The prediction of a flame response to plasma assistance requires extensive knowledge of discharge-induced plasma kinetics. Detailed studies of nanosecond discharges are common in N2/O2 and fresh combustible mixtures but are still lacking in burnt gases. To fill this gap, we define a combustion reference test case and investigate the effects of Nanosecond Repetitively Pulsed (NRP) discharges placed in the recirculation zone of a lean (Φ = 0.8) CH4-air bluff-body stabilized flame at atmospheric pressure. In this zone, the plasma discharge is created in a mixture of burnt gases. Quantitative Optical Emission Spectroscopy (OES), coupled with measurements of electrical energy deposition, is performed to provide temporally (2 ns) and spatially (0.5 mm) resolved evolutions of the temperatures and concentrations of N2(B), N2(C), N2 +(B), OH(A), NH(A), and CN(B) in the discharge. At steady state, the 10-ns pulses deposit 1.8 mJ at a repetition frequency of 20 kHz. Spatially resolved temperature profiles are measured during the discharge along the interelectrode gap. The temperature variations are more pronounced near the electrodes than in the middle of the gap. On average, the gas temperature increases by approximately 550 K. The heat release corresponds to about 20% of the total deposited electric energy. The electron number density, measured by Stark broadening of Hα, increases up to about 1016 cm-3. These characteristics allow to classify the discharge as a non-equilibrium NRP spark, as opposed to the thermal NRP spark where the temperature can reach 40,000 K and the degree of ionization is close to 100%. These measurements will serve (i) as a reference for future studies in the Mini-PAC burner at the same conditions, (ii) to test discharge kinetic models, and (iii) to derive a simplified model of plasma-assisted combustion, which will be presented in companion paper.

Plasma-Assisted Combustion (PAC) has shown potential in improving the ignition, extinction, and dynamic performance of combustion systems. In this work, Nanosecond Repetitively Pulsed (NRP) spark discharges are applied to extend the lean blow out limit of the SICCA-Spray burner. This laboratory-scale atmospheric test rig is equipped with a swirl spray injector representing in an idealized fashion a single sector of a gas turbine. Three fuels and injection conditions are considered: perfectly premixed methane-air, liquid heptane, and liquid dodecane injected as hollow cone sprays. The optimal electrode position that extends the LBO limit is found to be near the external edge of the outer recirculation zone (ORZ). Spectroscopic measurements show that the NRP sparks produce atomic species and heat the gas above the adiabatic flame temperature. High-speed chemiluminescence images of blow out sequences indicate that the flame evolves similarly for all three fuels from "M" or "V" shapes prevailing at φ = 0.9 to a configuration where chemical conversion also takes place in the ORZ at φ = 0.63. A low frequency combustion oscillation arises near the LBO limit (φ = 0.57). Spray flames blow out at this point, while the plasma-assisted ones continue to burn. It is shown that PAC provides a significant improvement of the extinction performance, in particular when operating with liquid fuel spray injection.

Non-equilibrium plasmas are known to improve ignition and extend flammability limits. In this study, NRP discharges of 2 mJ are applied at 20 kHz in the recirculation zone of a premixed methane-air bluff-body burner with a lean equivalence ratio of 0.8. The operating point is close to the lean blow out limit and NRP discharges efficiently enhance the combustion process. Discharge effects are investigated and the role of N2(C) is quantified with spectroscopic measurements.

This article presents a joint experimental and numerical analysis of a lean turbulent premixed methane-air flame stabilized by nanosecond repetitively pulsed discharges. In the experiments, the transient effects of the discharge on combustion are quantified by optical diagnostics to characterize their impact on flame stabilization. The flame shape is studied with OH* chemiluminescence imaging and the temperature is measured by optical emission spectroscopy. In parallel, Large Eddy Simulation (LES) of the turbulent premixed flame is conducted. Combustion chemistry is modeled by an analytically reduced mechanism whereas the plasma discharge is described by a semi-empirical model. The comparison between experiments and simulations validates the numerical methodology.

This work presents an experimental study of the stabilization of lean methane-air flames by nanosecond repetitively pulsed (NRP) discharges. The experimental facility consists of a gas turbine model combustor with a Lean Premixed Prevaporized multipoint injector. The fuel is injected through two stages, each stabilized by swirl. This facility is representative of a single sector of a gas turbine combustion chamber. The NRP discharges are found to significantly extend the lean blow-off limit for a wide range of operating conditions, with flame thermal powers up to 100 kW, and with an electric power less than 0.2% of the flame thermal power.

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