Air velocity from small aperture

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Discussion Overview

The discussion revolves around the design of a drying mechanism that utilizes a vacuum pump to direct air through a small aperture for the purpose of removing water from a window. Participants explore the implications of air velocity, flow characteristics, and the effectiveness of the proposed design, with references to existing technologies like hand dryers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the flow can be assumed to be incompressible given the calculated air velocity of 107 mph, suggesting that compressibility may not be a concern until reaching around Mach 0.3 (approximately 220 mph).
  • Another participant raises a question about the method used to determine if the air velocity is sufficient to remove water from the window.
  • A participant notes that the air velocity from the referenced hand dryer is 400 mph and suggests that a fraction of that velocity may suffice for the drying mechanism.
  • Concerns are expressed regarding the limitations of the pump's power and the potential for increased friction losses when forcing air through a much smaller orifice than it is designed for.
  • One participant inquires about the rationale behind choosing an air blaster over a simpler method, such as using a scraper.

Areas of Agreement / Disagreement

Participants express varying opinions on the assumptions regarding flow characteristics and the effectiveness of the proposed design. There is no consensus on the best approach or the adequacy of the air velocity for removing water.

Contextual Notes

Participants have not fully explored the assumptions related to the pump's performance, the impact of friction losses, and the specific calculations for determining the effectiveness of the air flow in removing water.

Who May Find This Useful

Individuals interested in fluid dynamics, drying mechanisms, or engineering design may find this discussion relevant.

pdfmech
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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..
 
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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.
 
How are you calculating whether or not you can remove the water from the window?
 
RandomGuy88 said:
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.
 
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.
 
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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