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De laval valve

  1. Sep 19, 2008 #1
    the de laval valve comprehensively reduces the pressure, temperature and increases velocity of passing gases. but it is mostly employed in areas where gases are at very high speed. could it give same results for speeds as low as, say, 50km/hr?
     
  2. jcsd
  3. Sep 19, 2008 #2

    stewartcs

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    C-D nozzles are designed to allow gases to flow at supersonic speeds. For them to work, the flow must be choked in the throat. Choked flow just means that the flow is sonic (i.e. M = 1). For a choked flow to occur, the mass flow rate and pressure must be set correctly for the specific gas and nozzle. The exit pressure is another variable that will affect the operation of it. The throat is specifically designed so that a choked flow will occur. Downstream in the divergent part of the nozzle the expansion of the gas drops the pressure and temperature of the gas. The temperature directly effects the velocity of the exiting gas.

    Here is some more info on them:

    http://exploration.grc.nasa.gov/education/rocket/nozzle.html

    CS
     
  4. Sep 19, 2008 #3
    [QUOTE Choked flow just means that the flow is sonic (i.e. M = 1). [/QUOTE]
    so does that mean that they don't give desired results at lower speeds?
     
  5. Sep 19, 2008 #4

    stewartcs

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  6. Sep 19, 2008 #5

    FredGarvin

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    Where do you want the speed of 50 km/hr? At the throat? There's no need to use a C-D nozzle if you don't need to accelerate the flow past M=1.
     
  7. Sep 20, 2008 #6
    50 km/hr is speed at which the air enters the valve.
     
  8. Sep 22, 2008 #7

    FredGarvin

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    OK. So your inlet speed is about 14 m/s. Again, I ask you what do you want to do with the nozzle? What is the point of your question?
     
  9. Sep 28, 2008 #8
    with the valve, i want to cool the incoming air. all i want to know is that if this formula- v=sqrt.(RT/M*2k/(k-1)[1-(p/P)^(1-1/k)]) holds for such low speeds or not
     
  10. Sep 28, 2008 #9

    FredGarvin

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    Considering that the equation you list id for compressible flow, you should run the numbers and look at whether or not this falls into the realm of compressible flow. Since we're talking about air, chances are that won't be an issue. In general, the rule of thumb is if you are above M=.3 you have compressible flow.
     
  11. Sep 28, 2008 #10
    at my speed, M=.036. so it's not compressible according to the thumb rule. so is the equation not valid?
     
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