Experimental study of a steam turbine blade wake

The flow downstream the trailing edge of a steam turbine is very complex. The behavior of the flow can, at this moment only be studied experimentally, since even for one phase flows the conventional mixing length and 2-equations turbulence models are unable to simulate the wake flow correctly. In this paper an attempt is made to decompose the complex problem into smaller separate problems. Therefore a model blade is tested in a steamtunnel. The thickness of the model blades trailing edge is chosen such that the ratio between the blade chord and this thickness is 0.02 (what is a typical value for real sized turbine blades), however the training edge thickness of the model blade equals 5 mm such that the flow phenomena are more visible: in size as well as in frequency. With a very fast visualisation technique the unsteadiness of the flow is shown when the flow at the trailing edge is in near sonic (sub- and super-) conditions. For supersonic Mach numbers (above 1.2) the base pressure region appears much more stable and can be seen as a combination of two backward facing step flows. Therefore the backward facing step was tested in the steam tunnel and the position of the reattachment point as well as the strength of the aerodynamic shocks determined. Finally the combination of the backward facing step and the model blade allows a more realistic simulation of the flow at the trailing edge of a real transonic steam turbine blade since on one side of the blade an aerodynamic shock interacts with the boundary layer.