Re: turbojet riddle

[William Beaty <billb@ESKIMO.COM>]

On Sun, 2 Jan 2000, John Denker wrote:

> This question bothered me for years, starting when I was a little kid and
> first heard the "explanation" of how a turbojet worked.


Heh.  Me too.

I first became suspicous of the "simple" explanation when I realized that
the gas EXPANDS upon heating, and a greater volume must exit the exhaust
port than enters the intake port.  If the internal diameter through the
engine is constant, then the gas must speed up as it goes through.  My
books didn't say anything about this, and they always showed diagrams
where the internal diameter was NOT constant.


> Hint:  There is a 100% compelling answer in terms of basic physics.  You
> don't need to know anything about the Navier-Stokes equations of fluid
> dynamics, and you don't need to know anything about supersonic shock waves
> or anything like that.

I don't know the simple answer, but the "intuitive" answer is that the
turbine assembly cannot resemble the compressor assembly.  That, or there
must be a gearbox between them, so that numerous rotations of the turbine
will cause just one rotation of the compressor. Without the gearbox, the
turbine would need to have fewer blades or fewer layers.  More expanding
gas escapes via the low-friction path than via the high-friction path.


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