The rpm estimate is based on his experience that I have no intuitive feel for, so I’m a bit of a sceptic. If it actually reaches those rpms in loaded use, it corresponds to pressures of 300-1,200 psi, given reasonable pressure recovery. I’m starting to think it possible that the unit will actually push water up to over 75 psi. If it does, then it seems that the air mass will be less than the water mass being pumped.

Of course I am going to blast straight shop air through a nozzle one time also.

I think I am going to get fair entertainment value for my money.

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||^|__| V |_____| V |__|^||
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||____| V _|^|_ V |____||
|_____| |_| |_| |_____|
V |^| V

If this doesn’t look good either, then feel free to delete this comment.

33,900 rpm is fairly low compared to some of the RC jets out there. I think they use real bearings and not a bushing oiled before each test.

This rig will be cheap enough to play with that I expect to do a lot of fiddling (HH) in the manner suggested (Eric) to get any useful information. I think getting multiple “I told you so” is in my future.

Delta P = rho v^2 / 2g [Bernoulli, applied to water in turbine blade]

Set Delta P to 4 bar ~ 60 psi ~ 400kPa

rho = 1000 kg/m3
g = 9.817 m/s2
then
v~88.6 m/s

v = omega *r; r~.025m

Omega = 3544 radians / s = 33900 RPM

Which seems a bit high.

You will get *some* pumping action. I have to go to the NewSpace 2009 opening talk, but the relevant hurdle is whether you will get enough power out of the turbine to elevate the pressure of the water at the blade exit above the pressure of the air at the blade exit.

BTW, do you mind my asking which machine shop, and how much they charge for machinining the turbine?

As I have mentioned before, I have no turbo-machinery experience, but I can see a couple of potential problems with this design just by looking at it.

First of all, by your own numbers you are attempting to inject water at a lower pressure (50-75 psi) than that of the chamber (125 psi). Unless you mean 50-75 psi above chamber pressure, I think you may have a problem coaxing the water out of the pump.

Secondly, as Habitat Hermit noted, you are severely restricting the amount of useful surface area where momentum may be imparted to the blades. Also, I think your high-pressure air will escape around the ends of the blades just as much as through them.

You may need to design a separate chamber for the turbo-pump that will allow you to isolate the flow paths of your shop air and your injected water. Perhaps something like this may work: (If you can’t make this out, try copy/pasting it into a fixed width text editor, like notepad.)

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||^|__| V |_____| V |__|^||
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||____| V _|^|_ V |____||
|_____| |_| |_| |_____|
V |^| V

The trick here is to prevent, as much as possible, the leakage of the high-pressure gasses from the main chamber into the pump injection chamber through the turbine. This may require the use of a solid ring around the outer edge of the turbine blade (with injector holes appropriately located). For the outlet of the main chamber to the turbine blades, expose as much of the turbine’s surface area as possible.