Numerical study on the impact of Nanosecond Repetitively Pulsed discharges on the lean blowout limit for a hydrogen/air swirled flame

Plasma-assisted combustion (PAC) using Nanosecond Repetitively Pulsed (NRP) discharges is an efficient method to extend the lean blowout limit of flames, as shown in numerous experiments. In this study, the underlying mechanisms by which NRP discharges extend the lean blowout limit in hydrogen-air flames are analyzed numerically for the first time. The computations are performed using Large Eddy Simulations (LES) coupled with a semi-empirical model for the NRP discharges. The blowout is triggered at constant hydrogen mass flow rate (i.e., constant flame thermal power) through a very slow increase of the injected air mass flow rate, resulting in a decrease of the global equivalence ratio. A PAC configuration featuring NRP discharges at a frequency of 15 kHz and a 1.5 mJ deposited energy per pulse is computed, for which the experiments have shown a 20% reduction of the lean blowout equivalence ratio, by using NRP discharges. The case without plasma is also computed. These two configurations are first compared in terms of combustion efficiency . Next, results of blowout simulations are presented. The LES is able to predict the blowout equivalence ratios accurately for both the case without plasma (0.9% error) and with the NRP discharges (2.8% error). In the case without plasma, blowout is triggered through the dilution of burnt gases by fresh gases, which penetrated the inner recirculation zone at sufficiently low global equivalence ratios. Plasma triggers the oxidation of these pockets of fresh gases, resulting in the production of radicals and heat, which stabilizes the flame. Novelty and Significance Statement This is the first time that simulations are performed of a plasma-assisted combustion (PAC) experiment in pure hydrogen flames. Also, for the first time, a PAC phenomenological model (Castela et al., 2016) is used to predict the lean blowout (LBO) limit of a flame assisted by plasma and the results are evaluated through comparisons with experimental data. The interactions between the flame and the Nanosecond Repetitively Pulsed (NRP) discharges at the LBO limit are studied numerically. The NRP discharges promote the oxidation of pockets of fresh gases, resulting in the production of radicals and heat, which stabilizes the flame and extends the LBO limit. • The impact of plasma on the Lean Blowout (LBO) limit is analyzed numerically. • The LBO limit of a plasma-assisted flame is correctly predicted by the Castela model. • Plasma extends the LBO limit by producing heat and radicals in the recirculation zone.