Building a Disk Turbine
Side view of the turbine. The the tapped hole was used to lock the housing to the shaft blank while machining. It is used with an oiler when running tests.
|April 16, 2003 Cont'd.
According to the formulas I've seen, the kinematic viscosity value decreases with an increase in temperature. What this means is, higher temperature gas requires a wider gap than lower temperature gas. On the other hand, as a disk turbine accelerates, the gap width requirement reduces. And by a substantial amount. With these two factors at work, it isn't easy to determine a single optimum disk gap.
According to the formulas a disk turbine's efficiency may vary substantially at different speeds. It also means that a compressed air-powered turbine may favor a different gap width than a steam powered version, or an internal combustion product turbine, since the temperatures of operation are each quite different, even if the turbine runs at the same speed in each case.
April 20, 2003
Some Practical Calculations
I had planned to make a list of calculated disk gap values for different temperatures and rotational speeds, but when I actually plugged in the numbers an interesting thing happened. The required gap stayed similar for different modes of operation within a specific disk size.
The formula I used was adapted from a paper by Glen A. Barlis, which essentially says that the disk gap in inches is equal to
for kinematic viscosity measured in sq. ft./sec
As I wrote earlier, increasing the rotational speed of a disk turbine decreases the ideal gap size. And increasing the temperature of the gas increases the ideal gap size.
A compressed air turbine like mine, as it runs now, at 68 F, and 10,000 RPM would ideally have a disk gap of .015" according to this equation (I actually am running at .017").
If the temperature remained the same and I ran at 70,000 RPM, the ideal disk gap would be .006", which is a substantial difference.
(c) Copyright 2003, Stephen Redmond, all rights reserved
However, to achieve that speed, I would probably be thinking in terms of an internal combustion engine with a temperature of say 1000 degrees (and of course heat resistant materials for the disks).
I plugged in approximate figures for an IC gas turbine and a steam powered turbine of the same size and got the following results:
For the IC turbine at 1000 F and 70,000 RPM the ideal gap would be .012". That's actually pretty close to what I am running now.
A steam powered turbine, running at 30,000 RPM and 400 degrees F, would require a gap of .013" -- again, pretty close.
I had expected a bigger divergence between these different cases, but the increase in speed is almost completely negated by an increase in operating temperature. And so a gap of about .013" would seem to be close to ideal across a range of operation for a disk of this size.
Some of the 9 and 10 inch turbines illustrated on the web, and Tesla's original turbine models have a disk gap of .032". Of course, bigger disks will run at slower speeds -- in proportion to their diameter, and so require larger gaps. But how large?.
Well, according to the formula considered, a gap of .024" would be optimal for a 9 inch disk run at 10,000 RPM on steam at 400 degrees F, and a figure of .023" would be optimal for an IC gas turbine running at 20,000 RPM and 1000 F.
Unless higher temperatures or lower RPM are anticipated as the normal operating range, it appears that a gap of .032" is 50% oversized for good efficiency in a large disk, and a gap of .024" might be a better choice.
The Cairns turbine has a diameter close to my own model, yet has a disk gap of about .020", again, oversized according to the Barlis formula by about 50%.