Airflow control, every single stage of a compressor needs to develop precisely the very same pressure ratio because most of the additional stages.
However, as stated in the preceding pages, when the engine is slowed down or the compressor inlet temperature climbs. The front stages supply too much air for the rear stages to handle and the rear stage will choke.
These airflow control conditions can be improved by:
- Introducing a bleed valve into the middle or rear of the compressor and use it to bleed air and increase the airflow in the front of the compressor at lower engine speeds.
- Dividing the compressor into two sections or rotors and design the front rotor speed to fall off more than the rear rotor at low speeds.
- Placing variable guide vanes at the front of the compressor and variable stator vanes in the front of the first several compressor stages, so that the angles of attack can be reset to low angles by moving the variable vanes at low engine speeds.
Some advanced engine designs also use the variable stator concept on the last several compressor stages.
BLEED VALVES IN AIRFLOW CONTROL
The compressor bleed system provides stall prevention by overcoming unstable airflow conditions in the compressor.
This is achieved by bleeding air out from the intermediate stages of the compressor in order to reduce the volume of the airflow particularly during low RPM airflow conditions and thus prevent the choking of the rear compressor stages.
The bleed system often operates in conjunction with variable inlet guide and stator vane systems to maintain an adequate compressor stall margin and a surge free engine operation.
During your study of the compressor. You will have identified some of the engine operations that push the compressor operating line towards the stall and surge line. These are:
- Engine starting and low RPM operation.
- Thrust reverse
During engine start and at low RPM the compressor pressure ratio is low. which means that the air is not compressed to the correct volume to be accommodated in the fixed annulus section of the compressor.
If the excess air volume is not bled away from the compressor the airflow velocity rises to choke the rear stator stages.
The result should now be familiar to you, mass flow falls, pressure ratio rises and the compressor front rotor stages stall.
Air bleed valves are provided at the LP compressor outlet and in the HP compressor section and these are open during engine start and low RPM operation.
As the engine accelerates the air bleed valves are scheduled to close at a given RPM. This will be when the compressor pressure ratio and mass flow reach the normal operating line.
During a rapid deceleration of a multi-spool engine.
The slowing HP compressor cannot accept all of the air being delivered by the LP compressor.
This results in a pressure rise at the LP compressor outlet that reduces the upstream air velocity and moves the LP compressor front rotor stages towards stall.
Air bleed valves are provided between the LP compressor outlet and HP compressor inlet. These open during engine start, low RPM operation, and rapid deceleration to bleed off the excess air.
On some advanced design engines, the loss of propulsion is reduced to some extent by deflecting the intermediate stage bleed air aftward.
At idle and at 100% RPM, with the bleed valves closed, the compressor will operate at a higher pressure ratio.
At the intermediate thrust settings, the opening of the bleed valves reduces the effective pressure ratio. Causing the compressor operating line to follow the compressor stall curve.
To avoid unnecessary thrust loss, bleed valves are actuated only during acceleration or deceleration conditions. Permitting the compressor to provide optimum performance at take-off thrust and also making the compressor operation compatible at lower thrust settings.
BLEED VALVE OPERATION
Bleed valves may be pneumatically or hydraulically operated depending on the engine type.
Positions and their control vary with engine design.
Pneumatic bleed valves are normally spring loaded in the open position and are closed by compressor delivery control air pressure.
A pressure ratio bleed valve control unit senses the HP compressor outlet to intake pressure ratio and directs control air pressure to close the valves or vents control air pressure to allow the valves to open as required. At low compressor pressure ratios, the valves are directed to open.
At high compressor pressure ratios, the valves are directed to close.
In order to ensure surge free operation during rapid decelerations, the inter compressor valves are directed to open.
A deceleration bleed overrides fuel pressure signal may be used to initiate this.
During thrust reverse operations a reverse actuated bleed system (RABS) may be installed to improve the compressor stall margin during reverse thrust operations.
This is actuated by a solenoid switch in the reverser system that signals one or more of the bleed valves to open.