Tip Clearance Effects on Loss Generation and Stall Inception of Forward-Swept and Radial Blade Flows in Subsonic Compressor Rotor

摘要

Low subsonic flow driven by a forward-swept blade rotor has been investigated in compared with a radial blade rotor of flow by setting the blade tip clearance in four cases between 0.25 mm (0.5% blade chord) and 1.1 mm (2.2% blade chord). The blade tip Mach number is 0.35. In the rig test, the throttle margin of the swept blade rotor is between 65% and 40% in this range of the clearance, with a considerable gain from that of the radial blade rotor ranging between 52% and 27%. This gain results from a particular feature of a span-wise loss distribution in the swept blade flow, indicating that the total pressure loss is rather low in the tip region and rather high in the mid span region. In the radial blade, the most of the loss is caused by the tip leakage flow, which increases remarkably as the flow rate decreases. Then, a rotating stall takes place in a flow range while the static pressure rise is still increasing, probably triggered by a short-length scale disturbance. However in the swept blade flow, the tip leakage vortex is swept back interacting weakly with the follower blade, and the mid span loss increases gradually as the flow rate decreases. The rotating stall takes place in the flow range where static pressure rise is decreasing, probably caused by a modal wave. In case of the wide tip clearance of 1.1 mm, the tip leakage flow loss of the swept blade flow much increases, and the rotating stall takes place in the flow range of increasing static pressure rise. It is shown by a precise Navier-Stokes analysis that the low energy hub flow streaming upward along the suction surface of the swept blade does not mixed with the tip leakage flow interrupted with an adverse pressure gradient as a particular feature in the rear tip region. This is one of the reasons of the low loss in tip region and of high total pressure rise in the flow range around the design point. The interrupted low energy flow grows up as the flow rate decreases to form a secondary span-wise vortex, which causes the high loss in the mid span region.