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FLOw COntrol TEChniques Enabling Increased Pressure Ratios in Aero Engine Core Compressors for Ultra-High Propulsive Efficiency Engine Architectures

Periodic Reporting for period 1 - FloCoTec (FLOw COntrol TEChniques Enabling Increased Pressure Ratios in Aero Engine Core Compressors for Ultra-High Propulsive Efficiency Engine Architectures)

Reporting period: 2018-10-01 to 2019-09-30

To reduce the environmental impact and enhance the overall efficiency of an aero engine, both its propulsive and thermal efficiencies need to be increased. A propulsive efficiency improvement can be achieved by increasing the aero engine bypass ratio, while its thermal efficiency can be improved by applying higher turbine entry temperatures and higher overall pressure ratio cycles. Therefore, the research activities of FloCoTec are associated with the development of novel high pressure compressor (HPC) rear stage concepts to achieve a high overall pressure ratio within the engine core. However, a further increase in core engine pressure ratio leads to an inevitable reduction of the core engine size and cross sectional area, which introduces new HPC design challenges, particularly for the compressor rear stages. Since the rotor tip and stator seal clearances are limited on an absolute scale to avoid rubs, a decrease in blade height results in larger relative blade clearances, which lead to increased secondary flow phenomena, including stronger blade tip vortices, shroud leakage flows, and an increased boundary layer growth in the endwall regions. These detrimental aerodynamic effects penalize the operational behaviour (stall margin) and aerodynamic performance (efficiency).

To overcome the detrimental effects arising from pronounced rotor tip leakage flows, casing treatments (CT) are commonly applied. However, while CTs are known to strengthen the flow in the rotor tip region, they typically cause a radial re-balancing of the flow and weaken the downstream compressor flow at lower span heights. This phenomenon will be particularly pronounced within the compact rear stages of future high-pressure ratio HPCs with small blade heights, and considerably increases the risk of a premature compressor stall due to weaker flows at lower span regions.
To tackle these aerodynamic challenges, GEDE investigates innovative HPC technologies including an advanced 3D blade design for HPC rear stages within CS2 Joint Undertaking. TUM aims to contribute to the research of GEDE through:

OB 1. The development of compressor flow treatment technologies that strengthen the flow across the entire span, enhance the stability in a multi-stage compressor environment, and maximise the potential of the CT-technology. The aim is to deliver an efficiency-neutral HPC rear stage design with an increase of compressor stall margin by ≥ 5%.

OB 2. The provision of a compressor rig test facility that allows for a validation of the HPC rear stage technologies developed by GEDE and TUM, including a detailed quantification of the HPC performance and operability, under engine representative conditions.

OB 3. The development and application of advanced unsteady pressure and temperature measurements that allow for a time-accurate entropy estimation and thus provide a detailed understanding of the flow physics and aerodynamic loss mechanisms within the developed HPC rear stage concept.
The work within the first reporting period of FloCoTec covered numcerical as well as experimental aspects. The numerical work scope covered the development and parameterisation of a novel flow treatment (FT) concept to complement CTs for a strengthening of the HPC flow across the entire span height. Furthermore, the blading of highly loaded compressor stage representative for modern aero engine HPCs has been developed based on a numerical test geometry available at TUM. The blading comprises high total pressure ratios, high relative rotor clearances, low aspect ratio blades and shrouded stator blade rows. The experimental work scope covered the acquisition of advanced measurement concepts to potentially asses the total pressure and temperature with a high spatial and temporal resolution. By combining the two measured flow quantities, an estimation of the entropy generation in a test rig is provided.
TUM and GEDE cooperatively acquired a novel FT concept to complement CTs. The FT is projected to compensate the radial rebalancing effects in HPCs caused by CTs through an increase of the axial momentum in the hub regions of the compressor, leading to a strengthening of the flow across the entire span height. The FT is expected to leverage the full potential of CTs in HPCs and to deliver an efficiency neutral combined FT with a significant stall margin increase allowing for a further reduction of the core engine size in future engine concepts. Therefore, the treatment concept will eventually enhance the thermal efficiency of future engine concepts and therefore reduce the environmental impact of aero engines enabling a more environmental friendly air travel.