The Flow in a Second-Stage Nozzle of a Low-Speed Axial Turbine and Its Effect on Tip Clearance Loss Development

The flow field in a one-and-a-half-stage low-speed axial turbine with varying levels of rotor tip clearance was measured in order to compare the behavior of the second nozzle with the first and to identify the manner in which the second nozzle responds to the complex tip clearance dependent flow presented to it and completes the formation of tip clearance loss. The tangentially averaged flow relative to the rotor blade in the tip clearance region was found to differ radically from that found in cascade and is not underturned with a high axial velocity. There is evidence rather of overturning caused by secondary flow. The axial velocity follows an almost normal endwall boundary layer pattern with almost no leakage jet effect. The cascade tip clearance model is therefore not accurate. The reduction in second-stage nozzle loss was shown to occur near the hub and tip, which confirms that it is probably a reduction in secondary flow loss. The nozzle exit loss contours showed that leakage suppressed the formation of the classical secondary flow pattern and that a new tip clearance related loss phenomenon exists on the suction surface. The second-stage nozzle reduced the hub endwall boundary layer below that of both the first nozzle and that behind the rotor. It also rectified secondary and tip clearance flows to such a degree that a second-stage rotor would experience no greater flow distortion than the first-stage rotor. Radial flow angles behind the second-stage nozzle were much smaller than found in a previous study with low-aspect-ratio untwisted blades.