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Periodic Reporting for period 3 - CORNET (CORE NOISE ENGINE TECHNOLOGY)

Reporting period: 2018-09-01 to 2019-04-30

This project aims to improve understanding of core noise from aero engines with low emission combustors and provide industry with analytical tool that can be used to predict the noise when designing future engines, thereby enabling quieter designs.
The Horizon 2020 goals are a 75% reduction in CO2 emissions per passenger km and 90% reduction in NOx, while the perceived noise is reduced by 65%.
However lean burn combustor technologies introduced to reduce NOx are proving to be inherently noisier than conventional combustors, generating broadband noise that can be heard external to the aircraft. Without careful design and optimisation, the low emission cores may cause aircraft engines to exceed the noise requirement.
The research in this project aims at understanding the flow physics involved in generation and propagation of core noise in low emission cores. It includes both direct noise of combustion, pressure waves generated directly by unsteadiness in the rate of combustion, and indirect noise generated as entropy waves accelerate through the Nozzle Guide Vanes (NGVs) and propagate through turbine blade rows.
The objectives are to:
• predict the turbulent reacting flow field in a low-emission combustor at engine conditions
• predict the generation of indirect noise and the propagation and interaction of acoustic and entropy fluctuations in a high-pressure turbine stage in an aero-engine
• develop a combustion noise prediction tool that can be used by industry to design quieter engines
• quantify experimentally the combustor unsteadiness in an aero-engine representative low-emission combustor and validate the modelling.
The conclusions of the project are:
• The FlaRe combustion model incorporated within the industry partner’s in-house LES (Large Eddy Simulation) code has been has been extensively validated through comparison with unsteady temperature data.
• The comparisons of LES results for single and double sector combustors show that single sector models and experiments should be sufficient to investigate indirect noise.
• High fidelity computations of the unsteady flows in turbine blade rows have identified the main causes of redistribution of temperature fluctuations as they propagate through a turbine. These can be captured in a low-order model that gives excellent agreement with the computational results.
• The experiments have demonstrated for the first time high-speed optical measurement of temperature profiles in a high-pressure combustion environment representative of an aero-engine. These have been validated near combustor exit by excellent agreement with the LES.
An experimental campaign carried out at DLR Cologne has demonstrated the feasibility of using high-speed OH-spectroscopy to obtain unsteady temperatures in the rough operating environment of a high-pressure combustor. Data with sufficient signal-to-noise ratio was obtained at a sampling rate of 10 kHz and gave detailed information on the unsteady flow at combustor exit. Excellent agreement between the experimental data and the results of LES have validated both the combustion modelling and the experimental technique. The project has also gave for the first time high-speed particle image velocimetry in a high-pressure combustor representative of an aero-engine.
Large Eddy Simulations have been made of both a single and a double-sector of a low-emission engine combustor operating at realistic engine conditions. Comparisons of the results showed that the double sector temperature fluctuations at combustor exit are practically the same as those in the single sector.
A novel approach using Proper Orthogonal Decomposition (PODs) to capture the unsteadiness at inlet to the turbine has been developed and implemented in a simulation of a turbine stage. This identified the main mechanism through which entropy waves propagate and are dispersed within turbine blade rows as being due to differences in convection time along different streamlines through the turbine blade rows, with a secondary redistribution mechanism due streamline curvature and buoyancy effects. Simplified models have been developed for this entropy redistribution and give excellent predictions for the entropy/temperature fluctuations downstream of the turbine blade rows. The low-order model provides transfer functions which can be incorporated into semi-actuator disk models of the type used in industry for preliminary design.
The model of turbulent combustion developed by UCAM and being used in this project includes finite-rate chemistry effects with no tunable parameters. Implementation within a LES of an aeroengine combustor has enabled partially premixed combustion noise sources to be studied. These simulations at high pressure are challenging pushing the boundaries of LES. The experimental validation enables improved confidence in the modelling.
Laser combustion diagnostics have evolved to indispensable for understanding and improving combustion technology but until recently higher pressure measurements were restricted to sampling frequencies of a few Hertz. In this project data with sufficient signal-to-noise ratio was obtained from OH-spectroscopy at a sampling rate of 10kHz in the rough operating environment of a high-pressure combustor. Laser absorption spectroscopy performed simultaneously with OH-LIF along a one-dimensional probe volume gave absolute OH concentrations, which were used to infer temperature. The concentration measurement of OH-radical as basis for temperature measurement near the exit of a lean-burn combustor was validated through comparison with the LES. The project also delivered the first time demonstration of high-speed particle image velocimetry in a high-pressure combustion environment representative of an aero-engine.
This project has shown that the temperature and flow profiles at combustor exit are highly unsteady and 3D. A novel way of using PODs to describe this unsteadiness was developed and implemented in high-fidelity simulations of the unsteady flow through the turbine. The high-fidelity turbine flow modelling gave physical insight but practical application by the ITD partners requires a quick way of capturing the important effects. Network models are a convenient way of doing this: they are quick and can be applied before the full turbine geometry has been defined. Analysis of the high-fidelity simulations has identified the main cause of redistribution of temperature fluctuations through a turbine. This knowledge has been captured in an advanced validated analytical tool. This tool has been transferred to industry, together with a manual describing its use.
The validated tools produced in this project have improved accuracy through the inclusion of important physics. They can be used during the design of lean-burn aero-engines to give reliable estimates of the noise from a particular lean-burn combustor. Therefore it is expected that better combustors and turbines can be designed and the whole design process will be quicker and cheaper. This will obviously have a direct effect on the profitability of European engine manufacturers. Society will benefit from low noise aircraft, with low emissions.
Parts of the project have potential impact wider than the aeronautical sector. The improvements in the modelling and measurement of high-frequency unsteadiness in turbulent combustion is relevant to a range of combustion systems.
Root-mean-square entropy in the turbine stage (analysis at rotor exit is in the rotating frame)
Large Eddy Simulation of a single sector low-emission combustor
Time-averaged entropy in the turbine (time-average at rotor exit is in the rotating frame)
Photograph of the OCORE high-pressure combustion rig at DLR
High-speed dye laser system of TU-Darmstadt during operation
entropy contours downstream of the NGVs at two time instances predicted by low-order model and CFD
Mean and rms temperature profiles near combustor exit, exp oooo, LES ___