Numerical investigation into the unsteady effects of non-axisymmetric turbine endwall contouring on secondary flows

Abstract

Turbine manufacturers are continually striving to improve turbine performance, and thus reduce emissions, which has been accelerated with the inception of the Kyoto protocol. One of the areas that have received attention is the controlling of secondary flows. The current investigation looks at the use of endwall contouring to reduce the effect of secondary flows. Endwall contouring has been shown to have promise by several researchers. The numerical investigation was based on the experimental geometry which was based on the cascade geometry of Ingram. The same boundary conditions were used, but the numerical investigation was unsteady. The steady state experimental and numerical results were also used as a basis for comparison of the isentropic stage total-to-total efficiency. The experimental time averaged velocity magnitude plots show reasonable correlation, but fail to capture the steep gradients between 25% and 35% span and between 75% and 85% span. Looking at the time dependent streamtubes it becomes obvious that there was little if any difference to the secondary flows due to wake propagation. The streamtubes show that the upstream stator wake had a minor effect on the secondary flow; however the downstream stator's stream tubes did show some oscillations. Thus there could be a greater advantage to contouring blade rows downstream to the first rotor; however this remains to be investigated. It was found that the computed efficiencies of the unsteady and the steady state analysis were different. It is evident that there are no correlations to be seen, the difference appears to be random. Upon inspection it was found that using isentropic efficiency for comparison was not appropriate at such low Mach numbers, due to its sensitivity to errors in temperature and pressure. Snedden et al. highlighted this as well.