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Bernoulli effect at spray nozzle

  1. Jan 1, 2016 #1
    First post so I hope I have the correct thread location.

    I am reverse engineering a 40+ year old vertical slide carburettor design i.e. the venturi is horizontal. The current spray nozzle protudes vertically into the airflow but is straight sided and flat across the exit.
    Option 1.
    Internally taper the nozzle (15°included angle) so the metering annulus remained the same near the base but the exit is now as wide as the OD and knife-edged.
    Option 2.
    Externally taper the nozzle so the metering annulus remained the same for the height of the nozzle, the exit retains the original ID but is now knife-edged.

    The question:
    Wikl option 1 or 2 improve spray spread and atomisation at the exit for a given airflow? or option 3 : do nothing.
    20160102_083527.jpg 20160102_083820.jpg
  2. jcsd
  3. Jan 3, 2016 #2


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    If the spray nozzle is perpendicular to the stream, it's relying on the vortices that form at the open end to further reduce pressure. I'm not sure how to predict how the vortices are affected by nozzle shape, other than sharp edges probably help.
    Last edited: Jan 3, 2016
  4. Jan 3, 2016 #3
    Are you able to make the various different shapes? The best way to check the effects is to build the variants and see if/how they run differently. Unless there is some problem with the original untapered version I would stick with it at least at first, if for no other reason than it is simpler to build. What kind of engine is this for? What have other tuners done over the past 40 years? Innovation is good, so try it and see what you gain.
  5. Jan 3, 2016 #4
    Thank you for replying. My thoughts are that the fuel will leave the sharp edge better (smaller).
  6. Jan 3, 2016 #5
    Thank you for replying.
    By asking, I hoped to find that a technical assessment of nozzle exit tapers had already been done by researchers.

    In the last 40+ years there has been almost no development by the manufacturer of british motorcycle carburettors except for:
    1. a hard coating to the zinc throttle slide to prevent galling with the zinc carb body (now only the carb body wears),
    2. an adjustable pilot circuit jet.

    From feedback I received from indignant 'tuners of high renown' it would seem they "...fit new parts..." and vary the jetting and aircleaner.
    So far I have installed 5 mods as part of phase 1 for a 32% increase of generated low pressure, and earlier signal, at both fuel nozzles. To taper the main spray nozzle (int., ext., or not) is mod 2 of phase 2. Three phases in total.
    Ta again.
  7. Jan 15, 2016 #6
    The internally tapered spray nozzle is the more responsive to the perpendicular airstream across it. I attribute this to a greater volume of air flow passing across the wider cross-section at any given air speed. At a constant air speed at the same slide opening, the generated low pressure at the main jet was significantly stronger than the other two options.
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