For the experimental and theoretical development of bladings for axial flow turbomachines the simulation of the two-dimensional flow field by a straight cascade is a very helpful and versatile tool. An infinitely long linear cascade is obtained by geometrically developing a cylindrical (blade-to-blade) intersection of the blade row into the plane (see the following figure). A real straight cascade, installed into a winndtunnel, consists of a finite number of profiles taken from that infinitely long linear cascade.
|From a turbine rotor to a linear cascade|
The straight cascade tunnel at DLR Göttingen is of the blowdown type with atmospheric inlet. A sketch of the main flow path of the windtunnel is shown in the following figure.
Sketch of the Straight Cascade Tunnel (EGG)
The ambient air first enters a silica-gel dryer, passes subsequently two screens and a honeycomb flow straightener and enters the cascade after a contraction. Downstream of the cascade the flow passes an adjustable diffuser and the main butterfly valve and enters at last a huge vacuum vessel (10000 m³). This vessel is evacuated by two sets of sliding-vane vacuum pumps.
The inlet total pressure of the cascade is equal to the ambient pressure. Inlet total pressure and temperature are measured in the settling chamber in front of the contraction. The Reynolds number cannot be varied independently, but is a function of the Mach number. Each cascade is mounted between two circular disks establishing the side walls of the flow channel. The inlet angle is adjusted by turning this assembly. The test section dimensions are 400 mm × 125 mm, allowing the cascades to consist of 8 to 20 blades depending on the upstream flow angle and the blade gap.
The flow downstream of the cascade is not guided. The downstream static pressure and therefore the Mach number is adjusted by the plenum pressure (i.e. setting of the diffuser).
EGG Test section
Some wind tunnel performance data are listed below:
|Test section size
||125 mm × 400 mm|
|Total pressure and temperature
||3 - 30 min|
|Downstream flow Mach number
||0.2 ... 1.6|
|Reynolds number based on blade chord
||~ 800 000|
The testing capabilities comprise conventional pressure distribution measurements on the blade or endwall surfaces and pressure probe measurements in the wake and profile boundary layer in order to gain information on the characteristic cascade values like flow angles and losses and on the boundary layer characteristics. Furthermore Laser velocimetry (L2F, PIV), measurements with heated thin films and with heat transfer sensors are available as additional measurement tools at the windtunnel. Flow visualization is provided by means of oil flow patterns and the Schlieren technique. To simulate coolant ejection there is an additional supply of air and/or CO2.
Measurements of heat transfer and film cooling effectiveness are done by surface bound foils combining heating and temperature measurement or by using heater foils together with an infrared camera.
Operation of EGG within a closed circuit:
Reynolds- and Mach number range of EGG when operated in a closed circuit
In future a much higher loading with transonic flow through the blade rows for low-pressure turbines in a jet engine is expected. In order to make the Windtunnel for Straight Cascades furthermore applicable for the support of design processes for turbine blade profiles and detailed studies of simple models for high and low pressure turbines, its test section was additionally connected to the new compressor unit of the NGTurb. Due to the high compressor pressure ratio in combination with a closed circuit, experiments with higher Mach numbers (e.g. for tip section blade profiles from steam turbines) and the independent variation of Mach and Reynolds number can now be performed at the EGG. In particular cascades with blade profiles from low pressure turbines can be studied under realistic engine conditions.