Mechanism of Stall in Low-Noise Duct Fans

Mechanism of Stall in Low-Noise Duct Fans

As air movers, low-noise duct fans maintain air circulation. It is well known that low-noise duct fans operate by increasing blade speed, but stall often occurs during operation. What is the reason for this? This article explains the stall mechanism of centrifugal fans. Rotational stall is a typical failure mode for low-noise duct fans. If left unchecked, it can lead to surges. Rotational stall disrupts the uniformity of the flow field within the impeller, generating additional aerodynamic loads. In severe cases, high stress points on the blades can trigger fatigue failure, reducing the fan’s efficiency. The housing and connecting ducts may experience violent vibrations, potentially leading to accidents. When the gas flow rate entering the propeller falls below the rated flow rate, the gas velocity at the propeller decreases, causing vortices to form near the trailing edge of the blades and the airflow to separate from the rear. As the airflow deteriorates at the blade and its trailing edge, lift decreases while drag increases. At this point, the rotating release group rotates from a reference frame fixed to the impeller. The angular frequency ω1, relative to the absolute coordinate system outside the propeller, is the angular frequency of the rotating blade assembly (ω - ω1). The direction of rotation is the same as the direction of rotation. Here, ω1 and (ω-ω1) are the two characteristic frequencies when a low-noise duct fan experiences rotational stall. From the calculation formula, the propeller behaves as follows. The stalling frequency (ω-ω1) falls within the range of approximately 0.5 to 0.8 times the speed frequency ω. Furthermore, the machine vibrations caused by rotational stall differ from those caused by other mechanical failures. Rotor imbalance and misalignment may increase the rotor’s amplitude, but the housing and ductwork may not exhibit noticeable vibrations. Rotational stall differs from airflow-induced excitation. In some cases, the amplitude measured at the rotor may not be significant, yet the housing and ductwork may experience severe vibrations. Additionally, the vibrations caused by rotational stall vary with changes in load, pressure, and flow rate.