**4. Results and Discussions**

The present analysis was conducted under fixed plasma conditions: EN = 200 Td, repetition frequency equal to 1000 Hz, and electron number density equal to 107 cm<sup>−</sup>3. The effect of varying H2 contents (xH2 from 0 to 0.2) on active particle production was studied in methane/air. Figure 4 shows the temporal evolution of active particles (H, OH, CH, and CH3) as predicted by ZDPlaskin simulations under fixed plasma actuation conditions, only changing the H2 content in the methane/air mixture. An increase in H2 concentration

led to a significant improvement in the mole fraction of active species. The improvement in active particles production was linearly proportional to the rise in H2 content, with the maximum concentration observed at 20% H2. The rapid decomposition of H2 <sup>+</sup> into H, (E + H2 <sup>+</sup> => H + H) due to its simple molecular structure and high reactivity and the subsequent reactions with other intermediate species, led to increased concentration of active particles. The maximum mole fraction of H was 0.00704 (Figure 4a), which was almost twice the OH species equal to 0.0038 (Figure 4b). The maximum mole fraction of CH3 was 0.006 (Figure 4c), slightly less than H but two orders higher than the molar fraction of CH (0.000041, Figure 4d).

**Figure 4.** Temporal evolution of (**a**) H, (**b**) OH, (**c**) CH, (**d**) CH3 concentrations at different contents of H2 in methane/air mixture using fixed plasma actuation conditions.

The H atoms were produced during the decomposition process when electrons reacted with the ions of CH+, CH2 +, CH3 +, CH4 +, and H3O+. The primary reactions contributing to the production of H, CH, and CH3 were E + CH4 <sup>+</sup> = CH3 + H and E + CH3 <sup>+</sup> = CH + H + H (reaction rates: 1020 and 1021 cm3 s−1, respectively). The OH radicals were produced through the reaction O(1D) + CH4 = CH3 + OH (reaction rate: 1024 cm3 s<sup>−</sup>1).

As shown in Figure 5, the mole fraction of active species was significantly improved with an increase in H2 content in the methane/air mixture. The highest mole fraction of O atoms was observed at a 20% H2 content (xH2 = 0.2) with a value of 0.0158 (Figure 5b). This was due to the decomposition of excited O2 species when reacting with H atoms and H2O molecules, which increased in concentration due to the presence of H2 molecules in the methane/air mixture. The dominant reaction path wasH+O2(V4) => O + OH (reaction rate: 1023 cm3/s). The O atoms produced began to reduce after 10−<sup>4</sup> s, likely due to the short reactive time of O atoms leading to their consumption during recombination and

intermediate reactions. Similarly, ozone concentration improved as shown in Figure 5c, with a mole fraction of 0.00729, close to that of H atoms (0.00704) at a 20% H2 content.

**Figure 5.** Temporal evolution of (**a**) CH2, (**b**) O, (**c**) O3, (**d**) NH3 concentrations at different contents of H2 in methane/air mixture using fixed plasma actuation conditions.

Ozone is primarily generated through the reaction of O atoms with molecular oxygen or with excited species of oxygen. The most significant reaction isO+O2 + N2 = O3 + N2, as it has a reaction rate of about 1023 cm3/s. Nitrogen acts as a third body, removing excess energy. Ozone can also improve combustion analysis as it increases flame speed [25]. The temporal evolution of ammonia was also found to improve when nitrogen seed particle was added to methane-blended hydrogen.

The kinetic and thermal effects predicted by ZDPlasKin were introduced into Chemkin to investigate the flame speed and maximum flame temperature. The reactant mole fraction of active particles and excited species predicted by ZDPlasKin were added to Chemkin to account for kinetic effects. This was executed with a 0.5 ms residence time, which is too short to affect the autoignition chemistry and the reactant composition. The flame speed and peak flame temperature were investigated using a pre-mixed laminar flame speed reactor at different methane-blended hydrogen mixture compositions with or without NSPD.

Figure 6a showed that flame speed improved with increasing hydrogen content and plasma excitation. At stoichiometric mixture, adding hydrogen (xH2 = 0.2) to the methane/air mixture resulted in a 14% increase in flame speed (ΔsL). A further improvement of 35% was achieved with plasma discharge. At leaner condition (φ = 0.6) and same H2 fraction, ΔsL was 16.7% without and 52% with plasma actuation. However, the same flame speed was observed for both cases of xH2 = 0.2 and xH2 = 0.05 with PAC at lean and stoichiometric conditions, similarly in case of xH2 = 0.1 and xH2 = 0 with PAC. It means the same range of flame speed could be reached by varying both hydrogen fraction and plasma discharge.

**Figure 6.** Comparison of (**a**) flame speed and (**b**) flame temperature at various equivalence ratios using different H2 contents with or without NSPD.

Figure 6b shows the predicted peak flame temperature Tf with or without plasma for various H2 fractions. The results showed that increasing H2 did not affect Tf without plasma discharges. However, with plasma, Tf was affected by H2 at fixed operating conditions, especially for the rich mixture (Φ > 1). A slightly increase in Tf was found at lean and stoichiometric conditions.

The study found that lean conditions at low flame temperature resulted in significant improvement in flame speed. With 20% H2 and NSPD, flame speed reached 37 cm/s at flame temperature of 2040 K at φ = 0.8. Similar results were observed with 0% and 5% H2 and a flame temperature of 2200 K at φ = 1. It was observed that the same flame speed can be achieved at lean conditions by reducing Tf, leading to reduced NOx emissions.

