Nicola Tesla's flat-disc boundary layer turbine

As the picture above shows, the turbine rotor is simply a stack of flat discs with ported centres, fixed to a shaft.  Pressure seals such as the labryrinth seals shown above are essential (as I found out the hard way).  The picture also shows the use of end discs as pressure seals, running close to the housing. Disc spacing and outer diameter to inner diameter are important parameters - a paper that I have read suggests the following formula for disc spacing: Pi = (disc spacing) x square root of (angular velocity / kinematic viscosity) Charts of kinematic viscosity are usually available in engineering thermodynamics textbooks.  Note that this property tends to change (for all fluids) with temperature and pressure. Another paper, by Prof. Warren Rice, points out that virtually all the useful energy imparted to the working fluid is given at the outer edge of the rotor, and that high flowrates between the faces of the discs simply result in high energy losses to friction.  There is a shock loss at inlet to the discs at high flowrate as well, since the fluid is asked to abruptly turn by 90 degrees.  Here is a picture of a simple test impellor I made out of paper and cardboard - it was relatively easy to spin up to 3000RPM in a lathe:

It had a relatively small inlet ports.  The end discs were cardboard, the working discs were paper, and disc seperation was achieved with four bolts and lots of washers.  The paper refused to be uniform, I should probably have gone with a much greater number of fasteners. This took a lot of work to make, which is probably the major reason that Tesla's device isn't made commercially.  It's not technical excellence they're after, it's money efficiency.  Aside from the fabrication of the discs - 28 of them - it took over an hour to simply assemble everything.  Contrast this with a cast, one-piece bladed unit, ready to go.

Performance seemed disappointing.  The flow from the impellor was much less than that from the jaws of the lathe's chuck. This illustrated the main difference between this device and a conventional bladed centrifugal blower - the bladed units work best at low pressure difference and high flowrate, while Tesla's device works best at low flowrate and high pressure difference.  If the pressure seals on the device aren't of high enough quality, the device won't seem to work at all. I also tried making a proof-of-concept water pump (next page).

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