Air velocity from small aperture

  • Thread starter pdfmech
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  • #1
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Hello. I am designing a drying mechanism for a window. Basically air is taken from the atmosphere by a vacuum pump and is ducted to a small width aperture. The aperture would be 18 inches long and as thin as we can make it (millimeters). Inspiration for this design is from the Dyson hand dryer which uses 400 mph sheets of air to blast water off hands. My question is when performing the calculations can i assume that flow is incompressible. I imagine i can not but this doe not have to be way too precise. I just need to know if it will remove water from the window. Calculations using conservation of mass with incompressible, uniform flow say that with my given vacuum pump the air velocity at the outlet will be 107 mph. I would appreciate any guidance in these calculations..
 

Answers and Replies

  • #2
406
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At 107 mph the air can certainly be considered incompressible. As a rule of thumb you don't have to worry about compressibility until the velocity is around Mach 0.3 which is somewhere around 220 mph think.
 
  • #3
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How are you calculating whether or not you can remove the water from the window?
 
  • #4
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How are you calculating whether or not you can remove the water from the window?
Thats a good question. The velocity from the hand dryer i mentioned was 400 mph and I figured a fraction of that would be good enough. The pump i found outputs 93 cubic feet per minute and i don't want to spend much more money on a pump than the $90 that cost. But i do need to do some more research/experiments on that.
 
  • #5
boneh3ad
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Just as a warning, a pump only has so much power, which translates to so much energy it can effectively put into a flow. When you squeeze that flow through a much tinier orifice than the pump is designed for, you will have more loss due to friction and thus most likely less mass flow.
 
  • #6
Out of curiosity, may I ask why you have chosen to use this kind of approach? I am wondering what the benefit would be of using a air blaster rather than the simple scraper!
 

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