Lean blowout model for a spray-fired swirl-stabilized combustor

Abstract

In aero-engine applications, the lean blowoff (LBO) limit plays a critical role in the operational envelope of the engine. The geometry of the combustion chamber primary zone plays a critical role in establishing LBO limits. This is especially true for advanced lean burn concepts which introduce the majority of the combustion air in a manner designed to enhance the rate of mixing with the fuel. In the present study, specific mixer hardware has been designed to develop a systematic, statistically sound test matrix to study the effect of mixer components (primary swirl vane, secondary swirl vane, Venturi, and co-and counterswirl) on LBO. A strategy is employed to develop, based on an existing model, a new predictive model for LBO which accounts for a heterogeneous swirl-stabilized reaction and explicitly relates the geometry of the hardware to the LBO limit. The model predicts the LBO fuel/air ratio at three operating temperatures to within 14% of the measured value. The multivariate experiments used to relate LBO to geometry were also further analyzed to establish the main hardware parameters affecting LBO. Specifically, the Venturi and swirl sense (co- versus counter-swirl) were found to impact LBO at lower air inlet temperatures (294 and 366 K). The Venturi and counter-swirl enhance the atomization and mixing processes which are more rate limiting at lower temperatures, and, as a result, improve stability. At a higher inlet air temperature (477 K), the secondary swirl vane angle also plays a role in determining LBO, with larger angles (75°) generating better stability, which is associated with a stronger recirculation zone. The hardware configuration with the best LBO performance over the different conditions was identified (45° primary swirler, 55° secondary swirler, counter-swirl with Venturi).