It would run a lot higher than spec for a short time, until the motor burned out. It would also have a huge increase in noise, but not as large of an increase in air movement. Might be useful for a TEC or something similar.
Project: Black and White i7 920 D0 | 3 x 2GB DDR3 | EVGA X58 SLI LE
XFX 4890 | Corsair HX750 | Corsair Obsidian
Hmm, thinking back to my BCIT dc motors class, I'd think that it is the actual fan blades and bearing noise that you'd hear. If the fan blades and bearing were identical, you'd hear the same db noise. You're not changing the RPM of output shaft, just the input voltage into a different motor.
I'd check rated CFM and db noise level on the fans you're interested in. Now, you may be able to find some higher CFM 24V fans and undervolt them if noise is a factor for you.
Interesting, thanks for making me think back to Class of 90 Robotics and Automation.
Edit: perfect timing, now my extra power cables have vibrated forward and are touching my bottom fan. Gotta take the cover off the 900. Probably wouldn't have happened if we weren't talking about fans. No one mention HD's please.
Quite often, a device using a higher input voltage can get slightly better efficiency (i.e. PSU's running 220V instead of 110V.), which would translate into less heat dissipated, but that's really not an issue where a fan is concerned. Noise-wise, I wouldn't expect the voltage to make any difference at all, although obviously different fan designs will matter. If there's some fan out there with an uber-magical noise/cfm ratio, that only runs on 24V, then that might justify the trouble. But I can't see it being worth the bother otherwise.
That said, you've got some rigs on water, right? If you're going grab yourself a 24V power supply, why not grab yourself an Iwaki RD-30?
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133