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
 
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