An Experimental Test Case for Transonic Low-Pressure Turbines - Part 2: Cascade Aerodynamics at On- and Off-Design Reynolds and Mach Numbers

Abstract

In this two-part paper, a novel test-case for transonic low-pressure turbines (LPT) is presented. The current study is a comprehensive report on the design, commissioning and testing of a high-speed LPT cascade. Part II reports the characterization of the aerodynamics at on- and off-design flow conditions. A detailed analysis of the steady aerodynamics of the highspeed low-pressure turbine blade is presented for a range of engine representative outlet Mach numbers from 0.70 to 0.95 and Reynolds numbers from 65,000 to 120,000. The blade 3D aerodynamics are characterized using an innovative set of traversable blades, enabling high-resolution radial measurements. A novel method to estimate the location of separation-reattachment based on pneumatic tap measurements is presented. The separation on the blade suction side is strongly influenced by the Reynolds number at the lower Mach numbers, while open separations were observed at transonic exit conditions independently on the Reynolds number. Downstream measurements by means of a five-hole probe, and pressure taps located in the passage endwall are employed to study the secondary flow development and structures at the cascade outlet. The results show that the losses follow different trends at high and low Reynolds numbers. The profile losses at Re = 70k decrease with increasing Mach number, contrary to what is observed for Re = 120k. The minimum secondary flow losses are found for an off-design condition with lower Mach number with respect to the nominal. The off-design comparisons presented in this paper indicate that at low Reynolds, operating at transonic outlet Mach numbers leads to beneficial effects on the performance.

RANS Prediction of Losses and Transition Onset in a High-Speed Low-Pressure Turbine Cascade

“…The cascade assembly includes 23 blades with a span of 165 mm. The Cascade C1 has been extensively investigated in a wide range of isentropic outlet Mach and Reynolds numbers [26]. During the tests, the freestream turbulence intensity (FSTI) was kept fixed at ∼2.40% using a passive turbulence grid.…”

“…This is caused by a change in the separation bubble formation and growth at a different Reynolds number. From a loading perspective, the size and the location of the separation bubble recovery point and reattachment point can be inferred from the distribution of the acceleration parameter [26], even though the exact location of the separation point and the reattachment point are analyzed better by resorting to the wall shear stress. The relative minimum of K S approximately indicates the initiation of a separation bubble (at S/S 0 ≈ 0.52 for Re 6,is = 70,000), while the peak (S/S 0 ≈ 0.76 for Re 6,is = 70,000) indicates the recovery point, where the separation bubble reaches its maximum size.…”

“…In this paper, predictions by both correlation-based (the γ-Re θt ) and physics-based (the k-ν 2 -ω [24]) transition-sensitive models are compared with experimental data obtained for a transonic low-pressure turbine blade studied in the framework of the EU-funded project SPLEEN [25,26]. While the first model is widely adopted for the prediction of low-Reynolds flows in the turbomachinery community, the k-ν 2 -ω has only seen limited applications for this type of flow condition.…”

PIV Measurements in a High-Speed Low-Reynolds Low-Pressure Turbine Cascade

“…Investigations of LPT blading tested at engine-relevant conditions were mostly conducted by means of aerothermal probes such as multi-hole pressure probes [6][7][8][9][10][11]. The immersion of probes in transonic flow fields encountered in turbomachinery applications introduces non-negligible effects in the flow being measured [12][13][14] as well as the testing article [15,16]. These effects become increasingly more impactful as the Mach number of the flow field in which the probe is immersed increases [17].…”

“…This work aims at extending the existing literature on PIV testing in low-density transonic turbomachinery flows. The measurements were performed in the VKI S-1/C transonic linear cascade equipped with the C1 blade exploited in the EU project SPLEEN [25,26]. The outlet Mach and Reynolds numbers were varied between 0.70-0.95 and 70−120 k, respectively.…”