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).


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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?