PREDICTION OF THE BECALMED REGION FOR LP TURBINE PROFILE DESIGN

Recent attention has focused on the so-called ''becalmed region'' that is observed inside the boundary layers of turbomachinery blading and is associated with the process of wake-induced transition. Significant reductions of profile loss have been shown for high lift LP turbine b lades at low Reynolds numbers due the effects of the becalmed region o n the diffusing flow at the rear of the suction surface. In this paper the nature and the significance of the becalmed region are examined u sing experimental observations and computational studies. It is shown that the becalmed region may be modeled using the unsteady laminar bou ndary layer equations. Therefore, it is predictable independent of the transition or turbulence models employed. The effect of the becalmed region on the transition process is modeled using a spot-based intermi ttency transition model. An unsteady differential boundary layer code was used to simulate a deterministic experiment involving an isolated turbulent spot numerically. The predictability of the becalmed region means that the rate of entropy production can be calculated in that re gion. It is found to be of the order of that in a laminar boundary lay er. It is for this reason and because the becalmed region may be encro ached upon by pursuing turbulent flows that for attached boundary laye rs, wake-induced transition cannot significantly reduce the profile lo ss. However, the becalmed region is less prone to separation than a co nventional laminar boundary layer. Therefore, the becalmed region may be exploited in order to prevent boundary layer separation and the inc rease in loss that this entails. It is shown that it should now be pos sible to design efficient high lift LP turbine blades.