For the experimental and theoretical development of bladings for axial flow turbines the simulation of the flow field by a straight cascade is a very helpful tool. But in a test facility the number of blades is limited and therefore it is very difficult to provide the required periodical flow field. Furthermore the instationary interaction of stator and rotor flow cannot be simulated in a straight cascade tunnel. An annular cascade is the test set-up assigned to overcome these deficits. The DLR tunnel for rotating cascades was in particular designed and built for the testing of annular cascades for turbines in a wide range of parameters. Nevertheless an essential design goal was to create a facility which is as versatile as possible. Consequently it is now mostly used to investigate turbine stages.
The wind tunnel operates continuously as a closed cycle facility with adjustable pressure and temperature level for independant variation of Mach and Reynolds number. The flow medium (dried air) is driven by a 4-stage radial compressor with a variable speed electric motor and equipped with an additional bypass valve. In order to maintain a constant temperature in the system a water cooled heat exchanger is installed downstream of the compressor. Part of the air can bypass the heat exchanger in order to achieve elevated temperatures at test section inlet.
Some wind tunnel performance data are listed below:
Before the fluid reaches the test section it passes a settling chamber with grids and honeycombs to straighten the flow. Different test sections serve to investigate flow fields on cylindrical stream surfaces as well as on conical ones. Smaller changes to the annular flow channel are carried out by exchangeable liner rings. The figures at bottom show the cylindrical test section with a turbine stage installed.
The speed-control of the test rotor is provided by an electric motor/generator which allows driving or braking in both circumferential directions. The flow conditions in the relative system are determined by the circumferential velocity of the rotor as well as by the pressure difference due to the windtunnel compressor.
Typical rotor data are listed below:
The testing capabilities comprise steady temperature and pressure measurements, measurements of unsteady pressures, measurements of the pressure distribution within the rotating system, wake flow measurements by different probes, Laser velocimetry and the determination of heat transfer.
Stage performance is determined by the simultaneous measurement of rotor torque, rotor speed, upstream, downstream pressures, temperatures and mass flow.