Experimental and Numerical Investigation of Optimized Blade Tip Shapes—Part I: Turbine Rainbow Rotor Testing and Numerical Methods

Abstract

Blade tip design and tip leakage flows are crucial aspects for the development of modern aero-engines. The inevitable clearance between stationary and rotating parts in turbine stages generates high-enthalpy unsteady leakage flows that strongly reduce the engine efficiency and can cause thermally induced blade failures. An improved understanding of the tip flow physics is essential to refine the current design strategies and achieve increased turbine aerothermal performance. However, while past studies have mainly focused on conventional tip shapes (flat tip or squealer geometries), the open literature suffers from a shortage of experimental and numerical data on advanced blade tip configurations of unshrouded rotors. This work presents a complete numerical and experimental investigation on the unsteady flow field of a high-pressure turbine, adopting three different blade tip profiles. The aerothermal characteristics of two novel high-performance tip geometries, one with a fully contoured shape and the other presenting a multicavity squealer-like tip with partially open external rims, are compared against the baseline performance of a regular squealer geometry. The turbine stage is tested at engine-representative conditions in the high-speed turbine facility of the von Karman Institute. A rainbow rotor is mounted for simultaneous aerothermal testing of multiple blade tip geometries. On the rotor disk, the blades are arranged in sectors operating at two different clearance levels. A numerical campaign of full-stage simulations was also conducted on all the investigated tip designs to model the secondary flows development and identify the tip loss and heat transfer mechanisms. In the first part of this work, we describe the experimental setup, instrumentation, and data processing techniques used to measure the unsteady aerothermal field of multiple blade tip geometries using the rainbow rotor approach. We report the time-average and time-resolved static pressure and heat transfer measured on the shroud of the turbine rotor. The experimental data are compared against numerical predictions. These numerical results are then used in the second part of the paper to analyze the tip flow physics, model the tip loss mechanisms, and quantify the aero-thermal performance of each tip geometry.

Characterization of the Unsteady Aerodynamics of Optimized Turbine Blade Tips through Modal Decomposition Analysis

“…At the outlet, the Reynolds based on the rotor axial chord was 2.76 × 10 5 , reached at high-subsonic conditions (relative Mach number of 0.78). A more detailed characterization of the experimental campaign boundary conditions was provided by the authors in [16].…”

“…The instantaneous rotor angular position was monitored by optical shaft encoders with an accuracy of ±0.25% of the rotor pitch at a rotational speed of 5900 rpm. Five blade passages per revolution were therefore considered for each single tip profile, leading to a total of 23 × 5 = 115 blade periods per test [16].…”

“…To maximize the experimental repeatability, a simultaneous testing strategy was adopted for multiple tip geometries, operating the annular cascade in rainbow rotor configuration [16]. Seven different tip designs were investigated, each one hosted in a dedicated sector of the rotor disk and all sharing a common blade profile.…”

Effects of inclined squealer rims on tip leakage vortex and loss in a transonic axial turbine

“…10,11 Then, more and more researchers focused on the geometric optimization of the squealer tip, such as the number and layout of rims, [12][13][14] cutback configurations, 15,16 and the width and depth of rims. 17,18 The researchers from the von Karman Institute for Fluid Dynamics (VKI) [19][20][21] tried to optimize the squealer-like tip geometry by using the differential evolution algorithm and finally obtained the double-cavity tip configuration with great aerodynamic and thermal performance. Prakash et al 22 found that the inclined PSSR could increase the blocking area caused by the flow separation on the top of the rim, controlling the TLF.…”

Variable Blade Tip Geometry Inserts for Combined Aerothermal Measurements for Bladed Disk Rotors

“…With this setup, they achieved blade-to-blade variations in the tip radius below ±0.1 mm. The same setup was used by (Cernat et al, 2018) for the combined aerothermal testing of tip designs, where high-frequency pressure and temperature instrumentation at the rotor casing was used. The time average pressure and heat transfer at the casing was found to vary significantly with blade tip geometry, as a function of blade passage and tip gap position.…”