of the tank you want it to flow into. Any restriction like a venturi will not help; it will make it harder.
     
  11. Jul 15, 2017 #10
    Ramjet and Scramjet don't need any kind of moving parts.
    800px-Nozzle_de_Laval_diagram.svg.png
    The picture from wikipedia clearly shows how both pressure and temperature falls with the increase in velocity. Inside a nozzle, the speed of fluid increases with decreasing radius of the pathway and in case of a convergent nozzle, the speed is at the peak at the throat. In case of convergent-divergent nozzle, velocity increases even after exit from the throat. It's also a known fact that both temperature and pressure decreases inside a nozzle as soon as it reaches the throat. This basic phenomenon is totally different from the scenario what you have described. in your described case, cooling comes because the compressed air has done some work by increasing its volume and that reduces its temperature.
     
    Last edited: Jul 15, 2017
  12. Jul 16, 2017 #11

    jim hardy

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    is this any help?

    https://en.wikipedia.org/wiki/Injector

     
  13. Jul 16, 2017 #12

    Nidum

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    These both depend on there being airflow induced by the forward motion of the engine .

    If what you say about nozzles were true then you could build an engine with no moving parts which produced continuous thrust while sitting static on the ground .
     
  14. Jul 16, 2017 #13
    Simply impossible. What nozzles can to is to multiply the effect of the input flow. Static engine means it has to start from zero. Now, whatever you multiply with zero, the end result would be zero.
    Kindly don't put words on me that I haven't said.
     
  15. Jul 16, 2017 #14

    Nidum

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  16. Jul 16, 2017 #15

    Baluncore

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    I have seen a ram jet generate thrust while being held static in a bench vice. It required a burst of compressed air to open the inlet valves, then it ran continuously without forward motion, or compressed air.
     
  17. Jul 16, 2017 #16

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    I have trouble believing that a restriction in the passageway between two different pressures can assist the flow from lower pressure to higher. If anything, it seems that the restriction must require more work to overcome. If it is true, I would like to see an experiment that shows it.
    This is very interesting. But I think it shows something different from the use of Bernoulli. It uses the latent heat of evaporation (when the water condenses and releases additional heat) to increase velocity and assist in the injection.
     
  18. Jul 16, 2017 #17

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    Heat is non-directional. If it is converted to directional motion, I think it would push backward as well as forward and require more work to force the "lower pressure" air through the venturi. I quote "lower pressure" because I believe that the total pressure must be raised above the other side before the air will flow. That requires work.
     
  19. Jul 16, 2017 #18

    Nidum

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    ' Water will flow down hill on it's own . To make it go up hill you need a big man with a big brush ' - Dr.Hooker at one of his famous impromptu drawing board tutorials .
     
  20. Jul 16, 2017 #19

    jim hardy

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    The diagram he posted makes use of internal heat as well.
    From its parent article https://en.wikipedia.org/wiki/De_Laval_nozzle
    We're all so accustomed to using Bernoulli for incompressibles that we forget (well at least I do) it has a term for internal energy.

    As an old broken down EE i seldom delve deep into Bernoulli . It always made me feel sympathy for Mechanicals - my Ohm's law is SOOO much simpler than theirs.

    http://geosci.uchicago.edu/~moyer/GEOS24705/Notes/Bernoulli.pdf
    upload_2017-7-16_9-10-15.png
     
  21. Jul 16, 2017 #20
    That's the inherent quality of a nozzle. It has been tested quite a long time and already accepted. In this thread, Just read what boneh3ad has said here.
    When heat is converted into velocity by a nozzle, it becomes directional.
     
  22. Jul 16, 2017 #21

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    There was work done to put the heat in there in the first place. There is work done to cause the heat to be converted into velocities directed in one direction. Air does not go from low pressure to high pressure just because of the shape of the opening.
     
  23. Jul 16, 2017 #22
    Heat is the lowest form of energy and doing mechanical work isn't always necessary to heat up something. Moreover, the fluid that will be passed has already its stored heat as internal energy.
    Some work is necessary to start the flow, but no extra mechanical work isn't necessary to convert the internal heat into motion. It's the inherent character of nozzle. And, by the way, no one here (including me) is saying that a convergent and/or c/d nozzle can in itself pump up air/gas from low pressure to high pressure. What I want to say is that they can reduce power consumption to do that by converting a part of internal enthalpy of the air/gas into motion. Just like the ramjet example, something is necessary to give a kick start (in this case a blower) and the nozzle will enhance the speed at the expense of internal energy.
     
  24. Jul 16, 2017 #23

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    In a typical experiment, the changes of temperature, pressure, velocity, etc. that occur in a venturi are due to the work done to force the air through the venturi. They are consequences of the work and do not reduce the amount of work required.
    One exception may be the example of the use of latent heat of transition that happens in @jim hardy 's steam injector example. But that requires energy to produce the steam and is an entirely different principle than Bernoulli's principle.
    I would consider providing heat to be equivalent to doing work. Otherwise, the simplest thing would be to warm the low pressure side till its pressure rose above the other side.
     
  25. Jul 16, 2017 #24

    russ_watters

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    @pranj5, note that your first post references a wiki article on incompressible flow. You would save everyone a lot of time and confusion if you described more accurately and specifically what type of scenario you are referring to.
     
  26. Jul 16, 2017 #25

    jim hardy

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    Quite so.
    Look at a boiler

    injector4pranjl.jpg

    They take out high energy fluid and use it to pump back in even more low energy fluid.

    A piston or turbine driven pump would be intuitive.
    But Thermodynamics is where one has to have faith in his math.

    I suppose if i'd ever worked on one of those things i'd be less amazed at the thought of it.
     
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