"Keith Willshaw" wrote in message ...
"The Enlightenment" wrote in message
...
Water Cooled Jet Engines: a possibility then and now?


At the present steam cooling is used on stationary power turbines. If
water cooling extended operating temperature such that fuel efficiency
rose 5% for a jumbo jet would we see it on a commercial airliner?


It seems unlikely given the weight penalty and rise in complexity the
water cooling system represents. This was looked at by most
of the gas turbine manufacturers in the late 1940's and the
consensus was that it wasnt viable. Given the increased
performance of high temp alloys since then I doubt thats
likely to change. Stationary turbines might utilise such a
system but most are derivatives of aircraft turbines.

My intuition agrees with you. I don't think that water/steam cooling
is prohibitive for jets though:- I just don't think its not worth the
extra costs in weight and complexity on an aircraft. For someone like
the Germans in WW2 who were facing raw materials shortages and had no
other way of making long lasting turbines except with ceramics or
water cooling it 'might' have found a niche.

This is the description of the evaporative steam cooling system that
was used on a Heinkel He 100 piston engined fighter from Wikipedia. I
think this type of system would have been used for a water/steam
cooled Jet engine:

http://en2.wikipedia.org/wiki/Heinkel_He_100
"For the rest of the designed performance increase, Walter turned to
the risky method of cooling the engine via evaporative cooling. In the
Heinkel system, designed by Jahn and Jahnke, the engine was run at 110
Celsius and the superheated fluid was then sprayed into the interior
of a centrifugal compressor, allowing the pressure to drop and steam
to form. The water, being heavier, was forced to the outside of the
pump by centrifugal force and returned to the engine. The weight of
the water forced the steam into the only available space, the inside
of the pump, where it was removed. The steam was then allowed to flow
into a series of tubes running on the inside surface of the leading
edges of the wings, where it would condense back into water and be
pumped back to the engine. A number of pumping systems were tried, and
eventually a system of no less than 22 small electric pumps (all with
their own failure indicator lamp in the cockpit) was settled on. "


One scheme that is being looked at for land based turbines
is using the waste heat from the turbine exhaust to produce
steam in a waste heat boiler. This steam is then used to drive
a chiller that cools the gas turbine intake air. This can improve
performance by as much as 20% in hot climates

I believe the steam is used to creat a vacuum via the venturi effect
and this low pressure is used to evaporate and thereby chill water.
It is a technique used in the chemical industry and on refrigerated
ships. It would be nice if it could be used to desalinate water as a
side effect.

The Swiss company Escher Wiess in 1955 introduced the worlds first
"closed cycle" gas turbine. This 5MW unit was apparently influenced
by the work of the German AEG company in WW2. In a closed cycle gas
turbine the working fluid (air in the case of Escher Wiess) is heated
indirectly through (metalic or ceramic) walls. After expanding in the
turbine the hot air is cooled in a heat exchanger.

The air is generally pressurised to make the turbo-machinery smaller.
the pressure of air can be varried in accordance with the load demand.
As a result the Escher Wiess turbine did not suffer an efficiency
drop at low loadings like open cycle gas turbines.

Its efficiency in cooler months was someting like 34% irresepective of
whether the turbine was at full load or 20%.

However in hoter weather efficiency dropped to 18% as the heat
echanger lost efficiency.

Open cycle turbines are not heavier than closed cycle gas turbines and
have the advantage of being able to burn coal or messy fuels. They
are however expensive becuase of the heater walls must suffer
corrosive and heat effects and the heat exhangers are elaborate.

I think they will have a niche where there is coal to burn but no
water for cooling a steam turbine.

Incidently the swiss might be regarded as the inventors of the
stationary gas turbine and still manufacture non aero-engine
detivative power plants. Swiss academics and German academics
developed the aixial compressor in the early 30s and this familiarity
was in part responsible for the selection of axial units on some of
the first Jets.