The need for reducing pollutant emissions is pushing towards lean combustion in gas turbine combustors. The drawbacks of such regime are reducing the operative ranges of the engines and the possible arise of detrimental phenomena in the combustion chamber. In particular, the ignition transient becomes prone to failures, which can deeply affect the performance and the design process of aero-engine combustors. To assess this complex phenomenon, this work investigates the behaviour of an annular combustor operating in lean-premixed conditions. The main objective is to determine the most favourable conditions for the safe operation of a complex combustor in lean conditions.

The experimental apparatus consists of swirl premixed bluff-body stabilised burners arranged into a cylindrical chamber. The combustor is placed horizontally to resemble realistic gas turbine combustors more closely. Two different configurations, comprising 12 and 18 burners, are employed, allowing the change of inter-burner distance. Ignition is obtained by a fixed number of small sparks (2 mm gap, 10 sparks of 10 ms each). The ignition probability (P~ign~) is computed from 20 tests for each condition, with "ignition" defined as successful ignition of the whole annular combustor, i.e. all individual burners ignited. The effect of spark location, mixture velocity, equivalence ratio and inter-burner distance is then assessed. Two different radial and three to four longitudinal spark locations are tested, with mixture velocity ranging from 10 to 30 m/s and equivalence ratio from 0.58 to 0.68. A successful ignition event is found to be dependent on the ignition of a single burner leading to flame propagation and lightround, while there is no possibility for a single flame to be stabilised without igniting the entire burner (P~ign~ = P~burner~).  The duration of the spark allows for the formation of a flame kernel in all investigated cases (P~kernel~=1), leading to possible failures only in the phases of flame growth, and flame propagation and stabilisation. High-speed imaging (10 kHz) of OH\* chemiluminescence is consequently utilised to shed light on the mechanism of flame propagation for failing and successful ignition events. This is obtained by two cameras simultaneously imaging the apparatus from the top (looking axially downstream towards the burners) and from the side (focused on the spark and the initial phases of the ignition transient). Main trends in the ignition and lightround time are obtained and compared to stable conditions. The stochasticity involved in the propagation process is enhanced by the harsh conditions, i.e. high velocity and low equivalence ratio, in the combustion chamber, leading to strong differences in individual tests for each case. At the spark location leading to the highest ignition probability,  
the stability of the system is assessed, obtaining lean ignition curves separating the different flame regimes obtained following a successful ignition. Finally, these are compared to the Lean Blow-off curves previously determined in the experimental apparatus, resulting in a comprehensive view of the stability limits of the system.

The findings of this investigation will provide useful data for validation of lean ignition models in complex geometries and help the design phase of realistic gas turbine combustors. 


Ignition Probability and Lean Ignition Behaviour of a Swirled Premixed Bluff Body Stabilised Annular Combustor

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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