As for the surge of axial flow fans, it is actually a common problem in the use of axial flow fans. Surge is caused by internal structure, so the problem needs to be analyzed from the internal principle!

Due to its advantages such as small size, light weight, high efficiency in low load areas, wide adjustment range, and fast response speed, adjustable blade axial flow fans have been widely used in large thermal power plants in China in the past decade. Due to the hump shaped performance curve of axial flow fans, it is theoretically determined that there is an unstable region in the fan. The fan cannot operate stably at any operating point. When the operating point of the fan moves to an unstable area, it may cause the fan to stall and surge.

Currently, most axial flow fans typically use high-efficiency twisted wing blades. When the airflow flows in along the inlet end of the blade, it splits into upper and lower branches along the two ends of the blade. Under normal operating conditions, the angle of attack is zero or very small (the angle between the airflow direction and the blade chord α is the angle of attack), and the airflow bypasses the airfoil blades to maintain a stable streamline, as shown in Figure 1a. When the airflow forms a normal incidence angle with the blade inlet, i.e. α>0, and this normal incidence angle exceeds a certain critical value, the flow condition on the back of the blade begins to deteriorate, the boundary layer is destroyed, and the vortex zone appears at the end of the blade back, known as the "stall" phenomenon, as shown in Figure 1b. The greater the angle of attack α is than the critical value, the more severe the stall phenomenon and the greater the fluid flow resistance. In severe cases, it can block the blade path and cause a rapid decrease in fan pressure.

In the manufacturing and installation process of axial flow fan blades, due to various objective factors, it is impossible for the blades to have exactly the same shape and installation angle. Therefore, when the flow direction deviates due to changes in operating conditions, the impact angle at the inlet of each blade cannot be exactly the same. When the inlet angle of attack α of a certain blade reaches a critical value, stall may first occur on the blade. Not all blades will stall simultaneously, but stall may occur in one or several areas, which may also include one or more blades. Because the stall zone is not static, it spreads from one blade to another or a group of blades.

When the fan operates in an unstable operating condition, the impeller will generate one or several rotational stall zones. Each time the blade passes through the stall zone, it is subjected to an excitation force, causing resonance in the blade; At this point, the dynamic stress on the blades increases, and in severe cases, the fan blades may break, causing major equipment damage accidents.