Why Is Fan Suction/Intake So Weak Compared to Exhaust?

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Discussion Overview

The discussion centers on the differences in suction and exhaust strength of fans, exploring the underlying physical principles and design considerations. Participants examine the mechanics of airflow, entropy, and the implications of fan design on air movement, with a focus on theoretical and conceptual aspects rather than practical applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that a fan's intake is designed to draw air from all directions, while the exhaust is more directional, creating a stronger airflow.
  • One participant explains that the fan lowers pressure at the inlet, causing air to flow towards it and gain momentum as it exits.
  • Another participant raises a philosophical point about the symmetry of physical laws, questioning why the output is directional despite the omnidirectional input.
  • Entropy is discussed as a factor influencing the behavior of airflow, with references to energy conversion in the context of power generation.
  • Some participants propose that the design of fan blades contributes to the directional flow of air, as they are optimized for axial acceleration rather than uniform distribution.
  • There is mention of the potential for powerful air currents on the suction side if a duct is fitted, suggesting that design can influence airflow characteristics.
  • Several participants speculate on the theoretical limits of airflow, including discussions about Mach 1 and the capabilities of centrifugal fans.
  • One participant argues that the pressure difference on the exhaust side allows for stronger airflow compared to the intake, while another counters that for desk fans, the pressure change is negligible.
  • There is a suggestion that placing a fan in a duct could equalize flow rates on both sides, indicating that design and context matter significantly.
  • Another participant discusses the energy dynamics involved in drawing air into the fan, comparing the energy required for different flow configurations.

Areas of Agreement / Disagreement

Participants express a range of views on the mechanics of fan operation, with no clear consensus on the primary reasons for the differences in suction and exhaust strength. The discussion includes competing explanations and interpretations of physical principles.

Contextual Notes

Some arguments depend on assumptions about airflow dynamics and pressure changes, which remain unresolved. The discussion also touches on theoretical limits and design implications that may not be universally applicable.

  • #31
jbriggs444 said:
In principle, the laws of physics are symmetric and time reversible. If the input to a fan is omnidirectional, there is no reason in naive principle why the output of a fan could not also be omnidirectional.
The thread title is too vague for a proper answer.
I imagine the discussions above assume knowledge of the context of jet engine technology.
Context could make a huge difference but the simplest model would be appropriate here because the OP leaves the subject wide open. I had to start with a small inlet aperture and possibly the same size of aperture out. At the input there cannot be more than a pressure difference of 1Bar in still air. More than a very small distance away from the aperture, air will flow from a hemisphere and there's nothing that can change the flow out there. With a large aperture you could perhaps tinker with the air flow in the plane of the aperture but you'd still have the basic 1Bar limit. Is there any way that the pressure profile across the inlet could be changed to make it directional? That's simple 'suck' dealt with.

There can't be any basic limit to the amount of Momentum that the fan / pump could transfer to air in the centre of the jet. So you could say that the possible Blow has no basic limit, given the right design. There has to be a limit to the volume available (mass flow) but squeezing the shape of the jet would allow more 'blow' than the 'suck'.

You can't provide supersonic flow at the inlet ( without starting with fast moving air) but you could, over part of the outlet.
 

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