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Dynamic pressure along a duct

  1. Sep 26, 2011 #1
    In an air supply duct after the fan

    static pressure is maximal directly after the blades and is zero at the duct outlet,
    volume flow is minimal after the blades, maximal at the outlet.
    mass flow is equal at all points,
    speed of air constituents is minimal after the blades (where air is being compressed) and is maximal at outlet (the air being expanded, that is, pushed out).

    But what about the dynamic pressure ?

    It seems - according to kinetic energy = 1/2 MV square - it should be maximal at the outlet, as the same mass is moving faster?
    Last edited: Sep 26, 2011
  2. jcsd
  3. Sep 26, 2011 #2
    Am I correct ?
    Last edited: Sep 26, 2011
  4. Sep 27, 2011 #3
    The formula from Wikipedia :

    dynamic pressure

    q = \tfrac12\, \gamma\, p_{s}\, M^{2},

    where (using SI units):

    p_{s}\; = static pressure in pascals,
    M\; = Mach number (non-dimensional),
    \gamma\; = ratio of specific heats (non-dimensional) (1.4 for air at sea level conditions),

    It is becoming clear now: at the outlet there is zero static pressure and only the Mach number left : increase in the dynamic pressure dramatic.
    Last edited: Sep 27, 2011
  5. Sep 27, 2011 #4
    As you flow along the duct:

    pressure drops => density decreases => velocity increases => volumetric flow increases

    Which is what you mentioned.

    The dynamic pressure increases since:

    P-dynamic = (1/2) density X velocity^2

    Although the density is dropping, the velocity is squared so is more significant.

    Also, in an ideal case: P-static + P-dynamic = constant. So at the end of the duct (in an ideal case) the static pressure is low and the dynamic has increased.
  6. Sep 28, 2011 #5
    Speed of air particles at fan blades should be maximal in the system. The system takes in for instance 1 kg/sec of air and throws out the same 1kg/sec from the outlet (supposing no losses to outside in the duct). Strangely why the same mass moving faster at the fan blades is said to have less dynamic pressure than it has at the outlet.
    Air also should heat up, then cool down, how does this conversion of energy influence the system?
  7. Sep 28, 2011 #6
    I was just looking at the ducting after the fan. In the fan, the dynamic pressure is highest in the blades and then converted to static pressure in the diffuser.
  8. Sep 29, 2011 #7
    Yes, there should be more complex processes within the fan.
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