Abstract
Based on Shear Stress Transport (SST) turbulence model for unsteady simulation of an axial-flow fan, this paper studies the time-frequency information in the hump region, and investigates the disturbance information of spike and modal wave under different flow coefficients based on continuous wavelet transform (CWT). The results show that before the hump point, the low-frequency modal wave occupies the main disturbance form and circularly propagates at 1/10 of the rotor speed, and the axial-flow fan does not enter the stall stage; while after the flow coefficient reduces to the hump point, the spike wave with higher frequency replaces the modal wave as the main disturbance mode while the axial-flow fan enters the stall stage. Through in-depth investigation of unsteady flow characteristics under the hump point, it is found that after experiencing the emerging spike, with the sharp increase of incidence angle, some flow distortions appear on the intake surface, and further induce some flow paths to form stall vortices. When a path goes into stall stage, the airflow state is greatly affected, the inverse flow and air separation phenomenon in the rim region increase significantly, and the flow capacity decreases significantly, so the flow capacity in the hub region increases correspondingly. The flow path distortion of tip leakage flow (TLF) and leading edge (LE) spillage caused by the stall vortices are the main inducements of rotating stall.
Highlights
As a typical rotating machine, axial-flow fan is widely used in the field of national economy due to its advantages of large air volume, good ventilation effect, high efficiency and energy savings [1,2,3].With the development of high speed and high power in the field of rotating machinery, the axial flow fan needs to meet the design indexes of high efficiency, low noise and stable operation under multiple working conditions
The results show that when the incidence angle of the hump point does not reach the critical incidence angle value, the disturbance mode belongs to the modal disturbance; while the rotor incidence angle before the hump point or at the hump point is higher than the stall critical incidence angle, the spike disturbance mode appears
Where p is the dynamic pressure of the monitoring points; p is the average pressure in the rotation period points; and u2monitoring is the peripheral theaverage impeller outlet; 1int*the represents where pofisthe themonitoring dynamic pressure of the points;speed p isofthe pressure rotation the time required for impeller to rotate one revolution in the circumferential direction
Summary
As a typical rotating machine, axial-flow fan is widely used in the field of national economy due to its advantages of large air volume, good ventilation effect, high efficiency and energy savings [1,2,3]. Spike disturbance is a kind of sudden precursor disturbance with high amplitude, fast circumferential propagation speed and small disturbance coverage. It belongs to the early form of the stall cell [14]. Modal disturbance belongs to the harmonic disturbance, which has small amplitude, extremely slow circumferential propagation speed and great disturbance coverage. It is not a stall disturbance mode, but only an unstable flow field disturbance of rotating machinery under off-design conditions [15]. The transition process from weak disturbances to unstable strong disturbances under spike-stall mode is investigated
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