Large Eddy Simulation of Flame Flashback by Combustion Induced Vortex Breakdown

Abstract

Flame flashback is a major concern during the design of lean premixed gas turbine combustors. One particular mechanism of flame flashback in swirl stabilized combustors is the combustion induced vortex breakdown (CIVB), where the flame can propagate upstream against flow velocities far higher than the local turbulent flame speed. In the present work flame flashback by CIVB has been investigated for two combustors of different scale using large eddy simulation. The LES offers full access to a well resolved flow field and allows a detailed analysis of the fluid mechanical processes happening around the flame tip. The method is shown being able to reproduce the operating points limiting stable operation. The analysis of each flashback process then reveals, that the driving mechanism is different for both combustors depending on their size and the swirl velocity field. Either a baroclinic push or a flame propagation by stretching of the flow can be observed. INTRODUCTION Lean premixed combustion in gas turbine burners is an effective way to reduce NOx emissions. However, a major concern is the stability of the combustion process, since ignitable gas is present upstream of the combustion zone bringing the risk of flame flashback, and since the lean flame needs to be prevented from extinction by sophisticated stabilization mechanisms. Several ways of flame stabilization are common, usually they feature the recirculation of burnt, hot gas to provide activation energy for the ignition of fresh gas. The recirculation can be realized in the wake of a body or by fluid mechanical instabilities like a vortex breakdown, the latter of which has been used in the presently investigated configurations. The premixed flow enters the burner from a plenum through a swirl generator into a mixing tub, where a perfect mixture of the reactants is achieved in the swirling flow. A vortex breakdown is forced at the entry of the combustion chamber caused by the sudden change in diameter of the confining walls. The thus created recirculation zone transports hot burnt gas upstream igniting the arriving fresh gas. The vortex breakdown can also be caused by an obstacle located on the centreline. Inside a recirculation zone the flame front can act as such an obstacle and influence the vortex breakdown. The so called combustion induced vortex breakdown (CIVB) has been described by Fritz et al. (2004). If the flame front is located far enough upstream within the swirl tube it can push the vortex breakdown further upstream, which itself pulls the flame even further upstream. The recirculation zone propagates against the flow at a higher velocity than the local turbulent flame speed. Due to the confinement the experimental access to the propagating flame is very limited. Most of the process takes place inside the mixing tube. By using a silica glass mixing tube optical measurement can be used with some restrictions (Fritz 2004, Konle 2008). The curved surface leads to bending effects of the light and parts of the images are superimposed by reflections and thus become unusable. Numerical simulation offers a far more detailed view to the flow, each component of the flow field can be accessed at every time step regardless of geometrical restrictions. This allows to deeply analyse the fluid mechanical phenomena and to identify the mechanisms which are the reason of the flame flashback by CIVB. Several studies have investigated the phenomenon previously to identify the fluid dynamical mechanism of the CIVB. The dominant production terms of circumferential vorticity, which induce the flow deceleration upstream of the vortex breakdown, have been assessed. From the correlation between flashback and swirl velocities observed in his experiments, Fritz (2004) deduces that the stretching around the flame drives the CIVB. Kiesewetter (2005) has performed RANS simulations of the process and extracted the vorticity production terms from CFD data indicating a baroclinic mechanism. From LES of the CIVB in an unconfined vortex Kröger (2010), in agreement with the model by Fritz, shows the stretching around the flame as the dominant term. Kröger (unlike Kiesewetter) has investigated the flow deceleration induced by the production terms, while Kiesewetter only has regarded the local vorticity production. In the present study two swirl stabilized combustor models without a central body have been investigated using June 30 July 3, 2015 Melbourne, Australia 9 1C-5