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Design a converging/diverging nozzle

  1. Nov 3, 2016 #1
    I am intending to design a CD nozzle that produces 20mN thrust under atmospheric conditions.
    The reservoir pressure will be 1.4 bar, and the ambient pressure will be 1 bar. The other variables are adjustable. This will in the end be sent out for fabrication.My process is as follow,
    May I know if this procedure is reasonable? The throat area to exhaust area ratio looks dubious though.
  2. jcsd
  3. Nov 3, 2016 #2


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    This does not look correct to me. Your first step should be checking to see if your pressure ratio is even large enough to choke the flow. It isn't. Therefore, there is no reason to use a converging-diverging nozzle since you won't actually be going supersonic with your exhaust anyway.

    Next, you appear to be using isentropic relations between the pressure ratio and the Mach number to calculate the velocity, which may be okay here, but you are using the wrong temperature. You have to use the temperature at the location at which you are caclulating the velocity if you want to convert the Mach number this way. Using the temperature in the reservoir will just give you the Mach number at the exit relative to the speed of sound in a completely different location, which is clearly meaningless.

    Last, your mass flow/throat area calculation assumes the flow is choked, which, as I mentioned before, it is not.
  4. Nov 3, 2016 #3


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    As boneh3ad said, your pressure ratio is too low to choke the flow, so you'll need to rethink your design in some way (either increase your pressure ratio, or switch to a converging-only nozzle)
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