Plasma-assisted combustion with nanosecond discharges. I: Discharge effects characterization in the burnt gases of a lean flame

Abstract

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.

Numerical analysis of turbulent flame enhancement by nanosecond repetitively pulsed plasma discharges

“…According to [19], the fraction of energy α responsible of the ultra-fast heating and the ultra-fast dissociation of species when NRP discharges are applied in lean burnt gases is similar to what was obtained for NRP discharges in pure air [12]. The fraction of energy that goes into ultra-fast heating (α -η) is also the same and is equal to 20%.…”

“…by dissociative quenching with O2 molecules leads leads to an ultra-fast increase of gas temperature and O2 dissociation into atomic oxygen [12].The Relaxation of vibrational states of N2 occurs on a much longer times scales, milliseconds scales, and results in slow gas heating. The recent measurement performed by Minesi et al [19] showed that when applied in a mixture of lean flame burnt gases the NRP discharges induce similar effects.…”

Combustion, Chemistry, and Carbon Neutrality

“…572 In addition to the concentrations of the main and intermediate combustion species, including excited and charged particles, the electric field and electron density distribution should be determined as a function of the fuel−oxidizer−bath gas system and of the relevant discharge parameters, on the time scales of the related processes. 570,571,574 Current research in plasma-assisted combustion is devoted to study ignition, 575−580 flame stabilization and combustion enhancement, 578,581−586 kinetics and mechanism development, 575,579−581,587−590 process visualization, 576,577,582,591,592 species diagnostics, 575,582,584−586,588−590,593−597 electric field and/or electron density measurements, 586,596,598 and numerical simulation, 575,[577][578][579][580]584,591,592,597 using different types of plasma generation, e.g., with repetitively pulsed nanosecond, [576][577][578]581,585,586,[591][592][593]595,599 dielectric barrier discharge, 577,579,588,590,594,596,597 gliding arc, …”

“…570,571,574 Current research in plasma-assisted combustion is devoted to study ignition, 575−580 flame stabilization and combustion enhancement, 578,581−586 kinetics and mechanism development, 575,579−581,587−590 process visualization, 576,577,582,591,592 species diagnostics, 575,582,584−586,588−590,593−597 electric field and/or electron density measurements, 586,596,598 and numerical simulation, 575,[577][578][579][580]584,591,592,597 using different types of plasma generation, e.g., with repetitively pulsed nanosecond, [576][577][578]581,585,586,[591][592][593]595,599 dielectric barrier discharge, 577,579,588,590,594,596,597 gliding arc, 582,583 and microwave 584 activation. Fuels include hydrogen, 577,597,…”

“…576,585,592 Further aspects have been studied regarding the ion chemistry, 600 influences of the balance gases (Ar, He, N 2 ) on the chemical kinetics, 601 and spectroscopic signatures of excited states. 586 Some of the contributions have also addressed pyrolysis 589,595,596 and partial oxidation. 587 To improve an in-depth understanding of plasma strategies for combustion processes, experimental investigations can build on numerous diagnostic techniques (Section 2.1).…”