Analysis of the Causes and Mechanisms of Stall in Moving Axial-Flow Fans

Analysis of the Causes and Mechanisms of Stall in Moving Axial-Flow Fans

When a moving axial-flow fan is operating normally, the angle of attack is very small (the angle between the direction of the airflow and the chord of the blade is the angle of attack), and the airflow flows around the aerofoil-shaped blades to maintain a streamlined state. When the airflow forms a vertical angle of attack at the blade inlet, and this angle exceeds a specific critical value, the flow conditions on the blade’s trailing edge begin to deteriorate. The boundary layer breaks down, and a vortex region—known as the “shawl” phenomenon—forms at the trailing edge of the moving-axis axial fan blade. When the angle of attack exceeds the critical value, the stall phenomenon becomes more severe, the drag of the fluid flow increases, the blade passage becomes blocked, and the fan’s static pressure drops rapidly. During the processing and installation of axial flow fan blades, for various reasons, the blades cannot be made to have exactly the same shape and installation angle. Therefore, if operating conditions change or the flow direction deviates, the angle of attack at each blade inlet cannot be exactly the same. When the angle of attack at a specific point on a blade reaches a critical value, that blade will stall first; not all blades will stall simultaneously. u is the circumferential velocity at a specific point on the blade, w is the relative velocity of the airflow toward the blade, and α is the angle of attack. As a result of the flow diversion in a moving-axis axial fan, if the blades rotate toward the U side, the surrounding flow improves, and the likelihood of stall decreases or disappears. However, if the flow diversion and stall shift to one side, stall occurs due to the increased angle of attack at the blade inlet, leading to blockage and stall in the moving-axis axial fan passage. If this phenomenon persists, the blockage caused by the stall moves in the direction opposite to the propeller’s rotation—a phenomenon known as “rotational stall.” When the fan operates within an unstable region, one or more rotational stall zones are generated by the propeller. As the blades pass through these zones, they are subjected to excitation forces that can cause the blades to resonate. At this point, the dynamic stresses on the axial flow fan increase, leading to fan failure and causing a major machinery accident.