Session: 35-04 Hot Streaks and Sealing Flows

Paper Number: 78057

78057 - Influence of Purge Flow Injection on the Performance of an Axial Turbine With Three-Dimensional Airfoils and Non-Axisymmetric Endwall Contouring 

In this paper, we present the evaluation of the aerodynamic robustness of an optimized axial turbine configuration (named „F3D“) against rim seal purge flow. In a preceding project the 1.5 stage axial turbine operated by our institute was aerodynamically optimized by applying a bowed profile stacking to the first stator, and an endwall contouring (EWC) to the hub and shroud of the first stator as well as the rotor (cf. Poehler et al. [1]). Our experiments and numerical simulations show that the efficiency benefit gained by the F3D configuration is reduced, but not eliminated, through the injection of rim seal purge flow through the cavity downstream of the first stator.

Studies by other authors showed that the behaviour of such EWC configurations can be unpredictable. Schuepbach et al. [2] investigated two different EWC configurations: the first one experienced a decrease in efficiency gain by about 50% compared to the base configuration when purge flow was injected while the second one was barely affected. In contrast to these results, Regina et al. [3] even reported a reduced efficiency relative to the axisymmetric base configuration once the injection rate exceeded 0.6% of the main mass flow. In difference to the aforementioned authors, the design of the EWC configuration of Jenny et al. [4] took the purge flow into account. Thus, it exhibited a better performance than the base case when subject to purge flow.

Using performance map measurements, we found that our F3D configuration provides an efficiency advantage over the base configuration of 0.37 % when purge flow is injected. Without purge flow, the F3D configuration provides an efficiency advantage of 0.50 %. Measurements with five-hole probes and 3D hot-wire probes give detailed insights into the flow downstream of the first stator and rotor. Supported by unsteady RANS simulations, we were able to identify the mechanisms behind this decrease in efficiency gain. The flow angles at the trailing edge of the first stator remained practically unchanged when purge flow is injected. This indicates that the F3D intended homogenization of the flow entering the rotor prevailed, regardless of purge flow injection. Major differences are, however, visible in the rotor flow: without purge flow injection, the F3D configuration is able to diminish the formation of loss-inducing secondary flow structures near the hub. However, the injection of purge flow takes away the F3D effect at the hub and the secondary flow structures near the hub are practically indistinguishable from the vortices found in the base configuration with purge flow injection. As the influence of the purge flow is limited to the lower 20% of the channel height, the endwall contouring at the casing is still providing an increase in efficiency by suppressing secondary flow structures.

In summary, this paper presents a detailed investigation of the influence of purge flow on the performance of an optimized turbine configuration. Our research demonstrates that even though purge flow was not considered during the optimization of the configuration, it still provides a substantial efficiency benefit under the influence of purge flow injection.

 

[1]:  T. Poehler et al.: „Investigation of Nonaxisymmetric Endwall Contouring and Three-Dimensional Airfoil Design in a 1.5-Stage Axial Turbine—Part I: Design and Novel Numerical Analysis Method,“ Journal of Turbomachinery, 2015.

[2]: P. Schüpbach et al.: „Influence of Rim Seal Purge Flow on Performance of an Endwall-Profiled Axial Turbine,“ in Proceedings of the ASME Turbo Expo 2009, Orlando, Florida, USA.

[3]: K. Regina et al.: „Aerodynamic Robustness of End Wall Contouring Against Rim Seal Purge Flow,“ in Proceedings of ASME Turbo Expo 2014, Düsseldorf, Germany.

[4]: P. Jenny et al.: „A Low Pressure Turbine With Profiled Endwalls and Purge Flow Operating With a Pressure Side Bubble,“ Journal of Turbomachinery, 2012

Presenting Author: Lukas Schäflein RWTH Aachen University - Institute of Jet Propulsion and Turbomachinery

Presenting Author Biography: PhD Student at the Institute of Jet Propulsion and Turbomachinery, RWTH Aachen<br/>M.Sc. in Aerospace Engineering from Technical University Munich (TUM)<br/>Research areas: compressor and turbine aerodynamics, aero-engine combustors

Authors:

Lukas Schäflein RWTH Aachen University
Johannes Janssen RWTH Aachen University
Henri Brandies RWTH Aachen University
Peter Jeschke RWTH Aachen University
Stephan Behre MTU Aero Engines AG