I have a question for you Monty. Is it better to design a 12 inch

diameter

blow down

direct drive turbine  running at 6000 to 7500 RPM or use a geared down
turbocharger type turbine?

Paul Lamar

Short answer-I don't know.

Long answer- it depends.

Swag-you are probably going to have way too much in the way of

tip/boundary

layer loss with the large dia turbine. The flow area will be very small
relative to the leakage path. In addition the pressure ratio you can

support

with an axial design is less than the radial inflow design. The large

dia

will be very heavy, but you are replacing a flywheel with it. The only

way

to know for sure is do a design study on both options and compare the
numbers. If you do a 12 in design, to make the blading work, you will
probably have to use a partial flow design where only a portion of the
blades see exhaust gas, rather than the whole annulus. More leaks, more
weight. This is done on some tip driven fan designs to make it work. It

is

not the best way to go and can only be justified where the

inefficiencies

and weight gain are overcome by getting rid of a gearbox, and other
complexity.

My guess is the large dia is not going to work well, especially at part
throttle cruise. I remain open minded, but not very optimistic. Turbines
like to spin fast-just not too fast ;-).

Monty

Thanks Monty. Got any pictures of that small turbine engine you are

working on

and showed me at Rotary Roundup in Florida last October?

Here is a chart from the new Garrett catalog that has me a bit mystified.

Very little

explanation on what is going on here.  I think these curves are of a

turbine connected

to a single compressor wheel design. Since the turbine radius and design

is not

changing basically what is going on here with the different A/Rs is the

nozzle

area is changing. The larger the nozzle area the higher the A/R ratio.
It appears on the flat part of the curves on the right side an increase
in mass flow has little effect other than increasing the back pressure.
Does this mean that the load from the compressor wheel is at a max
and no increase in exhaust gas mass flow at a given A/R (nozzle area) will

increase

the compressor wheel RPM?

Paul Lamar

Once the flow in the exducer or the nozzle chokes (reaches Mach 1), the mass
flow doesn't increase. There is an island in the middle of the flow graph
with the highest efficiency and J shaped operating curves that pass through
it. Of course we have no idea from this chart where that island is.
Generally it is in the middle. There is not enough information in this graph
to be of much use. The same is true of most of the compressor maps out
there. You really need to know what type of diffuser was used and what the
housing looked like. Not enough data out there to be able to actually mix
and match these components with any certainty. Bottom line- testing is
required.

Sorry, no pics of the engine. It's not ready for public consumption, but
there has been progress. You wouldn't recognize it anymore. And-no; the fuel
consumption is not as good as the rotary. Not even close. ;-)

Monty

Thanks Monty.

Paul Lamar
 
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