Abstract

Modern gas turbines usually adopt very lean premixed flames to meet the current strict law restrictions on nitric oxides emissions. In such devices, strong combustion instabilities and blow-off susceptibility often prevent from achieving a stable flame in leaner conditions. Numerical models to predict the lean blow-off in turbulent flames are essential to prevent such instabilities, but the simulation of blow-off still represents a challenge, requiring the appropriate modelling for the turbulence-chemistry interactions and the highly transient behaviour of the flame near the extinction limit.

The present work explores the capabilities of the widely-used Flamelet Generated Manifold model in predicting the lean blow-off of a turbulent swirl-stabilized premixed flame within LES framework. An atmospheric premixed methane-air flame, experimentally studied at the University of Cambridge, is firstly analyzed in three operating conditions approaching blow-off to validate the numerical setup. An extended Turbulent Flame Closure (TFC) model, implemented within the FGM framework in Fluent to introduce the effect of stretch and heat loss on the flame, reproduces the evolution of the key flame characteristics. Then, the chosen setup is used to study the blow-off inception and the dynamics in two conditions with different flow rate. An accelerated numerical procedure with progressive step reductions of equivalence ratio is used to trigger the blow-off. The extinction equivalence ratio is predicted quite accurately, showing that the Extended TFC is suitable for the study of the blow-off, without an increase in computational cost. The validity of the model could be extended, allowing the study of lean blow-off in realistic conditions and complex flames of gas turbine combustors.

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