What is the effect of a diffuser or nozzle placed upon the inlet of an axial fan?

New to physics, I'm studying a mechanical method to increase the distance at which a standard axial fan can be more effective pulling smoke from a larger distance, while reducing the negative effect of a filter (for particles) installed over the fan.

A nozzle or a diffuser have the property to change the fluid velocity and pressure, so I thought that they might have the right properties to accomplish the goal.

Diffuser concept Intuitively seems the more appropriate concept. As Venturi's effect states, a smaller tube section area has a greater fluid velocity but less pressure.

• Hypothesis 1: a faster fluid velocity may help the air overcome the filter impedance more easily.

• Hypothesis 2: a faster fluid velocity may help create a longer smoke attraction distance.

• It's been a long time since I've been involved with this sort of thing, but putting a restriction on either the inlet of outlet of a fan will reduce its efficiency, which could potentially increase its operating cost.
– Fred
Commented Jan 11, 2023 at 0:13
• Unfortunately, the smoke simply doesn't know about the fan geometry, and the inflow dynamics will not be any different more than a few inches from the fan. All you can do is stir the room air and have sufficient air changes to handle the number of smokers. getthesmokeout.com/content/CommercialCatalog.pdf Commented Jan 11, 2023 at 2:03
• @Fred Efficiency depends on what you care about and so most things will depend. Hoods and other ductwork can reduce backflow from output to intake, increasing "efficiency" in some cases where the objective is to move something specific from one point to another.
– Abel
Commented Jan 11, 2023 at 2:03
• Every change in section causes a head loss to be included. If you want the minimum losses when changing from one diameter to another the angle needs to be about 6 degrees - you can prove this with dimensional analysis and some differentiation, at least that’s how our prof proved it. Commented Jan 11, 2023 at 5:47