I have a pump provinging a constant airflow through a pipe with a diameter of 3 milimeters. The pipe will then Go trhough a larger pipe with a diameter of 11 milimeters.

I have the possibility to leave a distance between the tend of the smaller pipe and the entrance of the larger one as show in the image below:

image 1

By leaving a certain distance between the 2 pipes instead of keeping them together, will some additionnal exterior air get sucked in with the air being pumped in the larger pipe, thus increasing the final airflow? If yes, what would be the optimal distance between the 2 pipes?

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    $\begingroup$ You are trying to design a device known as an eductor. What are the upstream and downstream pressure constraints for this system? Without knowing quite a bit more details, your question probably can't be answered to your satisfaction. See shipfever.com/what-is-an-eductor $\endgroup$ Dec 4, 2023 at 3:09

1 Answer 1


The answer will depend on backpressure.

Its trivial to see that were you to cap the right side of the larger pipe, with a teeny-tiny hole, the airflow will be nearly 0 because all of the air will escape at the gap. Pressure in the large tube would be almost exactly the ambient air pressure. Were you to seal the connection between the small and big pipes, you would at least get some flow through the hole because you could build up a larger pressure.

On the other hand, that construction, with a gap, is exactly how many burners are constructed. With very low back pressure, this is a very efficient way to pull in air. In fact, it's precisely how a bladeless fan works.


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