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
 
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