STUDY OF TOPOLOGY AND VORTEX STRUCTURE IN TURBINE CASCADES WITH TIP CLEARANCE UNDER DIFFERENT INCIDENCE ANGLES

Three sets of conventional straight, positive curved and negative curved turbine cascades with tip clearance were tested with experimental measurement. The impact of incidence angles and blade bending on the tip leakage was studied under larger clearance (0.036 of span). An ink trace visualization of the wall flow and topology theory was adopted and thus the topological structures of the blade surface, the upper and lower end wall were analyzed in this paper. It was found that, compared with the same cascade under the zero incidence angle, the saddle points all move to the upstream, the scope of the separation expands along the flow and span direction, and the separation line of the upper and lower passage vortex climb to the middle span of the blade when the flow incidence angle increased from zero to 20 degrees. Under the condition of zero and 20 degrees incidence angle, the positive curved blade will eliminate the upper passage vortex. The numbers of singular points are reduced and the interaction loss between the passage vortex and the leakage vortex is greatly reduced too. On the other hand, it also strengthens the blocking effect of the end wall cross flow on the leakage flow, thereby reduces the relative leakage flow rate.