Sunday, December 14, 2008

Compressor Stall

A stall can happen within the compressor if the air shifts from its general direction of motion (also known as the angle of attack). At this point the low pressure on the upper surface disappears on the stator blade. This phenomenon is known as a stall. As pressure is lost on the upper surface, turbulence created on the backside of the stator blade forms a wall that will lead into the stall. Stall can be provoked if the surface of the compressor blade is not completely even or smooth. A dent in the blade, or a small piece of material on it, can be enough to start a turbulence on the backside of the blade, even if the angle of attack is fairly small.

Each stage of compression should develop the same pressure ratio as all other stages. When a stall occurs the front stages supply too much air for the rear stages to handle, and the rear stage will choke. If the angle of attack is too high, the compressor will stall. The airflow over the upper airfoil surface will become turbulent and destroy the pressure zone. This will decrease the compression airflow. Any action that decreases airflow relative to engine speed will increase the angle of attack, increasing the tendency to stall.

If there is a decrease in the engine speed, the compression ratio will decrease with the lower rotor velocities. With a decrease in compression, the volume of air in the rear of the compressor will be greater. This excess volume of air causes a choking action in the rear of the compressor with a decrease in airflow. This in turn decreases the air velocity in the front of the compressor and increases the tendency to stall.