Sunday, 13 January 2008

The new compressor

I'm pretty proud of this, My first attempt at using Carbon Fibre.

I got more resin on myself than on the wheel but it turned out pretty well in the end.

This picture is about life-size, its 60mm diameter. There are 8 vanes and they're simple radial paddles, with the inner slot height of 10mm reducing to 6mm at the edge .

That makes the cross-sectional area of each void constant as the gas flows toward the edge. (According to theory, no significant pressure/volume change takes place in the compressor wheel)

It's a little over-large but I can adjust it down during testing.

More Testing

By now the bearings were in a pretty rough state but I decided to go ahead anyway and get what life I could out of this prototype. .

I made up a temporary diffuser assembly and stuck everything together with string and masking tape.

When I turned on the blower (hairdryer) it was obvious things were not well. I couldn't get more than 1500rpm, accompanied by faint rattling and scuffing noises from the turbine..

But I fired up the propane gas anyway.

Speed went up to 9000rpm!

Unfortunately it became clear that at this speed, the faster flow of gas was shifting the flame position in the combustion chamber. Combustion was taking place right at the back of the turbine.
Eventually, the whole of the exhaust pipe aft of the turbine was glowing bright red. I didn't dare step round the engine to get a view of the turbine blades (forgot to set up a mirror). It would have been attempted suicide to get inline with that turbine.

Finally, with the turbine audibly rattling in it's bearing mount and speed dropping, I decided to call a halt.

Post Mortem

It's the end for this engine. The rear bearing has siezed solid and reamed itself a larger seat in the housing - about 20 'thou' oversize..

Things learned?

I definitely need to provide lubrication somehow and the combustion chamber is not good enough hopeless.

Fortunately, my back ordered copy of 'Model Jet Engines' (Thomas Kamps - Modeller's World Series) has arrived and I can see that the engine design given there is pretty much along the lines I have been experimenting with.
The rear bearing assembly is anchored by the turbine stator blades just like mine. In fact it's almost exactly the layout I had been sketching out for the next version.

The combustion chamber on the other hand is FAR more sophisticated, with air/fuel injected from the rear to set up and maintain a toroidal flow of the combustion gasses.

No question about it - I need to make a fresh start and build something more on the lines of Thomas Kamp's engine.

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