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Dynamics and combustion instabilities of Hydrogen/Air flames

MEGEP (Mécanique, Energétique, Génie civil & Procédés) – [Subject to defense authorization]

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Combustion instabilities are encountered in many practical burners, where they can damage systems and compromise operational safety. Understanding and controlling instabilities in hydrogen-air flames is therefore essential to support the transition of the energy sector toward zero-carbon fuels. The objective of this manuscript is to investigate combustion instabilities in hydrogen-air flames and develop a robust numerical framework for their analysis, with a particular attention to longitudinal thermoacoustic instabilities. 

The first part of this work addresses the treatment of acoustics at boundary conditions, which often leads to long convergence or large acoustic reflections that destabilize simulations. To overcome these limitations, a new Non-Drifting Non-Reflecting (NDNR) boundary condition is proposed, allowing both a rapid convergence towards the target state and an improved acoustic treatment. 

The numerical method is then applied to the study of a pratical dual-swirled hydrogen-air burner (HYLON). Characterizing the acoustic response of this burner is of practical relevance but requires an analysis of the flame stabilization before-hand. This study reveals the presence of a hydrodynamic instability downstream of the hydrogen swirler, identified as a Precessing Vortex Core (PVC). Simultaneously, a high-frequency intermittent flame anchoring is observed, and the role of the PVC in this destabilization mechanism is investigated. This analysis highlights the importance of considering for flame dynamics in the stabilization regime, often simplified as a lifted or anchored regime.

Following the investigation of flame stabilization, the experimental framework and hypothesis used to characterize the flame response to acoustic perturbations require dedicated validation through numerical simulations. To this end, the LES framework is first validated under forced-flow (hydrogen line), and is subsequently used to assess the two experimental assumptions: the acoustic model employed to estimate the reference velocity for the Flame Transfer Function (FTF), and the use of OH* as a marker for the heat release rate in thermoacoustic studies. The numerical results support these experimental assumptions, providing confidence in the experimentally determined FTF of the HYLON burner.

Finally, a self-sustained longitudinal instability is investigated in a laminar burner. Intrinsic ThermoAcoustic (ITA) modes are identified in methane-air and hydrogen-air flames, operating at similar laminar flame speed. Their comparison reveals a significantly higher ITA frequency for hydrogen, attributed to preferential diffusion effects that enhance local fuel consumption. An analytical model is developed to describe these ITA modes and is subsequently used to propose two novel mitigation strategies. While previously reported strategy is shown ineffective for pure hydrogen flames, combining preheating and equivalence-ratio reduction, as well as increasing the combustion chamber width, are demonstrated to be effective in mitigating hydrogen-air ITA modes and appear promising for methane-air flames.

Jury

M. Wolfgang Polifke	TU Munich	Reviewer
M. Sébastien Ducruix	EM2C	Reviewer
Mme. Annafederica Urbano	ISAE-SUPAERO	Examiner
M. Guillaume Legros	Sorbonne Université / CNES	Examiner
M. Thierry Schuller	IMFT	Examiner
M. Thierry Poinsot	IMFT	Thesis supervisor
M. Quentin Douasbin	Cerfacs	Invited / Thesis co-supervisor