**5. Conclusions**

This paper investigated the impact of NSPD on enhancing the flame propagation of CH4/H2/air mixture under ambient temperature and pressure. A reduced electric field experimentally estimated was used for numerical investigation. An extended version of the plasma and combustion kinetic mechanism was applied and validated using available experimental and numerical data. ZDPlasKin was used to predict the temporal evolution of active particles and the results were integrated into CHEMKIN to enhance the flame speed. The numerical study was carried out with varying H2 contents from 0 to 20% in methane/air with or without plasma actuation. It was noticed that with the enrichment of H2 concentration in the methane/air mixture at fixed plasma, the mole fraction of active species was significantly improved. However, the improvements in the production of active particles were linearly increased with the increase in H2 contents. The highest improvement in flame propagation was observed at 20% H2/Plasma reaching 35%.

Flame speed improvement was significantly higher at lean conditions and low flame temperatures. For instance, at an equivalence ratio of 0.8, 20% H2/Plasma resulted in a flame speed of 37 cm/s at a flame temperature of around 2040 K. This same flame speed was also observed in the case of 0% and 5% H2 with a flame temperature close to 2200 K, meaning that high flame speed can be achieved at lean conditions and low flame temperatures, reducing NOx emissions. Figure 7 shows the comparison of flame speed improvement at lean, stoichiometric, and rich conditions with xH2 = 0.2 with or without NSPD. The improvement in flame speed was higher at lean conditions (equivalence ratio of 0.6) with the addition of xH2 = 0.2, reaching 15%. Combining xH2 = 0.2 with plasma discharge significantly increased flame speed by more than 50%. Furthermore, the combination of H2 blend (xH2 = 0.2) and NSPD improved the flammability limit to equivalence ratio 0.35 at a flame temperature of 1350 K, allowing for reduced fuel consumption.

**Author Contributions:** Conceptualization, M.G.D.G.; Methodology, M.G.D.G. and G.M.; Software, G.M.; Validation, G.M.; Formal analysis, G.M.; Investigation, G.M. and S.B.; Data curation, G.M., G.C., Z.A.S. and S.B.; Writing—original draft preparation, G.M.; Writing—review and editing, M.G.D.G.; Supervision, M.G.D.G. and A.F.; Project administration, M.G.D.G. and A.F.; Funding acquisition, G.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was supported and funded by the PON R&I 2014–2020 Asse I "Investimenti in Capitale Umano" Azione I.1 "Dottorati Innovativi con caratterizzazione industriale"—Corso di Dottorato in "Ingegneria dei Sistemi Complessi" XXXV ciclo—Università degli Studi del Salento"— Borsa Codice: DOT1312193 no. 3. This project is also received funding from the Clean Sky 2 Joint Undertaking (JU) under the grant agreement no. 831881 (CHAiRLIFT). The JU received support from the European Union's Horizon 2020 research and innovation program and the Clean Sky 2 JU members other than the Union.

**Conflicts of Interest:** The authors declare no conflict of interest.
