TLC - Towards Lean Combustion
Overview
Background & policy context:
The mitigation of aviation emissions, in terms of their environmental impact, is a priority for both air quality (local impact) and the greenhouse effect (global impact). The main margin for progress is in the field of combustor technology. Lean combustion technology is the breakthrough that should enable high-level reductions in NOx emissions, both at airports and in cruise. In addition, lean combustion also assists the reduction of particulates. Injection systems are the most critical issue in achieving a satisfactory level of lean combustion, and are the focus of this project.
Objectives:
The project aimed to achieve sufficient maturity in lean combustion for the single annular combustor application. The objectives were a 80% reduction in NOx emissions in relation to the CAEP2 regulation limit during the LTO (Landing and Take-Off) cycle, and low NOx emission indices at cruise speed (EINOx=5g/kg as a target).
From the technological point of view, two essential features of TLC were that:
- the five manufacturers involved would be all exploring lean combustion applied to single annular combustor architecture;
- an ambitious reduction in NOx levels during LTO cycle emphasising on NOx levels at cruise speeds, particulates reduction during LTO and at cruise speeds.
In relation to the general scope, the detailed objectives were as follows:
- To intensively explore various fuel staged LP or LPP injection systems covering various engine compression rates (OPR ∈ [15, 35]) and all operating regimes. Emission performance characteristics were measured. Risk of auto-ignition, flash-back and stability was also assessed.
- To develop appropriate non-intrusive measurement techniques to enhance knowledge and understanding of all the phenomena occurring in the combustor and the injection system. To acquire high diagnosis capability for:
- NOx measurements: gas analysis;
- Particulates: SMPS on samples and LII;
- Combustion process (OH, kerosene, T, velocities, turbulence): LIF, CARS, PIV, LDA, PDPA. These techniques are based on laser use and are non-intrusive measurements.
- To calibrate CFD tools (RANS or LES codes, combustion and emission formation models) for the purposes of combustion and emission prediction. To acquire or assess prediction capability. The choice was made not to carry out numerical developments. Development activities are largely covered in recently completed or still on-going projects.
- To develop systematic optimisation procedures: definition of a lean injection system concept and associated design parameters, definition of design criteria and constraints, definition of design objectives, application of advanced optimisation algorithm.
- To extrapolate real engine performance for the future. To assess what is technologically feasible in the long term.
Methodology:
The project consisted of four Work Packages:
- WP1 treated the validation and the adaptation of non-intrusive measurement techniques to the conditions foreseen for the tests, in particular for HP tests (up to 30 bar).
- Highly instrumented experimental campaigns were carried out under realistic conditions (pressure up to 22 bar and high temperature) within WP 2. Advanced experimental diagnosis developed in WP1 were applied and complemented by more conventional measurements like gas analysis. The whole set of measurements permit to have a complete evaluation and understanding of the system performances: Spray characterisation, pre-mixing, pre-vaporisation, combustion process and efficiency, NOx emissions, particulates, radiation, lean extinction limit.
- An important part of the tasks within WP 3 was to demonstrate the applicability and effectiveness of optimisation methods applied for lean injection systems. Based on the results of the injector combustor optimisation design criteria was derived for lean injection systems using either combined experimental/ numerical investigation or empirical/ systematic approaches. Five different fuel injector/ combustor applications have been investigated by means of CFD and partially by the use of optimisation techniques.
- The scope of the WP 4 was the calibration of CFD tools (RANS or LES codes, combustion and emission formation models) for the purposes of combustion and emission prediction and to acquire or assess prediction capabilities.
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