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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SPLEEN (Secondary and Leakage Flow Effects in High-SPeed Low-PrEssurE TurbiNes)

Teaser

Summary

Geared Turbo Fan (GTF) engines with by-pass ratios beyond 15 are identified as a key-enabling technology to CO2 and NOX emission reduction by more than respectively 20% and 80% with consistent reduction in noise levels compared to a reference engine operating in 2000. One of the key technologies to enable efficient Ultra-High By-Pass ratio geared turbofans is the low-pressure turbine (LPT). While the geared engine architecture allows a large reduction in LPT stage count and weight, the LPT operates at transonic exit Mach numbers and low-Reynolds numbers. Within this range of operating conditions, there is a critical shortage of aerodynamic and performance measurements. A lack of relevant experimental data in these engine-like conditions also concerns the interaction of the secondary-air and leakage flows with the mainstream. SPLEEN aims at filling up this gap with an extensive experimental undertaking that investigates the aerodynamics of high-speed LP turbines of geared-fan propulsion systems. SPLEEN addresses this challenge with detailed flow measurements in two world-class turbine rigs: a large scale, transonic, low-Reynolds number linear cascade including periodic incoming wakes (termed at VKI S1 test facility), and a high-speed 1.5 stage turbine rig (termed at VKI CT3 test facility). The project first investigates the effect of cavity geometries and purge flow rates on the local flow features and turbine performance in the linear cascade (Workpackage 1) and following accounts for rotational and curvature effects (Workpackage 2).The SPLEEN project will validate new high-speed LPT technologies in engine-relevant environments (TRL up to 5) delivering new critical knowledge and unique experimental databases of major importance for turbomachinery designs.

Work performed

Regarding WP1, substantial technical efforts were performed concerning the investigation of cavity flow effects in a large-scale high-speed linear cascade at engine-representative Mach and Reynolds numbers inflow conditions. The tasks performed for WP1 are mainly related to the adaptation and the design of the cascade facility to allow testing with 3 different upstream cavities. VKI verified the capability of the test facility to reproduce the needed aerodynamic boundary conditions and performed investigations regarding the modular design of the cavity installation and instrumentation. An optimization of the test sequence to complete timely and efficiently the parametric test experimental campaign is proposed. 3D flow calculations have been performed to evaluate the ingress/egress coming from the wake generator cavity and to estimate the so-called 3D flow Region Of Interest (ROI). Additionally, VKI developed and validated in-house tools to determine the maximal bar lengths of the wake generator in order to reduce the relative perturbation on the 3D flow ROI due to the wake generator bar tip effects. The manufacturing and quality control of the cascade rig parts, of the rig specific instrumentation and of the cavity cassette is also currently on-going. Three innovative technologies were created by VKI and are expected for joint-patenting with SAE. Regarding WP2, the project aim and restrictions were captured in a novel VKI Systems Engineering Tool developed for the purpose of guiding the design and implementation work in a traceable manner. An important design challenge, which is the viable integration of a LPT turbine stage in a meaningful way on the existing VKI turbine rig CT3, was overcome with the buy-off from the topic leader of the proposed integration approach. The approach using previously published studies to support the decoupling the near wall flows from the mid-stream flow, thus especially allowing the LPT blade to be transformed into a HPT environment without compromising the near-wall regions of aerodynamic interest. One-dimensional preliminary studies were conducted to early understand impact on the CT3 rig. Preparation for the outsourcing of the rig mechanical analysis commenced. Dedicated meetings were held with each of these companies to assess their structural modelling suitability for the task, and to inform about the rig. Currently, all three companies are waiting for the RoQ from VKI for submitting their bid.

Final results

Progress beyond the state of the art
The approach proposed in the SPLEEN project is new and ambitious. A systematic research activity on the interaction between the mainstream and the cavity flows at high speed, in representative engine-similar conditions, has not been conducted yet to understand. Despite several contributions, experimental generalization to compressible flow at high Mach number and low Reynolds number in the presence of incoming wakes, still lacks. The progress beyond the state of the art is listed as follows:
- Investigation in both a high-speed and low Reynolds number environment,
- Investigation on high speed low pressure turbines,
- Correctly simulated velocity triangles and flow coefficient due to periodic inlet wake profiles and their interaction with the airfoil boundary layer,
- Wide variation of leakage cavity shape to limit leakage/mainstream detrimental interactions,
- Leakage cavity design rules at engine representative operating conditions, and variations of the latter,
- Highly space and time resolved interaction and performance database at representative, engine-like operating conditions.

Expected results until the end of the project
The SPLEEN project aims at strongly contributing to the further development, refinement and validation of the most modern methodologies for the understanding and control of the secondary flows induced by leakages in high speed low pressure turbines. The SPLEEN approach will provide an in-depth evaluation of this high speed, low Reynolds number interaction, including incoming wakes (for the cascade tests) and 1.5 stage environment (for the annular geometry). This extensive, space resolved, time averaged and time resolved data set could be used for:
- A direct assessment of the performance of the studied geometries,
- An extension of the correlations used in the todays design and analysis methods for the
- Leakage/mainstream flow interactions and their impact. The extension to the environment of high
- Speed turbines will be particularly welcome,
- New design and modelling rules for the various leakage cavities present in high speed turbines,
- Validation of high fidelity numerical predictions in the high speed, low Reynolds number
- Environment of low-pressure turbines.
The dataset will therefore contain a thorough description of the mainstream and leakage flows in terms of time-averaged and time-resolved quantities (pressure, temperature) and inlet turbulence (turbulence intensity and scales). These parameters are needed as accurate and complete boundary conditions to numerical simulations.

Potential impacts
The detailed analysis of the aerodynamics of high-speed LP turbines will contribute to the improvement of the aerodynamic efficiency of this component, with a direct impact on the thermodynamic efficiency of the engine and a straightforward reduction of its emissions. From an indirect point of view, SPLEEN will generate data of structural and mechanical relevance (i.e. time resolved heat-transfer and time-resolved pressure) which could then be employed to pave the way for a weight reduction process of the low-pressure turbine component, further reducing the specific fuel consumption.