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Generating MIST from still water

  1. Oct 25, 2011 #1
    Generating MIST from still water

    How much velocity / acceleration (linear) is needed to be imparted to water lying at rest (at atmospheric pressure) to convert it into MIST ?

    I know that Water is not converted to mist by velocity or acceleration. It's converted to mist by turbulent air flow on its surface.
    Re-phrasing a bit as ;
    " I put water in open conduit (half pipe section for example) and put it
    on top of Bullet Train (assuming conduit is welded to bullet train).
    Now will the water Lump ( mass) split into fine droplets (mist) ? "

    Dear Friends, I am also aware from my barbers shop that spray bottles can generate mist / mist-like phenomenon!

    Hence, my question is pertaining to CALCULATION ....!

    If I consider the drag equation :- F(drag) = 1/2 C*Rho*A*v^2 ;
    where Rho = air density A = cross section area v = velocity .

    Problem No 1 ; is what profile should I consider viz Speherical / Conic / Parabolic / irregular ? The value ranges from 0.5 to 2 according to SERWAY depending on the profile.


    Problem No 2 is
    What to equate the F(drag) to ; Van der Waal's Forces or Atomic Bond Forces ?
    The F(drag) must be > the Structural Stablizing force of the Water Bulk !
    And Van der Waal's Eq in simple form ; (p+ a'/v^2 )(v-b') = kT; where a' = inter particle force, b= volume of particles, v= voulme of container !! Considering p = 1 bar ( open to atmosphere) , T = (25deg C+ 273 ) Kelvin & k = 0.008314.

    Please help through this sequence of thought process
     
    Last edited: Oct 25, 2011
  2. jcsd
  3. Oct 25, 2011 #2
    The amount of energy it takes to turn a big lumpy mass of water into a mist depends upon how fine of a mist you are talking about. The energy is required to increase the surface area, because of surface tension. You want to use surface tension instead of Van der Waals Eq. A typical floating mist is composed of very small drops of water. The size of these drops of water is dependent upon the temperature of the surrounding air, because the drops are usually in equilibrium between evaporation and condensation. Any good book on cloud physics will have the details.

    Now, you are also asking about how to impart this energy to the water through some kind of air flow. This can be a complicated fluid dynamics problem. An open conduit on top of a bullet train making mist is not an easily solvable problem. The drag coefficient for open water is dependent upon a number of variables such as salinity and temperature. The drag on the water from the air will impart momentum which is likely to just whip the water out of the trough and onto the ground before misting. The way that a barber shop mist sprayer works is that the air is forced through a venturi which reduces the local air pressure and sucks the water into the air.

    Sorry this isn't much help for your calculation but hopefully it will point you in the right directions to go.
     
  4. Nov 6, 2011 #3
    I think your reply has provided me some direction ..which I put down as below

    1. How much fine mist :- for this , we can put the limits by say........

    " the Max size limit as small enough not to fall down to earth ...ie; not affected gravity i.e; stays afloat in air bed i.e
    the weight of drop should be lower than Surface tension force of Fluid it floats on ( the air ) "

    " the Min size limit as big enough not to become vapour ...ie; not become gaseous state "... But this leads to another
    question viz ; to numericalise the above words !!!

    2. For use of ST against VW-force :-
    ST ( surface tension) is the property of fluids because of which it resists external force.
    It arises out of cohesion property of water molecules. Which is again Vander Waal Forces (VW force) !!!
    So this means that first I will have to over come the Structure Force i.e; the SURFACE TENSION. This will cause
    break-up of water Lump in to smaller lumps and still smaller lumps as we supply further energy. this process has to
    continue till lumps become equivalent to fine droplets of water. The size of these being determined by the limits in
    point 1....!!!


    Hence , answer is EQUATE DRAG force to SURFACE TENSION force !!
    I will try to work the numericalised version of my words. Whether I succeed or not , I will still get back.

    But a big thanks ...!!
     
    Last edited: Nov 6, 2011
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