Mark Wrathall wrote:
Snip> >
To summarise, even given the wild assumptions and their inaccuracies:
6000rpm is far too slow for a efficient 14" turbine. You are be aiming
for
tip speeds around 280-300 m/s, not 109m/s. If you need to gear the
turbine,
then there is no further justifiction for the drawn location
The 13b's mass flow is too low to allow usable blade length on a 14"
turbine. This low mass flow is the real show stopper. It forces you to
small
turbines to get a usable blade length.
1174°K for an uncooled turbine blade temp is quite high (the GE90's
first
uncooled stage is LPT1 which is limited to 1258°K for continuous
operation).
Forcing a one per rev thermal shock with this kind of temp is not the
route
to hardware durablity. I can not picture how you would seal rotor to
stator
to even get a single nozzle design working. The aero drag of the 95+% of
blades which, at any point of time, are beating fresh air must offset
any HP
gains.
You will need something well designed to get a decent TBO at this temp
(not
home built).
As you are forced to gear a turbo's turbine, then the larger the
turbine,
the lower the speed, but the lower the efficiency (due to the reducing
blade
length) , and the easier the gearbox would be (right??). Finding an "off
the
shelf" large diameter, low flow, high temp turbine won't be easy. Marine
diesels? Is it even feasible to design a gearbox with an 80000 rpm
input,
6000 output, bolted one side to a hot casing an the other to cold end
housing , transmitting 15-20hp continuously?
Mark Wrathall
www.rotary.microcosman.com
OK Mark. Thanks a bunch. Looked like you did a lot of work on this.
I will give up on the idea.
Back to geared turbo charger parts. I kind of like the way Caterpillar
apparently did it. Looking at the lousy picture I have they appear merely
to have extended the shaft out of the compressor on what pretty much looks
like a stock turbo charger. Perhaps a final belt drive to the e-shaft
will mitigate any rotational inertia problems.
Paul Lamar
My numbers could be a fair way off, as the turbine inlet pressure is a wild
guess, as are the efficiencies. still close enough to getthe idea.
The low mass flow means that there is likely only a relatively narrow sweet
spot of turbine size (and thus RPM) which will give good efficiency, but you
could trade some turbine efficiency for gearbox efficiency by using a larger
slower unit. I mentioned a marine diesel as I figured that they are sized
for efficiency with no compromise for acceleration response.
I was think another nail in the coffin of compounding a NA 13B is that the
turbine would act as a exhaust flow restrictor, reducing HP of the core
engine (the 13b is rather sensitive to exhaust restriction right?)
At 80K+ rpm the turbo/turbine shaft has to be carefully designed for
critical bending (is that what it is called in english?
Biegekritischedrehzahl). One solution Kamp uses on his models running 100K+
rpm is to use a quite large hollow shaft. The ceramic bearings are 6mm ID
Compressor, and 8mmID Turbine, and are run at about 2-3x their rated speeds.
Bearing failure is the normal failure mode of a model gas turbine, once you
have the combustion chamber sorted to avoid hot spots.
The compounding extension shaft you have drawn would be quite a challenge.
Turbo charger turbines are shunk onto their shafts with enough interference
that you have to consider it a permanent joint. The standard turbine shaft
(or turbine bore) is also small diameter ~8mm with the compressor down
around 6mm, and a very short shaft. To build this design with an off the
shelf turbine, you would need that extension shaft either thin enough to
load the turbine over (where it would then have a critical bending rpm which
is too low), or a shaft coupling (which I can't picture).
The Automotive industry knows full well that they are pumping 40-50% of the
fuel energy down the exhaust pipe, but we haven't seen even a prototype
compounded small engine. I would guess that the cross over, point where the
engines mass flow starts making compounding realistic, is above the point
where todays turbines out perform intermittant combustion engines for
aviation use.
Mark.
www.rotary.microcosman.com
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/
Copyright 1998-2002 All world wide rights reserved.