Hurant Karibian wrote:

I belive I said this before. In the navy flight engineer school we

were told the R3350 turbo compund produced no more exhaust back pressure as
the space between the turbine blades equaled the area

of what would be a normal exhaust system for that engine.

Marc de Piolenc wrote:

The explanation is a little simplified, but the fact that no extra
back-pressure was produced is correct. NACA tests back in the 30's and
40's, aimed at getting direct thrust from exhaust stacks, showed that
exhaust passages could be restricted to a quite remarkable extent
without shaft-power loss. Likening the exhaust restriction to the
interblade spaces or "nozzles" of a reaction-type gas turbine predicts
that considerable power can be extracted from exhaust flows without
penalty to the core engine... which is exactly what extensive
experimentation later showed.

Work by Rauscher and Philips showed that it is possible to use EITHER
the thermal energy of the exhaust OR the mechanical (kinetic) energy,
but not both, so it makes sense, if you choose to use kinetic power,
to maximize it by running the exhaust flow through the maximum
permissible restriction, thus converting some of the exhaust enthalpy
to kinetic energy.

Marc de Piolenc

A slight clarification on the not both statement. Depends on where in
the exhaust stream you choose to exploit the thermal energy. If it is

after

the turbine it can be done. Your local gas turbine power generating

facility

is probably doing it right now as we speak. I agree if it is done before
the turbine the mass flow will be drastically reduced.

Paul Lamar

NACA Report 786 confirms these statements of no back pressure to speak of.
This was a mildly supercharged engine running at 33 inches of Hg.
The turbine was connected only to a dynamometer so no feedback from a

turbo

charger was involved.

BTW reading this report and others our kinetic energy calculation are way
off for the 13B as you do not get significant power out of a turbine until
the gas velocity is twice the blade tip velocity. Our calculations

indicate

around 900 fps for the exhaust gas but it would need to be near twice that
to rev a turbocharger turbine to 130,000 RPM and generate any appreciable
amount of boost.

Paul Lamar

I do not know if you noticed - but this data is another nail in the coffin
for
automobile useage of the turbo compound concept - at sea level output
they only recovered 9 percent extra- the big gain being at altitude where
the low exit pressure supports sonic flow in the turbine nozzle. These
data are all full throttle where the "blow down pressure" at valve/port
opening
is much higher than ambient. At cruise (even in overdrive) part throttle
operation the blow down pressure is significantly lower because you
have throttled the flow for a partial "charge" for both less pressure
and less weight flow in the exit gases. We have all seen this effect
when we pussyfooted the throttle to ease our straight pipe "rod"
past the local cop. - Vance

10% is nothing to sneeze at. That would raise the 70 MPH steady speed MPG
from 25 to 27.5. Most auto engineers would sell their children for that sort
of improvement.
 
I understand all that and for the car I am working on implementing a combined cycle
wherein the remaining heat is converted to steam and then injected
into the kinetic turbine. Only one turbine is needed. The main problem is 
recovering ALL of the water.

I am not advocating steam for an aircraft. You have enough kinetic energy in the
exhaust to build a conventional turbo compound for aircraft use based on the Connie model.
Seems like the R&D is simpler than the freon working fluid system.

For a car one has this long tail pipe that one could convert to a heat exchanger
and a large muffler just ahead of the rear bumper that could be replaced by a condenser.
the forward motion of the car would provide the cooling air for the condenser.
Just choose a turbine that works the best for both steam and exhaust gas.
All the steam does is increase the kinetic mass flow.

Large combined cycle gas turbines are approaching 60% over all efficiency
which is twice as good as any car engine. The problems are scaling it down
and recovering all of the water.

Paul Lamar

 
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