Flame Root Dynamics and Their Role in the Stabilisation of Lifted Flames

Abstract

AbstractThis chapter aims to provide a broad overview on the importance of lifted flames in turbulent flows with and without swirl in practical applications for energy production and propulsion. The stabilisation of lifted flames is governed by numerous physical processes that control the behaviour of the flame root or the leading edge. The flame lift-off height, which is the physical distance of the flame root above the burner, is a quantity of practical importance and obtaining accurate predictions using computational fluid dynamics (CFD) is challenging. The large eddy simulation (LES) paradigm has proven to be successful in accurately capturing the flame lift-off height in turbulent reacting flows in simple and complex geometries of practical relevance. The objective of this chapter is to present an overview of the stabilisation mechanisms that have been observed in simulations with relevance to modern applications. An overview of the LES framework for turbulent reacting flows and different sub-grid combustion models are briefly discussed with a focus on the unstrained flamelet combustion model that is used in the case studies presented here. The first part of the simulation results focuses on the canonical jet flame configuration, where it is seen that the lift-off height is sensitive to the jet velocity and the fuel used. The second part of the results focuses on a more complex configuration, which is a gas turbine model combustor with two radial swirlers. The flame root in swirling flows is typically more robust, but failed ignition and local extinction cause the flame root to oscillate and leads to flame lift-off. The amplitude of this oscillation can lead to the occurrence of thermo-acoustic oscillations and flame blow-off under appropriate conditions. The discussion is presented on a physical basis and the observations are compared with measurements. The chapter concludes by summarising the role and importance of modelling flame stabilisation.KeywordsComputational fluid dynamicsGas turbinessLifted flamesPartially premixed flamesSwirling flowsTurbulent reacting flows