Study On The Influence Mechanism Of Axial Gap On Stall Inception In A Two-stage Axial-flow Fan

Axial fan is an important power plant auxiliary equipment, so the safe operation of fan is very important to the economics of power plants. The safe operation of the axial flow fan is facing some unstable flow phenomenon, such as rotating stall and surge, which affect the reliability of fan. Axial gap is an important structure parameter, so the study of the influence mechanism of axial gap on rotating stall has important significance.Based on the throttle valve function and Realizable k-ε turbulence model, the flow field in the stall condition of a two-stage axial fan with adjustable vanes of a certain power plant was numerically studied. The influence mechanism of different axial gaps on initial positions, forms and the stall inception of three dimensional unsteady evolution process were investigated. The results show that the stable operation range of the axial flow fan increases as axial spacing decreasing; The stall inception shows spike type under different axial gaps and the stall inception both firstly appears in the tip region of the second rotor, then the stall inception also appears in the first rotor. The time of stall inception which spread from the second rotor to the first rotor decreases with the distance of the two stages reducing. The study shows that when axial gap is relatively small, the separation vortex is formed by the interaction between the main flow and the trailing edge back flow. While the axial gap is relatively large, the formation of the separation vortex is related to the leakage flow of the tip clearance.The flow field in the stall condition of a single-stage axial fan of a certain power plant was numerically studied. The numerical results show that the tip clearance flow moves from suction side to pressure side of the blade and mixes with the main flow, it generates 3D leakage flow in the passage, the leakage flow is increasing gradually and propagates circumferentially and radially. The form of stall inception of the single-stage is the same as the way that the two-stage axial flow fan with large gap. And the mine noise is concentrated with the fundamental frequency. On the stall inception stage, discrete frequency increased significantly in the low frequency, and the mine noise is rotating noise.