858 Gas Turbines
The recuperator thus preheats the combustion air by recovering waste
energy from the exhaust, improving cycle efficiency and reducing fuel con-
sumption. Low-power efficiency is improved still further by power turbine
variable area nozzles (VAN); these maintain a constant power turbine entry
temperature, which in turn maintains recuperator gas side entry conditions and
improves recuperator effectiveness as power reduces.
The intercooler cools air entering the HP compressor, reducing the work
required to compress the air; the intercooler also reduces the HP compressor
discharge temperature, which increases the effectiveness of the recuperator.
Air enters the compressors via a composite radial intake designed to main-
tain uniform circumferential velocity at entry to the gas generator. The IP com-
pressor (so called because it reflects commonality with the parent three-spool
turbofan aero engine) includes six stages of compression; stages two to six are
in common with the RB211 engine, while the first stage is modified to suit the
increased flow requirements of the ICR cycle.
The intercase between the compressors transmits structural load from the
engine through two support legs to the sub-base; it also houses the five inter-
cooler segments in an outer casing and the internal gearbox within an inner cas-
ing. Both casings combine to form the air flow path between compression stages.
Incorporating an intercooler between the compression stages on a twin-spool
cycle increases the specific power of the engine. The amount of work needed
to drive the compressor is reduced, thereby increasing the net power available.
Bypass valves on the intercooler are modulated depending on the pressure and
relative humidity of the air so that condensation formation can be avoided.
Cycle thermal efficiency is approximately the same as that of a simple-
cycle engine, as additional fuel is required to offset the drop in compressor exit
temperature. Combining an intercooler with a recuperator, however, is attrac-
tive for higher pressure ratio engines, leading to high specific power outputs
and good thermal efficiency.
The HP compressor also has six compression stages and is aerodynami-
cally identical to its aero origin. Compression is split 30:70 between IP and HP
compressors, and both stages incorporate additional borescope holes to allow
greater flexibility for inspection.
The Marine Spey SM1C combustor was adopted as the basis for the WR-21
combustor design as a proven system in use worldwide. Although otherwise
conventional in its construction, the combustor features a Reflex Airspray
Burner method of fuel injection developed specifically for the marine versions
of the Spey. This achieves a controlled mixing of fuel and air, allowing a high
burner exit air–fuel ratio (AFR) to be maintained with adequate flame stability.
Based on previous experience, high (lean) AFR was considered an important
factor in reducing visible smoke when burning diesel fuel.
Preservation of the proven aero RB211 HP and IP spool lengths, which are
characterized as short, rigid high-integrity structures, was a principal design
objective. The requirement to remove compressor delivery air and return recu-
perated air within the length constraint dictated that the annular RB211 com-
bustor be replaced by a radially orientated turbo-annular system.