Numerical Investigation On The Tip Leakage Flow And Unsteady Flows Due To Rotor/Stator Interactions In A Micro-Axial Fan

For cooling purpose, micro-axial fans are widely used in the civil electrical equipment, medical, mechanical and electrical industry. In order to improve the cooling performance and reduce noise, it is necessary to have enough understanding with the inner flows of the fans. Although the flows in large-scale turbomachinery have been intensively investigated, only limited studies on the performance of this kind of fans have been reported. In this paper, numerical simulations are performed to investigate the flows in a macro-axial fan stage by utilizing computational fluid dynamics. The main research contents are as follows:In the environment of isolated rotor, the loss in the passage can be simply classified as boundary layer loss, tip leakage loss and endwall loss. The difference of the loss under different flowrates are shock loss and tip leakage loss. Generally, shock loss can be controlled to the minimum at design flowrate. With the increase of the flowrate, shock loss increases obviously. The size of the tip leakage vortex(TLV) gets smaller and vorticity gets stronger. Although the region affected by TLV gets smaller, the mixing distance in the axial direction gets longer. Accordingly the loss at large flowrate is higher than that at design flowrate. With the decrease of the flowrate, shock loss also increases obviously, and the size of the TLV gets bigger. When the flowrate decreases to some extent, secondary leakage flow can be observed, resulting in a sharp increase of the loss in the tip region, according higher loss is observed at small flowrate than that at design flowrate. Hence, the efficiency increases first and then decreases with the decrease of the flowrate. On the left side of the peak efficiency point, the loss as well as the work obtained increases with the decrease of the flowrate. When the increase of the work obtained is larger than that of loss, the total pressure drops. So the peak efficiency point always located on the right side of the peak pressure point.Near peak pressure point, the self-induced unsteadiness is observed in the tip region. An explanation based on the propagation of the low energy spot and its multi-passing through the high gradient zone of the relative total pressure, is proposed to clarify the originating mechanism of the unsteadiness. At large flowrate, the low energy spot can only pass through the interface once, accordingly the pressure fluctuation is very weak. At the flowrate near peak pressure point, the main flow is weaker than the tip leakage flow and the interface moves upstream. The low energy spot which propagates along in the close behind of the interface has opportunity to circulate in the circumferential direction and passes through the sensitive interfaces several times, a slight perturbation therefore maybe magnified significantly and develops into the self-induced unsteadiness.In the stage environment, the pressure fluctuation on the blade surface of the upstream rotor is mainly caused by the high pressure gradient around the leading edge of the stator. During the propagation process of the potential interaction, there exists an expansion region in which the pressure fluctuation is in phase with that on the suction surface. While in the other regions, obvious axial directivity of the propagation is observed. Hence, the pressure changes in phase on the suction surface, and a certain lag on the pressure surface. For the downstream stator, the rotor-wake changes the attack angle of the stator, because of the velocity deficit. However, the effect of this change is limited in around of the stagnation point. The unsteady response in the second half of the stator surface is weak.Through the research of this paper, the characteristic of the steady tip leakage flow in a micro-axial fan which has been studied rarely is shown comprehensively. Based on this, a conjecture of the low energy spot multi-passing the sensitive interfaces is proposed to clarify the mechanism of the self-induced unsteadiness. Finally, the characteristic and the originating mechanism of the pressure fluctuation on the blade surface due to rotor/stator interaction are revealed.