One way of reducing combustion emissions significantly is a lean injection system in which the fuel is directly mixed with a large amount of the combustor air. The operability of an aero-engine requires a high flame stability, which leads to a staged fuel injection arrangement for a lean burn combustion system. The pilot injector is fuelled at low power (engine idle and approach) and the pilot flame is anchored in an airflow recirculation zone. A save ignition of the fuel-air mixture at low pressure and low temperatures at the engine altitude conditions becomes more difficult, in particular in the case of a burner centred pilot injector, where the mixture formation takes place relatively far away from the ignitor. In this context the ignition characteristics of a lean burn injector system is examined within this project. Most of the available CFD-codes allow for the computation of the combustor air flow, the fuel spray trajectories and the combustion process. One of the unsolved problems of CFD codes is the computation of the ignition process within a two-phase mixture. In the following modelling approach the computed two-phase flow field inside the combustion chamber is the post-processing basis for a prediction of the ignition performance. The present paper describes the validation of this subroutine in a model combustor under atmospheric conditions. Two different spray shapes could be observed with one injector configuration at the same air and fuel mass flow conditions. This effect was observed at different combustor configurations. Because the ignition performance strongly depends on the spray shape, any stochastic spray change during the ignition period will influence the ignition probability. The investigation of the influencing factors, causing the spray forms mentioned is described in this paper including the airflow field, spray mass flow and atomization quality. For the investigation of the spray flip effect different laser measurements technique and simulations methods were used. Tree different parameters influence the spray form: air-, fuel mass flow and atomization quality. The interaction between the airflow and spray has dominated influence on the mixture formation and combustion. Two possible spray and flame shapes determine different stability- and ignition characteristics of the combustor. The combustion performance resulting from a specific spray shape will be changed by a possible spray-flip. Therefore, it is mandatory to design a lean burn combustion system in order to prevent an undesired change of spray and flame shape, while all operability requirements in terms of flame stability and ignitability are fulfilled.
Spray phenomena and their influence on the ignition performance of a modern aeroengine combustor
2008
7 Seiten, 12 Bilder, 1 Tabelle, 8 Quellen
Conference paper
Storage medium
English
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