Thèse Chauffage et Dissociation Induits par des Décharges Nanosecondes dans la Combustion du Nh3 H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Sciences de l'ingénierie et des systèmes École doctorale : Sciences Mécaniques et Energétiques, Matériaux et Géosciences Laboratoire de recherche : EM2C - Energétique Moléculaire et Macroscopique, Combustion Direction de la thèse : Gabi-Daniel STANCU ORCID 000000031743916X Début de la thèse : 2026-10-01 Date limite de candidature : 2026-04-21T23:59:59 This thesis is an experimental study on the production of atomic species in plasma-assisted combustion. The objective of plasma-assisted combustion is to offer engineers greater design freedom by generating a plasma capable of sustaining combustion under unfavorable conditions. Among plasma sources, Nanosecond Repetitively Pulsed (NRP) discharges are one of the most promising tools for the stabilization of flames. The efficiency of NRP discharges has been thoroughly demonstrated in hydrocarbon flames and, more recently, in carbon-free fuels (H2, NH3). However, several fundamental aspects of these discharges remain unknown, hindering the transition from laboratory experiments to industrial applications. Notably, the process responsible for the formation of atomic radicals in these plasmas is not completely known.
In this thesis, the formation of atomic hydrogen (H), atomic oxygen (O), and hydroxyl (OH) using Laser-Induced Fluorescence (LIF) and Two-photon Absorption Laser-Induced Fluorescence (TALIF) will be investigated. In parallel, Optical Emission Spectroscopy (OES) will be performed to quantify the heating induced by the plasma. The measurements will be performed in canonical nanosecond discharges stabilized in a laminar burner to facilitate method development and the interpretation of results. The formation of these radical species will be resolved spatially at a timestep of ~10 ns, providing important validation of reduced models of nanosecond discharges developed by the plasma group and employed by the combustion group of the EM2C laboratory.
Plasmas have been studied for more than a decade to extend the regime of efficient combustion from laboratory to semi-industrial scales. Among the sources employed for plasma-assisted combustion, Nanosecond Repetitively Pulsed (NRP) discharges are one of the most promising tools for the stabilization of flames. These discharges last for approximately 10 ns and are typically applied at frequencies of 10 to 100 kHz. One of their main advantages is their low power consumption (10-100 W) compared to the thermal power of the flames they stabilize. Experimental demonstrations have already been performed using multiple fuels (CH4, C3H8, dodecane, etc.) up to thermal combustion powers of the order of 100 kW [1,2]. In some conditions, NRP discharges can even allow regimes where the NO emissions are reduced compared to a flame without plasma-assistance [1]. Recent work has focused on using NRP discharges to stabilize NH3 flames, improving combustion efficiency, or reducing NO emissions [3,4].
Nanosecond discharges typically generate plasmas that are out of Local Thermodynamic Equilibrium (LTE) [5]. When non-equilibrium occurs, the electron temperature is typically above 10,000 K, while the gas temperature heating induced by the plasma remains below 1000 K. One particular regime of nanosecond discharges, the nanosecond spark, has been well studied in air [6]. Several investigations at the EM2C laboratory indicate that nanosecond spark discharges in air convert approximately 35% of the electrical energy into O2 dissociation and 20% into fast gas heating [7,8]. These experimental data, combined with numerical simulations [9], showed that the heating and the formation of atomic oxygen were due to the dissociative quenching of the electronic excited states of N2, as given by,
N2(X) + e- -->N2(A, B, C, a) + e-
N2(A, B, C, a) + O2 --> N2(X) + O + O + heat (R1)
A few studies were also performed in other mixtures than air. It was shown that for NRP discharges applied in burnt CH4 [10] and across a quasi-1D flame of NH3 [11], the share of electrical energy converted into heating was the same as in air (~20%). However, in these mixtures, less work has been devoted to investigate the formation of atomic species or radicals by NRP discharges. A recent review of the N2* quenching process in combustible mixtures, see reactions (R2), indicates that the quenching of N2(B) or N2(C) by H2O, CO2, CH4, H2, and NH3 could lead to the formation of atomic species [12,13]. In NH3 combustion, the dominant species in fresh or burnt gases are NH3, N2, O2, and H2O. The quenching of N2* by NH3 could produce NH2 + H or NH + H2.
N2(B, C) + NH3 --> N2(X) + NH2 + H + heat
--> N2(X) + NH + H2 + heat (R2)
An indirect study of these reactions at ambient temperature indicates that the reaction rate coefficient for the NH2 + H channel is approximately three times that for the NH + H2 channel [14]. For H2O, the main products are expected to be OH + H, see reaction (R3) [12].
N2(B, C) + H2O -->N2(X) + OH + H + heat (R3)
For plasma-assisted combustion of H2 or NH3, the reaction (R3) is expected to be of main importance in burnt gases, as the fuel will be converted into H2O. In NH3 combustion, (R2) will also play a role in burnt gases, as, depending on the conditions, NH3 can be partially burnt. However, no direct experimental evidence for these rate reactions or the nature of their products was obtained. Therefore, the first objective of this thesis will be to elucidate the heating and formation of OH and H following the quenching of electronically excited states, N2*, by H2O. The second objective will be to determine the products of N2* quenching by NH3, notably whether H atoms are produced. In case of lean flames (i.e. in air excess), (R1) is also expected to have an impact, but the relative importance of (R1-R3) is unknown and (R1) was only studied up to 1000 K [7]. Thus, the third objective is related to the formation of atomic O by O2 dissociation in burnt gases (of lean flames).

This thesis project is intended for engineer/M2 graduates in fields of engineering sciences or physics. Knowledge of plasmas, combustion, lasers, spectroscopy or transport phenomena is appreciated.