Here's a project that uses some of those dead hard drives you've got lying around.
In the Tesla Turbine, air, steam, oil, or any other fluid is injected at the edge of a series of smooth parallel disks. The fluid spirals inwards and is exhausted through ventilation ports near the center of the disks.
A regular blade turbine operates by transferring kinetic energy from the moving fluid to the turbine fan blades. In the Tesla Turbine, the kinetic energy transfer to the edges of the thin platters is very small. Instead, it uses the boundary layer effect, i.e. adhesion between the moving fluid and the rigid disk. This is the same effect that causes drag on airplanes.
To build a turbine like this, you need some dead hard drives, some stock material (aluminum, acrylic), a milling machine with a rotary table, and a lathe with a 4 jaw chuck.
Wikipedia has a good review article (http://en.wikipedia.org/wiki/Tesla_turbine
), as well as articles about
Nikola Tesla http://en.wikipedia.org/wiki/Nikola_Tesla
the boundary layer effect (http://en.wikipedia.org/wiki/Boundary_layer
and Reynolds number (http://en.wikipedia.org/wiki/Reynolds_number
(which determines if the fluid flow is laminar or turbulent).
I run my turbine on compressed air (40 psi), and it easily reaches speeds of 10-15,000 rpm. While the speed is high, the torque is low, and it can be stopped with your bare hand.
I have more details on my webpage (http://staff.washington.edu/sbtroy/turbine/turbine.html