"tonyp" wrote in message ...
"Dan Thomas" wrote
The turbine section drives the compressor, and extracts about 75%
of the energy from the gas flow in doing it. The remaining velocity
and pressure is what drives the engine forward. If I was to say where
the pressure is concentrated, I'd have to say it's against the
compressor disks.
Thanks for a truly informative post, Dan. A couple of questions:
1. Do you have a rough idea of the ratio of _axial_force_ on the compressor
disks vs. the turbine disks? Is the (backward) force on the turbine about 75%
of the (forward) force on the compressor, for instance?
Coudn't answer that with any accuracy, but I would assume
something along those lines.
Turboprop, turbofan and turboshaft engines have more turbine
stages to remove almost all the remaining energy and use it to drive a
fan or prop or helicopter transmission. In a high-bypass turbofan as
used on newer airliners, the fan produces most of the thrust. Four or
more times as much air goes around the engine as goes through it.
2. So, does this mean that in a turbofan engine the thrust really is indeed
mostly transmitted through the 10K+RPM spindle bearings? And are those
rolling-element bearings, or do they use some clever fluid-dynamical bearings in
modern engines?
They are all rolling bearings, operating in sythetic oils
squirted in at low pressure.
The fan doesn't turn at 10,000 RPM. More like 3,000, depending on
model and size of engine. There's a gearset to reduce the turbine
speed and increase torque for the fan. The fan has similar limitations
to the airplane propeller, where the blade tip speeds have to be kept
under the speed of sound. The engine air inlet is designed as a
divergent duct, like the diffuser, to take the high-speed ram air (in
cruise) and slow it down and increase its pressure even before it hits
the fan (which is also the first compressor stage). Supersonic
airplanes use special intake designs that create a shock-wave aross
the inlet to slow the air to subsonic speed before compresor entry.
Don't tell modern passengers this, but that fan is a throwback to
propeller technology. Contained within a duct, it becomes much more
efficient, though, and many more blades can be added than would work
on an open propeller, and higher cruise speeds are possible. Straight
turbojet engines are really inefficient and noisy, while large
turbofans have efficiencies as good or better than piston engines. The
secret is diameter: bigger is better.
Check out
http://www.mtu.de/Projects/wi/cda/Mt...0-1--0,00.html
The fan gearset isn't visible but would be inside the fan hub. Many
are driven by an independent set of turbine disks via a coaxial shaft
withing the main spool shaft.
A much better explanation of pressures and combustion processes is
found he
http://www.chevron.com/prodserv/fuel...t_engines.shtm
Dan