Numerical analysis of flame shape bifurcation in a two-stage swirled liquid burner using Large Eddy Simulation

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Abstract

A flame shape bifurcation in the liquid-fueled two-stage swirled BIMER combustor is studied using Large Eddy Simulations. This combustor, developed at the EM2C Laboratory to study Lean Premixed Prevaporized (LPP) burners, is composed of a two-stage injection system: a central swirled pilot stage fueled with a pressure-swirl atomizer, to sustain a piloting flame, and an outer swirled stage fed with a multi-point injection, to generate the LPP regime. After ignition in the pilot-only operating condition, a V flame is stabilized near the Inner Shear Layer (ISL). When switching to multipoint-only injection, a flame shape transition is observed and the flame bifurcates into a M-shape. In this work, we identify the mechanisms that lead to this bifurcation, and we show that the transition is driven by a complex coupling between the flame, the chamber acoustics and the ISL vortices. By switching to a multipoint-only injection, the fuel is essentially given to the ISL flame, which is mainly premixed. Because of the increased heat release rate and thanks to positive Rayleigh criterion, the quarter wave mode of the chamber is promoted. The ISL vortices, locked to this mode, increase in size until they are large enough to merge the flame in the CRZ, the radial momentum budget forcing the flow topology to switch to a bubble-like structure. Therefore, these results show that it is the existence of two possible flow topologies that renders this flame shape transition possible, the instability being responsible for transferring sufficient energy to the flow to enable the transitioning and the flame then changing its shape simply to adapt to the new topology.

Introduction

Lean Premixed Prevaporized (LPP) burners are prone to extinction, flashback and thermo-acoustic instabilities [1], [2]. A candidate for the control of LPP burners is staged injection, which splits the fuel supply in several swirler stages. The two-stage BIMER combustor of EM2C laboratory has been developed to expand the knowledge on LPP staged burners. It is composed of an outer stage fed with a multipoint injection system, whose goal is to generate the LPP regime thanks to fast atomization and evaporation; and of a central stage fueled with a pressure-swirl atomizer that sustains a rich diffusion-like pilot flame close to the injection system, aiming to stabilize the burner operation [3], [4].

Thanks to the versatility of this staged injection system, thermo-acoustic instabilities and flame shape bifurcations can be triggered and studied depending on the injection regime and a hysteretic behaviour can lead to multistable operating points. Indeed, experimental and numerical studies have exhibited three flame archetypes and three bifurcations between these flames when varying the fuel distribution between the stages [3], [4], [5], [6], [7], [8]: a V-flame, an M-flame and a Tulip-flame. In [4], [6], the authors studied two of these flame shape bifurcations. Starting from a V-flame with dual injection and ceasing to feed the pilot injector with fuel leads to an M lifted flame. Reinjecting fuel from the pilot burner recovers the V-flame by flashback. In [7], this transition has been numerically evidenced for the operating conditions of [3] and some mechanisms have been suggested to explain it. In the present study, the objective is to investigate further the mechanisms driving this type of transition using an updated LES numerical setup with a finer mesh and a Lagrangian description for the liquid fuel to better reproduce the phenomena observed experimentally in [5]. Compared to [7], we focus on the operating conditions of [4], [5], [6], for which a similar transition is observed and a larger set of validation data is available.

This paper is organized as follows. First, the combustor and the operating conditions are presented. The numerical setup is shown and a brief description of the flame ignition and initialization is given. Finally, the transition results are presented, and the main events and mechanisms controlling the transition are discussed.

Section snippets

Experimental setup and operating conditions

BIMER is a labscale combustor designed to investigate the operation of a swirling two-stage injector. An inlet feeding line leads the air to a cylindrical plenum, which passes through the injection system and then reaches a rectangular combustion chamber (500 × 150 × 150mm3). The chamber contains three water-cooled walls (entrance, top and bottom) and two large lateral silica windows for optical access. The swirling injector is composed of two stages (Fig. 1): the central stage is called the

Analysis of the flame shape transition

From the stable V state, α is reduced from 15% to 0% at a rate of 1% per ms, meaning that the pilot injection is progressively decreased as the multipoint injection is increased. In the following sections, the transition is carefully analyzed and the phenomena at stake are identified.

Conclusion

The LES of a two-stage swirl stabilized liquid-fueled burner have been performed to analyse a singular flame shape transition. This transition is triggered by a change in the injection regime to make the burner operate in LPP conditions. A strong coupling between flame, acoustics and flow dynamics is observed, creating an intricate mechanism. Changing the spatial distribution of fuel triggers a combustion instability, due to the wrinkling of the flame caused by the ISL helical vortices. The

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank CERFACS for sharing the AVBP solver. This work was granted access to the HPC resources of CINES under the allocation A0012B00164 made available by GENCI, and of the mesocentre computing center of CentraleSupélec and Ecole Normale Supérieure Paris-Saclay supported by CNRS and Region Ile-de-France (http://mesocentre.centralesupelec.fr/).

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