# Cumulative effect of two axial fans

If I already have a small DC axial 2m3/h fan blowing air at something, what will happen if I add a larger more powerful 6m3/h DC axial fan blowing from behind that one (i.e. its the larger fan first, then then smaller, then the object to blow air at)? Will the smaller one just hinder the airflow from the first, or are their outputs cumulative?

Many thanks!

• are they ducted? Commented Mar 28, 2020 at 22:22
• @SolarMike Yes they are Commented Mar 28, 2020 at 22:31
• @SolarMike Actually I meant each one is ducted - but the two are not connected in any way besides being positioned one behind the other Commented Mar 29, 2020 at 0:17

It depends on their aerodynamic properties.

Jet engines are multiple fans as well, but they can and do different functions.

Some work together to increase the pressure and reduce the speed of stream.

Some even drive energy from the stream of air.

In your case similar situations can happen.

The combination can increase the flow or the pressure. Or it may cause the entire stream to rotate in the cylinder and stall the system.

• So what you're saying basically is theres no simple answer to this and no way to know if the smaller fan is a benefit or hindrance? Commented Mar 29, 2020 at 0:23
• @OliverWalters, yes. that's true. it puts us in uncharted waters. unless you test it by playing with the size and position of the 2 fans. like a wind tunnel test, or if you have seen something that worked try to learn from it. Commented Mar 29, 2020 at 0:34
• The first fan will produce an output flow pattern that contains tangential velocities. This is not what the second fan was designed to expect in its input. Multi-stage "fans" that are all enclosed in a duct often have static vanes between the fans to cancel out the tangential flow, or alternatively two fans rotate in opposite directions (and with blades that are mirror images of each other). Commented Mar 29, 2020 at 2:55