Increasing airflow of fan

Question

I learned at at young age from a physics kit that one can blow up a large bag with your mouth by blowing into it from a distance and letting the air pick more air with it. Later in life when Dyson launched its fan, I understood the principle well. If I remember correctly Prusa also does something similar to help more air pass the heatsink of the hotend.

I am now looking to design a fan shroud/holder that will be used to move hot air out from the box. The box has vents at the front and rear but the hot air is practically still in the box and the content within is slowly overheating. The idea is to use a couple of PC fans in the rear to move air out of the container and let fresh air be pulled in through the front of the box. I need to design the part that make it possible to mount the fans to the rear vent of the box.

I therefore want to make use of this physical effect to increase the airflow. The idea is to keep the rpm, sound and power consumption down by making use of this effect. I have been searching for days now but I am kind of stuck. My questions are:

1. What is this effect called?
2. How do I maximize the airflow using this design? Are there any guidelines to follow?
3. Do you know of any drawings or similar that make use of this physical property?
4. How much extra air can be pulled by the extra drag created by the fan? What is the limit of this effect?

Basically I want to learn more about this and make the most out of this physical property. I need some guidance so I can conduct some more research, but if you happen to know of any drawings/designs/product that make use of this that I can take inspiration from, then that would save a lot of time.

Answer 1

The bag trick only works because the momentum of the air you're blowing into the bag is "knocking" it open. The force of the wind spreading the plastic apart is what draws more air in to fill in the space behind your gust, the same as if you were to jam a ring-shaped plunger down the bag, or spread open a bellows to draw air into its chamber. It's not the "Dyson air multiplier effect" dragging air alongside the wind from your breath to fill in the bag faster. If you were blowing a steady stream into a solid pipe of the same diameter that were already cylindrical, you wouldn't get an increased volume of air per second moving through the pipe if you didn't seal your lips around it versus if you did.

In fact, there is no "Dyson multiplier effect" and the fans don't actually "multiply" air. It's completely fake. I mean, the air being forced out of the holes in the ring does "drag" air from behind it through the center of the ring, but this comes at the cost of an equal and opposite reduction in the momentum of the air that was forced through the tiny holes. There is no free energy being generated by the Dyson fan. Regular fans drawing the same power will move a lot more cubic feet per minute than a dyson fan - you can easily look up these measurements for various fans.

Answer 2

Devices like this are ubiquitous in industry. To name a few:

• Venturi vacuum generator (https://www.smc.eu/en-eu/products/zh-a~138195~nav),
• propane torch/bunsen burner (where the gas flow induces air flow for combustion),
• Bernoulli effect blow gun (https://www.smc.eu/en-eu/products/knh-efficient-nozzle~50365~cfg)

In each case kinetic velocity of the fast-moving jet is exchanged with the surrounding air, lowering the jet velocity and increasing the air velocity. Net result, more moving air, albeit at a lower velocity than the jet originally had.

As a PC fan typically makes air move slowly, there's probably hardly an effect of making more air move expected.