You don't need counter-rotating blades to do what you want.
When you put 2 fans serially in-line (intake-2 to exhaust-1), you
generally effectively boost static pressure but not additively airflow.
o Technically you can occasionally boost cfm beyond that of a single fan
o More usually, you boost static pressure and so realise closer to free air cfm
For example, your enclosure has a static resistance 5Pa and you fit a single
fan of 25cfm in free-air, and you find your in-enclosure free air is 18cfm. If you
fit another 25cfm fan serially in-line, your in-enclosure free air is 24cfm.
This doesn't come at a cost however - noise :-)
o Additionally you need to vary the separation between 2 inline fans
o At some separation you will minimise noise for the given benefits
In reality this method is only used for redundant fans:
o You use tier-1 fans, NMB, Panaflo or EBM-Papst, some Nidec
o You fit them serially inline for redundancy
o Reason is not so much re risk of failure - but risk of service personnel
---- it allows YOU to choose when to replace the fan, not when the server does
---- it reduces the risk of disrupting other rack servers (common coloco downtime cause)
Many fans produce air in a Tunnel Vortex (eg, Panaflo) others more Radial.
The ideal fan type for making it work is Tunnel Vortex where noise is not so
critical, or fans which use an airflow straightener on the outlet - eg some Delta.
In general, you will suffer more turbulence noise, as the airflow impacting on
the leading edge of the foil will be at an angle it wasn't designed for. Thus the
blade will suffer more stall, moving it down the P-Q (Pressure-Airflow) efficiency
curve, with the resulting increase in noise from that inefficient blade turbulence.
Axial fans produce very little static pressure, but high airflow:
o Typical fan produces 0.1-0.2" elevation in a H2O (water) manometer
o Human beings can produce 16"+
Radial fans produce lots of static pressure, but low airflow:
o Typical fan produces 1-4" elevation in a H2O (water) manometer
o So whilst the cfm is less, *more* cfm is realised in high resistance applications
The only real use for radial fans (blowers) on PCs is for extremely high resistance
heatsinks comprising a very long, very dense, very deep, skived copper method.
Even here, axial fans can perform better where they are designed for such, an
example is the 40x28mm Sanyo high-velocity axial flow fan - and 40dB(A) too.
1U rack cases will be moving to a dual-motor, concentric shaft fan eventually.
It will give the cfm of an axial fan, but the static pressure of a radial fan - with
the result you get >25cfm at 1.2" of H2O for dense 1U applications, and 56dB(A).
Better to engineer a design to utilise the airflow in m/sec over a large surface
area, at least for lower noise, than rely on a pressure-orientated solution (noisy).
--
Dorothy Bradbury
www.stores.ebay.co.uk/panaflofan for quiet Panaflo fans & other items
www.dorothybradbury.co.uk (free delivery)