Position of the fan inside a convergent nozzle

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Discussion Overview

The discussion revolves around the optimal positioning of a fan or blower within a convergent nozzle structure, specifically comparing two scenarios: one where the fan is located at the throat of the nozzle and another where it is positioned at the inlet. The focus is on subsonic fluid velocity in both cases, with participants exploring factors that may influence the effectiveness of each configuration.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Exploratory

Main Points Raised

  • Some participants propose that the position of the fan/blower depends on various factors, including the desired outcomes such as flow rate, velocity, and pressure, as well as constraints like fan type and structural support.
  • One participant emphasizes the need for maximum velocity at the throat and clarifies that the fan power is the same in both scenarios.
  • Another participant challenges the assertion that there is no increase in pressure, arguing that a fan's purpose is to create a pressure difference to move air.
  • A suggestion is made that an axial fan should ideally have an inlet bell to improve efficiency by converging air into a coherent stream, which could influence the choice of fan position.
  • Some participants reference external links to support their claims regarding the effectiveness of fan positioning, particularly favoring the inlet position.
  • There is a clarification that the link discussing the inlet bell does not imply that the best position for the fan is at the inlet, but rather describes a component that aids fan performance.

Areas of Agreement / Disagreement

Participants express differing views on the optimal position of the fan/blower, with no consensus reached. Some argue for the inlet position while others highlight the importance of the throat position, leading to an ongoing debate about the factors influencing performance.

Contextual Notes

Participants have not specified the exact conditions or types of fans being discussed, which may affect the conclusions drawn about performance in each scenario. The discussion remains open-ended with various assumptions and conditions unaddressed.

T C
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TL;DR
What's the best position to set a fan/blower inside a convergent nozzle?
I want to present two scenarios here. First, there is a convergent nozzle shaped structure having a fan/blower fitted inside. In this case, the fan/blower is fitted at the throat of the nozzle. The inlet to throat ratio isn't important here. And the second scenario, the same nozzle is used but the blower is now fitted at the inlet. Just to mention that velocity of fluid is subsonic in both cases at every point. I want to know what's the best position of the fan/blower? The first one or the second one?
 
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T C said:
I want to present two scenarios here. First, there is a convergent nozzle shaped structure having a fan/blower fitted inside. In this case, the fan/blower is fitted at the throat of the nozzle. The inlet to throat ratio isn't important here. And the second scenario, the same nozzle is used but the blower is now fitted at the inlet. Just to mention that velocity of fluid is subsonic in both cases at every point. I want to know what's the best position of the fan/blower? The first one or the second one?
It depends on many factors, such as what your goals are (highest flow rate, highest velocity, highest pressure?) and other constraints, such as is this the same nozzle/fan in both scenarios, type of fan, structural support issues, space constraints, flow smoothness/uniformity requirements, etc. There is no general answer to the question.
 
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Correct! I just forgot to mention that in both cases the fan/blower power is the same too. And I have already mentioned that the velocity is subsonic in both cases, therefore no question of increase in pressure. And, to be precise, as you have asked, I want maximum velocity at the throat. And in my thread, I have clearly mentioned that the nozzle is the same in both cases.
 
Last edited:
T C said:
Correct! I just forgot to mention that in both cases the fan/blower power is the same too.
Ok...
And I have already mentioned that the velocity is subsonic in both cases, therefore no question of increase in pressure.
That isn't true. The whole point of a fan/blower is to move air by creating a pressure difference. But I guess what you are after is a just an axial fan in a nozzle, with nothing else attached (no ductwork system)? In that case, the static pressure requirement is close to zero, and the type of fan fits that requirement.
And in my thread, I have clearly mentioned that the nozzle is the same in both cases.
I see now...

So you want to know how two different fans that you haven't selected or specified will perform under conditions you also haven't specified. Well, ok, just to throw something at the wall and see if it sticks:

An otherwise uncased axial fan should at least have an inlet bell because the inlet side of the fan has to pull air from all directions and converge it into a coherent stream, which creates turbulence if there is an abrupt entry. This will make it operate more efficiently than the same fan with no inlet bell.

https://www.thefanguy.ca/wp-content/uploads/2017/01/Inlet-Bell-Brochure-Motion-Canada-2017.pdf
 
From the link, it seems that the best position is at the inlet.
 

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