Next generation aeroengines for civil aviation are required to produce fewer pollutants. For example, the European Strategic Research Agenda (SRA) calls for a reduction of nitric oxide by up to 80 percent. To reach this ambitious target, jet engine manufacturers are developing a novel combustion concept: Lean combustion. This strategy aims at a reduction of combustor temperatures, which is the basis for a step change towards a reduction of emissions and hence towards enhanced environmental friendliness.
The potential of this technique, however, can be exploited only at the expense of increased burner complexity. Because burners are designed to operate in the fuel-lean regime during takeoff and cruise, the fuel-to-air ratio becomes so small at low power conditions, like at idle, that the amount of injected fuel would become too low to sustain stable combustion: The flame would extinguish. Therefore lean combustors have to be equipped with pilot burners, in order to ensure flame stabilization at any time. In the new designs by Rolls-Royce and General Electric, these pilot burners are integrated into a larger main injector. This design is called Lean Direct Injection (LDI). Efficient piloting, minimum pollutants produced by the pilot burner, efficiency and other factors related to the performance of lean combustors depend crucially on an optimized aerodynamics and the design of the fuel injection system. The new lean combustor concept comprises a large number of design parameters, and therefore still lacks dependable design rules that would allow the transformation of a combustor at a given power specification towards smaller of bigger engines. In support of the development of such design rules, DLR and Rolls-Royce pursue a novel strategy: Lean combustors for future aeroengine generations are investigated at full size in an optically accessible combustion chamber at realistic engine conditions using laser-based measurement techniques.