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Water Cooled Jet Engines: a possibillity then and now?



 
 
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  #1  
Old December 17th 03, 11:48 AM
The Enlightenment
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Default Water Cooled Jet Engines: a possibillity then and now?

Water Cooled Jet Engines: a possibility then and now?

This post examines German water cooled gas turbine work in the period
1930-1945 and also looks at modern 'steam cooling' of gas turbine
blades.

I speculate as to the possibility of water cooled jet fighters with
radiators or evaporative steam cooling and the use of steam cooling in
modern aircraft.

Hopefully I will reawaken the air cooled versus water cooled debate!

In 1938 Professor Ernest Schmidt of the LFA in Germany began studies
to develop a water cooled gas turbine.

The turbine was to exploit the critical point of water at around 374
degrees Centigrade at which point there is a massive increase in the
thermal conductivity of water. To test the process a copper bar was
hollowed out and pressurized appropriately. The bar was found to have
20 times the conductivity of an equivalent bar of copper even without
circulation of the water.

A program of 3 test turbines was commenced T2, T3 and T4. T1 was an
ceramic turbine attempt by another group of researchers while T4 was a
4 stage turbine intended for a 5000kW stationary gas turbine to be
built by MAN. T3 was a multistage version of T2.

T2 ran very successfully. It was made out of low alloy steel (an
advantage for the nickel and chromium deprived Germans) and was able
to maintain a temperature of 400C in the bulk of its material and 500C
at its trailing edge in an impressive gas temperature of 1200C. (At
this
time the Jumo 004 was operating at 720C-750C). T2 was a 'blisk' or
" BLade diSK " in that the turbine blades and disks were integrally
machined out of one piece of metal. This before CNC was a job for a
craftsmen and naturally produced a lot of metal swarth which
needed to be recycled but it was not prohibitive. Blisks are today
common because the efficiency of a turbine is directly proportional to
its temperature and its worth the extra expense.

T2 produced enough steam to power a steam turbine 1/10th its own
power. In a 1000kg Jet engine of that day with a turbine inlet
temperature of 800C I think that would be about the same waste heat as
a 500kw (700 hp) piston engine. ( The radiator would be smaller
because of the higher temperature. )

Cooling the turbine stator "nozzles" was far more difficult because
the centrifugal force of the water in the turbine provided the
pressure required (this was about 110psi) and the density difference
under such intense centrifugal forces a large amount of circulation.
However stator nozzles have to endure far less stress and on the
German engines were an aluminum coated anodized steel with only about
0.5% chromium that was cooled by air and seems to have been
reliable.

The program was regarded as successful in its first stage and was to
lead to a 5000kw turbine built by MAN. The systems was seen as
having mainly as having marine and stationary applications but I
speculate it may have seen use possibly in land vehicles (The Germans
built a Panther Mk V Panzer with the GT101 direct drive gas turbine
that was to lead to the indirect drive GT102 and GT103 tank propulsion
units).


****************

Modern Japanese stationary gas turbines are now operating at 1500C.
These are metal blades, coated with a ceramic heat/corrosion barrier
and cooled internally by steam.

Steam may at first seem an odd cooling medium to use but it must be
remembered that compressor bleed air used for blade cooling can be at
hundreds of degrees centigrade. For instance compressor bleed air of
the Jumo 004B4 of the Me 262 was 150C (Compressor pressure ratio
3.3, efficiency 0.79). For an engine of 7 to 1 compression ratio the
compressor outlet temperature can be 250C. Steam has at least twice
the conductivity and heat capacity of air and the use of steam means
that precious compressor air does not need to be wasted. (the Jumo
used 3% of its air for bleed cooling of hollow turbine, blade roots
and combustors). Furthermore in a multistage turbine bleed air
dilutes the air and cools it down thus lowering the efficiency of
subsequent stages.

The most modern air cooled blades can operate at 1700C but these are
test bench models however I speculate that if combined with steam
cooling they might reach 1800 or more. State of the art for
stationary applications is presently 1500C.

It is not inconceivable that steam cooled engines might find
themselves hypersonic aircraft. The heat might be dumped into
vaporizing fuel. In this way a turbojet could have its operating
envelop extended into the hypersonic region for a short dash?

*********

As for the Germans they might have developed successful water cooled
gas turbines for the land based power plants and marine propulsion
systems
they were working on in the 1930s and 40s. (This was driven by the
desire to have powerful engines that could use non octane dependant
fuels).

The possibility of a Messerschmitt or Focker Wulf Jet fighter with
radiators in the second world war is I believe a plausible because
this capable cooling method might have greatly improved the
reliability and fuel efficiency of German jets at the expense of a
modest radiator and a rotary unions to carry the fluid onto the shaft
(such unions operating to the required speed and pressure do exist 'of
the shelf'; their main use at 16000 rpm is to supply machine tool
spindles) .

The turbine of the Jumo 004B was expected to have a life of 25 hours
(about 16 sorties, later to be extended to 60 hours) before being
x-rayed and either replaced or retained for another 10 hours. Some 4
turbines were to be produced per engine as spares. If water cooling
could extend the life of the turbine to say 200 hours it would be
worth the extra effort. Extending the life and temperature was not
possible since the requisite quantities of nickel were not available.

Rotary unions required to transfer the fluid to the shaft are leak
free at these pressures and temperatures but on the basis of the 1.5%
of the 25kg/sec air mass flow required to cool the hollow blades of a
Jumo 004B4 we might require a mass flow of water about the same i.e.
375 grams a second or about 1.5 tones per hour. If 0.5 % leaked a
header tank of 7.5kg would be required for an hour of flight.
Injecting water into a jet exhaust actually increases thrust (tested
on the Jumo 004)

Since the steam could be flashed into wing leading edges and recovered
by scavenge pumps a radiator might even be dispensed with.

Nickel, the metal which typically makes up over 80% of turbine blade
metal in the nimonic alloys used even today in British turbines was in
very short supply for the Germans and this would have been their
motivation.

On stationary or marine turbines steam cooling makes a lot of sense
since relatively modest steel alloys can operate at 1200C: a very high
temperature and the complexity is not a disadvantage.

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?




  #2  
Old December 17th 03, 12:33 PM
Keith Willshaw
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"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.

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

Keith


  #3  
Old December 18th 03, 02:03 AM
The Enlightenment
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Posts: n/a
Default

"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.
  #4  
Old December 18th 03, 10:41 AM
Keith Willshaw
external usenet poster
 
Posts: n/a
Default


"The Enlightenment" wrote in message
om...
"Keith Willshaw" wrote in message

...


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.


There is no might about it.

Brown Boveri of Baden, CH were manufacturing gas turbines
for commercial use in the 30's. One such unit was installed at
the Sun Oil Marcus Refinery in Pennsylvania in 1936, others
were installed in the municipal power station at Neuchatel in 1939

Keith


 




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