Ignition dynamics in an annular combustor for liquid spray and premixed gaseous injection

Ignition is of importance in many combustion applications and raises fundamental and practical issues. The light-round process corresponding to the flame spreading phase in the ignition of annular combustors is examined in this article by performing experiments in a model scale configuration “MICCA-Spray”. This system features 16 swirling injectors each comprising a hollow cone pressurized injector. Experiments are carried out with premixed gases as well as n-heptane and dodecane sprays. The flow, spray and flame are first characterized in a single injector configuration. Propagation from the initial kernel created by a spark plug is then observed using high speed light emission imaging. This provides flame structures at various times during the process and gives access to the time delays for flame merging. With n-heptane and dodecane fuel injection, it is found that the light-round process is similar to the one observed under fully premixed propane/air experiments but the duration of the process is augmented especially for the less volatile fuel. It is also confirmed that the delay is notably influenced by thermal conditions prevailing in the chamber at the moment of ignition, injection process and fuel composition. Making use of a flamelet like model of the combustion process, the relative changes in light-round time delay are found to be, to the first order, proportional to the relative changes in laminar burning velocity induced by the fuel spray in the air flow.