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Aircraft Noise. Getting to the core of the issue with CORNET

NOx, CO2 … and noise. Is it possible to tackle all three simultaneously? Aeroengines with lean burn combustors reduce NOx, but create noise concerns, and without optimised design there's the risk that low emission cores will cause aircraft engines to exceed the EU's targeted aviation noise thresholds. Clean Sky's CORNET project aims to improve understanding of the noise from engines with low emission combustors in order to provide industry with new computer-based methods to predict the noise when designing future engines. This, in turn, will provide Europe's aviation industry with the competitive advantage of being able to design quieter aircraft.

It's not every day that all three of the main high level objectives of the Clean Sky 2 programme – mitigating CO2, NOx and also noise – present themselves in a single project. To contextualise the challenge, Europe's Flightpath 2050 targets specify a 75% reduction in CO2; 90% reduction in NOx and a reduction of 65% in perceived noise emissions, relative to the levels of a typical new aircraft in 2000. The convergence of these challenges has prompted considerable anticipation of the results of the CORNET (CORe Noise Engine Technology) project as it approaches its conclusion this April. 

"Lean burn combustor technologies being introduced to reduce NOx emissions are proving to be inherently noisier than conventional combustors, generating broadband noise that can be heard external to the aircraft," says Professor Dame Ann Dowling at the Department of Engineering of the University of Cambridge. "Without careful design and optimisation, there is a danger that the low emission combustors will cause the aircraft engines to exceed the noise requirement". 
The University is the primary coordinator of the CORNET project which aims to improve European aviation's understanding of flow physics associated with generation and propagation of combustion noise. This means analysing ‘direct noise’ of combustion, pressure waves generated directly by unsteadiness in the rate of combustion, and the ‘indirect noise’ generated as unsteady hot spots accelerate out of the combustor and through the turbine blade rows. The research is relevant to Rolls-Royce's ALECSys (Advanced Low Emissions Combustion System) engine, a demonstrator that is testing a lean-burn system that improves pre-mixing of fuel and air prior to ignition, delivering a more complete combustion of the fuel, resulting in lower NOx and particulate emissions.

"This project is a nice illustration of looking at the combined problem of low NOx combustor interaction with a high-pressure turbine, because the temperature profile of a lean burn combustor is completely different from a normal combustor," says Jean-François Brouckaert, Clean Sky Project Officer. "The CORNET consortium is looking at the interaction of noise, particularly noise propagation of these types of low NOx combustors, because going towards the new generation of engines – which are less noisy in terms of fan and bypass jet noise – the level of the core noise becomes relatively more important with respect to what existed before. Therefore there's benefit to Europe’s aviation industry in evaluating this in order to update aerodynamic combustion and acoustic models of the combustor and the turbine, validated by experiments – so, a beautiful academic project!"

In terms of work carried out and results achieved so far, Professor Dowling reports that the CORNET team at the University of Cambridge has predicted the turbulent reacting flow field inside a low-emission combustor operating at representative engine conditions.

"Large Eddy Simulations have been performed for a low-emission combustor operating at realistic engine conditions. These have been validated through comparison with experimental data. The results of these simulations have been used to investigate the validity of using OH-spectroscopy to determine unsteady temperatures at the conditions in a gas turbine combustor exit" she says, adding that all this has been achieved despite the fact that "modelling of turbulent combustion at elevated pressures to a satisfactory level of accuracy, reliability and robustness is challenging". 

"The combustion model developed at the University of Cambridge and being used in this project does not treat the mixing and chemical reactions to be statistically independent, includes the finite-rate chemistry effects, and does not need tunable parameters. Implementation within a Large Eddy Simulation of a single sector of an aero engine combustor within the first 18 months of the CORNET project has enabled the characteristics of the combustion noise sources to be studied in partially premixed combustion. Subsequent work simulated a double sector with two fuel-injectors and this has been used to investigate additional physics such as burner-to-burner interaction that might be important in the annular combustor of an aeroengine gas turbine" she says. 

High-fidelity calculations of the propagation of flow unsteadiness through the turbine have been performed at the University of Cambridge, and the unsteady flow at combustor exit provides the inlet boundary conditions to these calculations. 

"The temperature and flow profiles at combustor exit are highly unsteady and three-dimensional. A novel way of using filtered Proper Orthogonal Decomposition to describe this unsteadiness has been developed and implemented in high-fidelity simulations of the unsteady flow through the turbine" says the Professor, pointing out that "The high-fidelity turbine flow modelling is giving physical insight but practical application by the industrial partners requires a quick way of capturing the important effects. The knowledge from the high-fidelity turbine modelling is being captured in an advanced, validated design tool that can be used by industry".

With regard to the high frequency temperature measurements in a high-pressure combustor, Professor Dowling explains that "the experimental campaign carried out at DLR Cologne as part of the CORNET project, with support from researchers at the Technical University Darmstadt, has demonstrated for the first time the feasibility of using high-speed OH-spectroscopy to obtain unsteady temperatures in the challenging operating environment of a high-pressure combustor facility. Data with sufficient signal-to-noise ratio was obtained at a high sampling rate". 

"Laser combustion diagnostics have evolved into an indispensable tool for understanding and improving combustion technology. Data with sufficient signal-to-noise ratio was obtained from OH-spectroscopy at a sampling rate of 10kHz in the operating environment of a high-pressure combustor. Subsequently, laser absorption spectroscopy has been performed simultaneously with OH-LIF recorded along a one-dimensional probe volume to give absolute OH concentrations, which is being used to infer temperature at sampling rates of up to 1kHz" says Professor Dowling, concluding that all of this effort will directly produce tools that can be used for the design of lean-burn aero-engines: "These tools will be useful in building reliable estimates of the noise from a particular lean-burn combustor. What will make this possible is not only the improved accuracy of the models, but also the inclusion of important physics such as injector-injector interactions. Therefore it is expected that better combustors 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, and society will benefit from low noise aircraft, with low emissions".