Friday, 8 February 2008

New Compressor

Compressor test rig revisited

Completely remade the compressor wheel and diffuser assembly. Put the whole thing together... and the difference? None!

It's really wierd that whatever I do seems to make no difference.

Well, no, it isn't really. The problem is that faced with a lump of engine that doesn't work I don't have any feel for what might be the problem. If it were a piston engine, there wouldn't be a problem but the jet turbine is something where I have no experience to go on.

I keep coming back to the fact that I don't have any facts or figures that relate to the actual engine. Calculations show that at such-and-such a blade angle and such-and-such temperature, the turbine wheel should generate yeay watts of power, compressing so many kg of air per second to such-and-such pressure. But there's no way to tell if it actually does.

Anyway, I do know that there is something wrong with the combustion. So I've worked out a way to drive the compressor section from an electric motor and run the combustion chamber in the airstream.
This avoids having a blower just blast air into the engine which doesn't tell me anything.

I've been trying to do this before by driving the shaft directly from a motor, which had to be placed right in the exhaust (putting it at the front reverses the sense of rotation and runs the compressor backwards).

For obvious reasons that wasn't too good. The motor was in the way when looking up the pipe and had to be on a long shaft to get it out of the exhaust. The long drive vibrated and shook the engine/motor, affecting the alignment and causing speed variations. Igniting the engine caused the shaft to expand, creating friction that had me on a wild-goose-chase for a while.(It looked like the bearings were starting to lock up when hot.)

Then I hit on the idea of driving the engine from the side using pulleys. It works fine!
The shot above shows the compressor running with the engine lit. I get a beautiful clean blue burn with the flame 'locked' in the fore-part of the chamber even at full pressure from the gas bottle.

Now I have to think about what this allows me to measure.


With the motor driving, I get a clean, blue burn. I reassembled the engine with the NGV and turbine in place, then tried again.

The motor drive spins the compressor fast enough to produce the same clean burn but the moment I take off the drive the combustion collapses into yellow.

Is it possible that I'm not getting enough power from the turbine wheel? When I shaped it, I left the blades rather thick - almost 3mm at the tips. Maybe that is affecting the turbine power.

Update - 12th Feb

Reshaped the turbine blades, making them much lighter and about 1mm thick. Also added extra vanes to the NGV assembly to smooth the airflow further.
Still no improvement.

A bit of foreign material got into the compressor so I had to strip it down. On checking measurements, I find that the wheel is rather thicker than I designed it for. Because it is broadly conical in shape, when I trimmed the diameter it widened out the blade tip height to almost 10mm instead of the 8 I was aimimg for. Since my calculations show an ideal height of 6.5mm, maybe I have gone too far?
I'll make a new one.

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