AXIAL FLOW COMPRESSORS

In axial flow compressors, the flow proceeds throughout the compressor in a direction essentially parallel to the axis of the machine. The unit consists of adjacent row of rotor blades and stator blades. One stage of the machine comprises a row of rotor blades followed by a row of stator blades. For efficient operation, the blades are of aerofoil section based on aerodynamic design. The fixed blades serve the following purposes.

• To convert a part of the K.E. of the fluid into pressure energy. The conversion is achieved by diffusion process out in the diverge blade passage.
• To guide and redirect the fluid flow so that entry to the next stage is without shock.

Surging: Surging is caused due to unsteady, periodic and reversal flow of gas through the compressor when the compressor has to operate at less mass flow rate than a predetermined value. Thus, when flow through the compressor is less than a predetermined value, a surge or pulsation begins and air surges to and fro through the whole compressor instead of giving a stream in one direction.

Choking: Mass flow rate reaches at maximum value when the pressure ratio becomes unity. This generally occurs when the Mach number (ratio of gas velocity and sound velocity) corresponding to relative velocity at inlet becomes sonic. The maximum mass flow rate possible in a compressor is known as choking flow. Choking means fixed mass flow rate regardless of pressure ratio.

Stalling: The phenomenon of reduction in lift force at higher angles of incidence is known as stalling. It is defined as aerodynamic stall or the breakaway of the flow from suction side of the blade aerofoil. The breakaway of flow from suction side may be due to lesser mass flow rate than designed value or due to non-uniformity in blade profile.