ENGINE CROSS SECTION – COLD SECTION
AIR – PRE – SWIRL SYSTEM
The pre-swirl system of the engine compressor is designed to improve surge
margin at low compressor speeds by changing the relative angle of attack of
the air entering the compressor.
It consists of the following:
• Compressor bleed valve
• Hollow inlet case struts with holes (Piccolo Holes)
COMPRESSOR BLEED VALVE
A compressor bleed valve is installed on the top side of the compressor,
between stages 2 and 3.
The main function of the bleed valve is to extract compressed air from the
compressor to supply the pre-swirl system.
The bleed valve is operated:
• pneumatically (by differential pressure)
• electrically by the EEC
OPERATION
When the bleed valve is open, pressurized air coming from the interstage, is
guided into the air intake through the Piccolo Holes.
The Piccolo Holes, located on the inlet case struts produce air jets that
cause swirling of the incoming air before it enters the compressor.
The swirl, turning in the direction of rotation of the compressor, helps
drawing air in producing an increased compressor performance at low
speeds.