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

Abstract

In this paper, we present the evaluation of the aerodynamic robustness to rim seal purge flow of an optimized 1.5-stage axial turbine configuration with a bowed stator profile and endwall contouring. Performance maps obtained by experiments and numerical simulations show that the efficiency benefit gained by this optimized configuration is partially reduced by the injection of purge flow through the cavity downstream of the first stator. Measurements with five-hole probes and hot-wire probes, as well as unsteady RANS simulations, give insights into the physical effects of the purge flow inside the rotor passage. There, when no purge flow is injected, the optimized configuration diminishes the formation of loss-inducing secondary flow structures near the hub and the casing. When purge flow is injected, however, strong secondary flow structures are induced near the hub. These vortices generate additional losses and thereby partially negate the efficiency benefits gained by the optimization. We found that this influence of the purge flow is limited to the lower half of the channel. The data also shows that the optimized configuration reduces the vorticity near the casing regardless of the purge flow injection, which in turn leads to an efficiency increase in this area. Together, these effects lead to a reduction of the previously gained efficiency benefit by the optimized configuration when it is subjected to purge flow injection. However, compared to a baseline configuration with cylindrical endwalls also subject to purge flow injection, the efficiency is still increased by 0.38%.

Aerodynamic influence of rim seal purge flow injection on the main flow in a 1.5-stage axial turbine with nonaxisymmetric end wall contouring

“…In order to investigate the behaviour of the design by Poehler et al (2015) (hereinafter referred to as "F3D configuration"), a secondary air system was integrated into the axial turbine test rig at the IST and detailed measurements of the efficiency and local flow conditions in the rig were carried out. The results of this measurement campaign can be found in Schäflein et al (2022). Together with numerical simulations, these experiments demonstrated that the F3D configuration loses parts of its efficiency gains when purge flow is injected.…”

“…In a previous paper on this project (Schäflein et al, 2022), it had already been demonstrated that the purge flow has virtually no effect on the flow inside the stator passages. It was also shown that the numerical simulations are able to reproduce the local flow measurements downstream of the rotor (MP2) with good accuracy.…”

“…In order to assess the performance of the F3D configuration under the influence of purge flow, the 1.5-stage axial cold air turbine test rig at the IST was equipped with a secondary air system (SAS) capable of injecting a mass flow of 128 g/s (equal to 1.6% of the main mass flow) through the cavity directly upstream of the rotor. A detailed description of this SAS is given in Schäflein et al (2022). Figure 1 shows the flow path (green arrow) of the purge flow.…”

“…Detailed, twodimensional flow measurements were conducted downstream of the stator (MP1) and rotor (MP2) by traversing the flow radially and circumferentially with a five-hole probe and a 3D-hot-wire probe. For a more detailed description of the turbine instrumentation, the reader is referred to Schäflein et al (2022). Additional measurement data of the unsteady pressure fluctuations inside the cavity were acquired by the use of an unsteady pressure transducer.…”