The performance of a one and a half stage axial turbine including various tip clearance effects.

Abstract

The necessary clearance at the tip of unshrouded rotors of axial turbines allows fluid to leak from the pressure to the suction side of the blade and produces an important component of loss that is ultimately responsible for approximately 25 % of the total turbine rotor losses. Leakage fluid can pass through the tip clearance gap with either high or low loss generation. It has been customary in turbine design to employ high loss designs since it is only by the creation of loss that the gap mass flow rate can be restricted. The present work, however examined the effect of streamlined tips that have low entropy generation within the tip and high leakage flows. An axial turbine followed by a second stage nozzle (ie one and a half stages) was designed, built and instrumented and used to evaluate performance with particular reference to the understanding of tip clearance effects in a real machine and possible benefits of streamlined low loss rotor tips. A radiused pressure edge was found to improve the performance of a single stage and of a one and a half stage turbine at the selected tip clearances. This was in contrast to previous cascade results where mixing losses reduced the benefits of such tips. Clearance gap flow appears to be similar to other turbine flow where the loss mechanism of separation must be avoided. Loss formation within and downstream of a rotor is more complex than previously realized and does not appear to obey the simple rules used to design for minimum tip clearance loss. For example, approximately 48 % of the tip leakage mass flow within a rotor may be a flat wall-jet rather than a vortex. Second stage nozzle efficiency was significantly higher than first stage nozzle

efficiency, and even increased with tip clearance. This was a surprising result since it means that not only was there a reduction in secondary flow loss but also that rotor leakage and rotor secondary flows did not generate significant downstream mixing loss. The manner in which the second nozzle responds to the complex leakage flows presented to it and how it completes the formation of tip clearance loss for various rotor tip clearances was identified. The tangentially averaged relative rotor flow in the tip clearance region differed radically from that found in cascades which was seen to be underturned with a high axial velocity. There was evidence rather of overturning presumably caused by secondary flow. Axial velocity followed an almost normal endwall boundary layer pattern with almost no leakage jet effect. Cascade tip clearance models are therefore not accurate in predicting leakage flows of real rotors. The reduction in second stage nozzle loss was seen to occur near the hub and tip confirming a probable reduction in secondary flow loss. Nozzle exit loss contours showed that the leakage flow suppressed the formation of the classical secondary flow pattern and that a new tip clearance related loss phenomenon existed 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 appeared to rectify the secondary and tip clearance flows to the extent that a second stage rotor would experience no greater flow distortion than the first stage rotor would. Radial flow angles behind the second stage nozzle were found to be much smaller than those measured in a previous study of low aspect ratio, untwisted blades.