The High-Speed Turbine Test Rig
The high-speed turbine test rig is designed as a pressurized open-loop wind tunnel and is used for film cooling investigations in nozzle guide vanes. The flow chart of the turbine test rig, depicted in the following figure, can be divided into the main flow and the coolant flow. The main flow is provided by a screw compressor that supplies a mass flow rate of up to 2.2 kg/s at a maximum pressure ratio of 2.5. The main flow can also be fed temporarily via a throttled pressure vessel to extend the operating range. Downstream of a Venturi nozzle for mass flow metering, the flow passes through a custom-made gas burner which is capable of heating the main flow up to 400 °C at maximum compressor output. The coolant flow is provided either from a compressed air supply system or from bottled gas, which is necessary for pressure-sensitive paint measurements.
The Annular Sector Cascade with Contoured Endwalls
Effective endwall cooling can be achieved by utilizing design-related gaps between turbine components for coolant injection. For dimensioning purposes, it is essential to understand the influence of slot geometry and coolant-related parameters on both film cooling effectiveness and heat transfer. To account for the influence of the radial pressure gradient on secondary flow and, thus, on film cooling performance, an annular sector cascade has been developed and integrated into the high-speed turbine test rig at the institute.
The cascade is equipped with four state-of-the-art nozzle guide vanes with axisymmetrically contoured endwalls. An exchangeable insert in the central passage endwall allows different upstream slot geometries to be investigated. It incorporates various measurement techniques such as five-hole probes, pressure-sensitive paint, and infrared thermography to investigate both the thermal and aerodynamic aspects of film cooling.
Secondary Flow and Filmcooling in Turbine Cascades
Secondary flow not only plays an important role in loss generation but also significantly influences the heat load on the vane and endwall. Moreover, film cooling is significantly impacted by secondary flow as it restricts the propagation of the coolant. This is particularly true when the coolant is injected from an upstream slot to protect the endwall. At the same time, the injection of coolant can alter secondary flow patterns.
A simplified model of secondary flow in a turbine cascade is depicted in the left part of the upper figure: The incoming boundary layer rolls up around the vane stagnation point to the so-called horseshoe vortex. Its pressure side leg combines with the passage crossflow near the endwall to form the passage vortex. The suction side leg of the horseshoe vortex is deflected around the leading edge to the vane suction surface and becomes the counter vortex.
Although statements about endwall coolant coverage cannot be generalized as it is highly dependent on the injection- and slot-related parameters, an inclined injection with medium-momentum coolant will usually result in a distribution that is schematically shown in the right part of the upper figure: While the stagnation pressure in the leading edge area counteracts a uniform coolant discharge, the coolant propagation in the upstream part of the passage is limited by the horseshoe vortex system. As a result, the area around the leading edge, the pressure side, and the downstream part of the passage do not experience significant cooling.
The Annular Test Section
The following figure shows the test rig from the shroud view, with the flow coming from right to left. A welded transition duct (1) converts the circular cross-section into an annular sector cross-section of 60°. This is followed by the first inlet segment for flow conditioning which houses a turbulence screen (2) as well as a rectifier (3) and provides measurement access for monitoring the inlet conditions by using a Prandtl probe. To set a defined level of turbulence at the inlet of the cascade, a turbulence grid (4), designed as an intermediate flange, is mounted between the first and second inlet segments, with the latter one containing a second measurement access. Immediately upstream of the cascade inlet, a hub- and shroud-side boundary layer suction (5) is installed. Here, the incoming boundary layer is separated via circumferential shear sheets, collected in cavities, and discharged via radial bores. The extracted mass flow can be measured and adjusted independently at the hub and at the shroud by means of orifices and control valves integrated into the outlet piping (11).
The annular cascade casing not only contains the five-passage cascade but integrates the necessary instrumentation for the five-hole probe measurements and the application of PSP and IR thermography. The contoured endwalls of the shroud with the central purge slot measurement module are designed as milled inserts (6) that are held in place by clamping jaws. By means of pressure taps, which are distributed around the perimeter at the inlet and outlet of the cascade, the operating conditions can be monitored. A numerically optimized plenum (7) allows the purge slot to be fed homogeneously along the entire length of the slot at both low and high blowing ratios. The design makes use of a porous sleeve by which the coolant is evenly introduced into the plenum. Downstream of the cascade, a linear transition (8) directs the flow to the outlet plenum (9), which collects the flow and guides it into the chimney (10).
The welded cascade casing houses both the vanes and the upper and lower cascade limitations with the fixed and adjustable tailboards. It further provides optical access for the PSP and IR measurements and integrates the probe traverse system that allows the five-hole probes at the inlet and outlet to be independently traversed in the radial and circumferential direction.
For questions about the test rig, please contact Christian Landfester.
Impressions from the Annular Test Rig
Acknowledgment
Essential parts of the annular test rig work were supported within the AG Turbo framework of EcoFlex-turbo KüpLe 3.2 of the Federal Ministry for Economic Affairs and Climate Action. We would also like to thank MAN Energy Solutions SE for the financial and technical support.