How to calculate change in pressure of air through a funnel

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Discussion Overview

The discussion revolves around calculating the change in pressure of air as it flows through a funnel in a closed system, particularly focusing on scenarios involving compressed air. Participants explore the implications of air compressibility and the application of fluid dynamics principles, such as Bernoulli's equation and the behavior of incompressible versus compressible flow.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks a formula for calculating pressure change when air is funneled, noting difficulty finding relevant information for compressible fluids.
  • Another participant questions the method of air movement and suggests that incompressible flow assumptions may apply unless the air is significantly compressed.
  • It is noted that unless airspeed is very high or there are significant pressure variations, the flow can often be treated as incompressible.
  • A participant mentions that friction and viscosity in the pipe reduce pressure over distance, converting pressure energy into heat.
  • The original poster clarifies that they are dealing with compressed air and seeks to understand the pressure relationship at the funnel's ends.
  • One participant suggests that if the velocity is not extremely high, pressure remains roughly constant throughout the funnel, with velocity increasing as the cross-sectional area decreases.
  • Another participant acknowledges that high velocity can lead to conditions where incompressible flow equations may not hold true.

Areas of Agreement / Disagreement

Participants express differing views on whether the flow can be treated as incompressible and the conditions under which this assumption holds. There is no consensus on the best approach to calculate the change in pressure in the described scenario.

Contextual Notes

Participants highlight the importance of airspeed and pressure variations in determining whether to apply incompressible flow equations, indicating that assumptions may vary based on specific conditions of the system.

fergusonc
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i am looking for a formula or text containing information on how to calculate the change in pressure when air is funneled, in a closed system, where the air is being forced through the system initially with no motion then forced through a pipe that has the same diameter for a relatively long distance then is suddenly funneled down to a much smaller diameter. any help is much appreciated. i can't seem to find anything related to this topic. most formulas i have found deal only with incompressible fluids where i am interested in air which, of course, can be compressed. thanks for any help.
 
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Welcome to PF.

I'm having trouble visualizing what you are talking about. How is the air forced to move? A fan? A diagram would help a lot.

Yes, air can be compressed, but that doesn't necessarily mean that it is compressed in yoru scenario. Unless you are dealing with compressed air (above, say, half a psi), the incompressible flow assumptions will work fine. It sort of sounds like you need Bernoulli's equation applied to a Venturi tube, but I'm not certain.
 
Unless the airspeed is very high (V > mach 0.2-0.3 or so, depending on the required accuracy), or there is significant pressure variation within your system, you can treat the flow as incompressible.
 
If you're trying to model a real world situation, note that the pipe does work against air flow by reducing it's pressure over distance. The rate of mass flow is the same everywhere in the pipe, but pressure is reduced due to friction with the pipe itself and viscosity within the air, with the pressure energy being converted into heat.
 
i am dealing with compressed air and it's forced out of an air tank or air compressor through the pipe toward the funnel section where the diameter is reduced. i was just wondering what the relationship would be in the pressures at each end of the funneled section.
 
If the velocity is not extremely high, then the pressure will remain roughly constant throughout the funnel, with the velocity increasing in inverse proportion to cross sectional area.
 
Yes, I phrased that poorly: it's when the velocity pressure pressurizes the air (which happens at high velocity) that the incompressible flow equations start to break down.
 

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