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Low Speed Cascade Tunnel, Göttingen (NGG)

Photo of windtunnel NGG . Test section of NGG with AITEB-1-blades . Turbulence grid in the inlet nozzle of NGG

The NGG is an intermediate running blow down type facility, built up in a similar way as the Windtunnel for Straight Cascades (EGG). Atmospheric air from the wind tunnel hall passes the inlet duct, the test section with the cascade, an adjustable choking diffuser and finally through a valve (o-ring flap) and a pipe system into a large vacuum vessel. The vessel with a capacity of 10000 m³ is constantly evacuated by a set of vacuum pumps, allowing measuring times of up to 20 minutes. The mass flow through the wind tunnel and hence the back pressure or isentropic exit Mach number of the cascade is set by an adjustable diffuser.

The test section air of the NGG is sucked directly from the test hall, i.e. in comparison to the EGG an air drier is missing. Accordingly it is not advisable, to exceed Mach number 1 in the test section, as condensation would occur. The main advantage of the NGG compared to the EGG is the possibility to install model blades of bigger size in order to enable more detailed investigations.

An active turbulence generator can be mounted on top of the test section inlet plane, allowing adjustments of the turbulence intensity upstream of the cascade. This is achieved by independent up- and downstream blowing of air through rows of small holes in the bars of the turbulence grid. With moderate blowing a turbulence above 10% has been achieved.

Pressurized air is supplied to feed the turbulence generator and to enable the simulation of leakage air and coolant flow in the test rig. Windtunnel, probes and data acquistion (pressures, temperatures) are PC-controlled. The software in use are SIMATIC from Siemens for the windtunnel control and LabView from National Instruments for the control of the data acquisition (PSI-system etc.). Mass flows of secondary air (turbulence generator, leakage flow) are determined by measuring the pressure loss of standard apertures.

The NGG was recently dedicated to measurements for the Europaen research task AITEB-1 (Aerothermal Investigations on Turbine Endwalls and Blades), accordingly the test rig (blades, flow path) is adapted to that task only. "AITEB-1" is a research task to investigate secondary and leakage flow in turbines and their influence on heat transfer.

The upper and lower test section walls are shaped according to streamlines through the cascade and were obtained from a CFD calculation. Two flaps are installed in these walls enabling a periodicity control of the cascade flow field. A plane sidewall is equipped with large windows for optical access at different wave lengths. Varying experimental instrumentations can be employed because the centre vane (including tip endwall) is easily exchangeable from the test section.

The effect of leakage flow can be simulated by blowing pressurized air through a leakage gap in the sidewall upstream of the vane inlet plane. Air is supplied to a settling chamber, where it is calmed before passing an orifice plate and a slot of about 100mm depth and 2mm height through which the air is finally ejected into the test section.

Some characteristic parameters of the test section with installed AITEB-1-cascade are listed below:

Inlet pressure and temperature	atmospheric
Maximum run time	20 min
Exit flow Mach number	0.1 ... 1.0
Reynolds number related to blade chord	2 ·106

Results of investigations at DLR in the course of the AITEB-1-task can be found in following publication:
Rehder, H.-J., Dannhauer, A.:
Experimental Investigation of Turbine Leakage Flows on the 3D Flow Field and End Wall Heat Transfer
ASME Technical paper GT2006-90173, 2006

Authors: Fritz Kost, Hans-Jürgen Rehder, Axel Dannhauer