ARTICLE IN PRESS JID: PROCI [m;June 27, 2018;14:14] Available online at www.sciencedirect.com Proceedings of the Combustion Institute 000 (2018) 1–8 www.elsevier.com/locate/proci Cumulative effect of successive nanosecond repetitively pulsed discharges on the ignition of lean mixtures Sara Lovascio a,c , Jun Hayashi b,∗ , Sergey Stepanyan a , Gabi D. Stancu a , Christophe O. Laux a a Laboratoire EM2C, CNRS, CentraleSupélec, Université Paris-Saclay, 3 rue Joliot Curie, 91190 Gif-sur-Yvette, France b Department of Energy Conversion Science, Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyoku, Kyoto 606-8501, Japan c CNR NANOTEC – PlasMI Lab., Via Amendola 122/D, 70126 Bari, Italy Received 30 November 2017; accepted 4 June 2018 Available online xxx Abstract This work aims to provide a better understanding of the cumulative effect of successive nanosecond repet- itively pulsed (NRP) discharges on the ignition process. Fast chemiluminescence imaging of both the post- discharge and the following fame was used to analyze the ignition of a lean propane/air mixture (φ = 0.7) by a train of 82 NRP discharges at a pulse repetition frequency (PRF) of 30 kHz, in comparison with the traditional spark ignition. The transition from the non-equilibrium plasma to the ignition kernel has been imaged. It has been found that, differently from traditional spark ignition and despite similar experimen- tal conditions, each NRP ignition develops in a unique way. NRP discharges generate both highly reactive species and thermal instabilities in the gap: the multi-pulse strategy produce a gas motion resulting in a jetting phenomenon, reported here for the frst time. An algorithm within ImageJ software was used to quantita- tively describe the observed jetting phenomenon. The results presented in this work suggest that jetting is the driver of the ignition initiated by NRP discharges, at least in the experimental conditions investigated here. © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Nanosecond repetitively pulsed discharge; Lean premixed mixture; Ignition; High-speed imaging; Jet formation 1. Introduction Even today, combustion of fossil fuels covers more than 80% of the primary energy production. ∗ Corresponding author. E-mail address: hayashi@energy.kyoto-u.ac.jp (J. Hayashi). Environmental concerns about sustainability, global warming and air pollution impose stringent worldwide emission regulation. To satisfy these de- mands, new ignition and combustion concepts are under investigation. In the automotive feld, lean combustion and exhaust gas recirculation (EGR) are regarded as possible strategies for increasing the thermal effciency of internal combustion (IC) engines. Nevertheless, these approaches lead to https://doi.org/10.1016/j.proci.2018.06.029 1540-7489 © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Please cite this article as: S. Lovascio et al., Cumulative effect of successive nanosecond repetitively pulsed discharges on the ignition of lean mixtures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.06.029