Investigation On The Unsteady Mechanism Of Tip Leakage Flow In Transonic Compressor/Fan Rotors

A numerical and experimental study was conducted to investigate the unsteady tip leakage flow in three transonic compressor/fan rotors, with a hope to illustrate the unsteady features, reveal the originating mechanism, uncover the 3-D structure of tip leakage flow, and explore the link between the unsteady tip leakage flow and rotating stall.First of all, the existence of the unsteady tip leakage flows in three different transonic compressor/fan rotors was validated by numerical simulation and experiment. The unsteady features of tip leakage flow such as oscillating frequency, distribution of fluctuating strength, and instantaneous flow fields were studied. It was found that for all the three transonic rotors investigated, there were two main unsteady features of tip leakage flow:(1) the high-pressure spot washed down the low-pressure spot along blade pressure side, and (2) the trajectory of tip leakage vortex oscillated and thus changed the location of the low-pressure spot on the blade pressure side. This kind of unsteadiness was called self-induced unsteadiness because it emerged without external unsteady excitation.The fluctuation of pressure distribution and tip leakage velocity indicated that the mechanism of fluctuating tip leakage flow was the dynamic interaction between the incoming main flow and the tip leakage flow. An in-depth analysis of the unsteady flow fields was performed, and two necessary conditions for the initiation of unsteady tip leakage flow were proposed. One was that the region influenced by the tip leakage flow should reach the pressure side of the neighboring blade, and the other was that the tip leakage flow should be strong enough to interact with incoming main flow in order to achieve the dynamic balance. It was also found that the momentum ratio between tip leakage flow and incoming main flow and the tip clearance size could be used to quantify the initiating condition for the unsteadiness of tip leakage flow.The three-dimensional flow structure of tip leakage flow was studied by utilizing the pathline of the particles artificially released from the tip clearance region. The in-depth analysis of three-dimensional flow structures revealed three features:(1) there existed an interface between the incoming main flow and the tip leakage flow, (2) the tip leakage flow could be divided into two parts according to the blade loading distribution, and (3) each part played a different role in the location of the interface between the incoming main flow and tip leakage flow. A model of three-dimensional flow structures of tip leakage flow was thus proposed accordingly.Finally, a surface streaking method was used to experimentally examine the surface shear pattern on the casing. A region of zero axial shear stress was found to move upstream while decreasing flow coefficient. This phenomenon was also captured by numeical simulation. The compressor stalled once the zero-shear-stress line moved upstream of the blade leading edge for all cases. Inlet distortion was experimentally and numerically varied to alter the momentum ratio at the blade tip and the resulting location of the zero-shear-stress line was exmined. For all cases tested, the zero-shear stress line was observed to be at the same axial location just prior to stall. At the same mass flow rate, the the zero-shear-stress line is closer to the leading edge of the blade with tip inlet distortion than with hub inlet distortion, perhaps due to that the momentum ratio of tip leakage flow to incoming flow with tip inlet distortion is larger than that with hub inlet distortion.