Figure 7 compares the improvement in flame speed (%) at lean, stochiometric, and rich conditions with xH2 = 0.2 with or without NSPD. It was observed that the improvement trend was φ = 1.4 > φ = 0.6 > φ = 1. At lean conditions (φ = 0.6), adding xH2 = 0.2 improved flame speed by 15%, however, using both xH2 = 0.2 and plasma resulted in a more than 50% improvement. At rich conditions, the largest improvement was seen with plasma due to the increased fuel causing more active particles to be produced during NSPD".

**Figure 7.** Comparative behavior of flame speed improvements (%) at lean, stoichiometric, and rich conditions.

Literature [26] showed that the molecular excited species oxygen O2(1Δ) increased burning velocity by 1% without plasma. The reaction path H2 + O2(1Δ) = H + HO2 was found to play a significant role. More than 5% of O2 was converted to O2(1Δ) in the presence of electric discharge at ambient pressure [34]. Thus, plasma discharge could produce significant amounts of O2(1Δ). Figure 8 analyzed the role of O2(1Δ) using hydrogen blends with or without plasma discharge. The results showed no change in O2(1Δ) production with hydrogen blends alone at various equivalence ratios. However, with the use of plasma discharge, there was a significant rise in excited species production, especially at higher hydrogen content.

**Figure 8.** Comparison of molecular excited species O2(1Δ) at various equivalence ratios using different H2 contents with or without NSPD.

Impact of atomic excited species O(1D) on flame speed was studied using plasma and hydrogen blends (Figure 9). Results showed low O(1D) concentration increased with hydrogen and plasma, but still had minimal effect on combustion. However, it could be increased with the increase in plasma amplitude.

**Figure 9.** Comparison of atomic excited species O(1D) at various equivalence ratios using different H2 contents with NSPD.

Free radicals such as O, H, and OH are active due to unpaired electrons and short lived in combustion [35]. They initiate chain reactions and branching. Figure 10a–d show mole fraction profiles of O, H, OH, and CH3 using hydrogen blends with/without plasma discharge. Adding hydrogen increased O, H, and OH mole fractions, but decreased CH3

slightly (Figure 10d). Using NSPD in hydrogen blends raised O, H, and OH concentrations and moved the reaction region upstream. CH3 mole fraction was also slightly increased with plasma discharge. OH particles had the highest concentration at 0.009 mole fraction. Main reactions producing O, H, and OH particles are described as follows in Equations (6) and (7).

$$\rm OH + H\_2 = H + H\_2O \tag{6}$$

$$\text{OH} + \text{O}\_2 = \text{O} + \text{OH} \tag{7}$$

**Figure 10.** Mole fraction profiles of (**a**) H, (**b**) O, (**c**) OH, and (**d**) CH3 with different blends of hydrogen without or with NSPD.

Figure 11 shows the production rate of Equations (6) and (7) with xH2 = 0 and xH2 = 0.2 with/without NSPD. The rate increased and the peak shifted upstream with hydrogen addition, but with xH2 = 0.2 and plasma, a significant impact was seen.

H2 and O2 mole fractions change with hydrogen blends and NSPD, shown in Figure 12. H2 transforms from intermediate species to initial reactant in methane flames with xH2 ≥ 0.2 and NSPD. H2 starts reacting upstream in xH2 = 0.2, confirmed by [36]. H2 promotes combustion and moves the reaction region towards upstream due to its higher reactivity than CH4.

**Figure 11.** Rate of production of O, H, and OH with different blends of hydrogen without or with NSPD.

**Figure 12.** Mole fraction profiles of H2 and O2 with different blends of hydrogen without or with NSPD.

Figure 13a illustrates the mole fractions of CH4 in different H2 blends with or without the NSPD. The addition of H2 and NSPD leads to a decrease in CH4 mole fraction, possibly due to the high reactivity of H2 and lower CH4 concentration. The oxidation of CH4 greatly increased and its profiles were shifted towards the upstream sides. CH4 was mainly consumed by reactions with active particles O, H, and OH. The dominant CH4 consumption reactions are listed below.

$$\text{CHI} + \text{CHI}\_4 = \text{CHI}\_3 + \text{H}\_2\text{O} \tag{8}$$

$$\text{HF} + \text{CH}\_4 = \text{CH}\_3 + \text{H}\_2 \tag{9}$$

$$\text{CO} + \text{CH}\_4 = \text{OH} + \text{CH}\_3 \tag{10}$$

**Figure 13.** (**a**) Mole fraction profile of CH4 (**b**) Rate of production of CH4 with different blends of hydrogen without or with NSPD.

The rate of production of Equations (8)–(10) using hydrogen contents xH2 = 0 and xH2 = 0.2 with or without NSPD is shown in Figure 13b. CH4 consumption was increased for reaction Equations (8)–(10) and the peak of the reaction region was shifted towards the upstream with the addition of hydrogen contents with or without plasma. However, when combining the H2 blends of xH2 = 0.2 with NSPD, a noticeable impact was observed. It was because hydrogen is more reactive, which promoted methane combustion. The concentration of active particles O, H, and OH were increased when methane was blended with hydrogen, mainly due to the chemical effects. Moreover, the NSPD further improved the combustion process due to the thermal (moderate gas heating) and kinetic effects (excitation, ionization and decomposition of fuel and air molecules occurred, which resulted in the production of intermediate fuel fragments and active particles).

Finally, the lean flammability limit is discussed as the minimum equivalence ratio for flame propagation. Figure 14 shows the lean flammability limit using hydrogen contents XH2 = 0 and XH2 = 0.2 with or without NSPD. The flammability limit remained at φ = 0.6 without H2 and plasma but improved to φ = 0.5 with the addition of XH2 = 0.2. Plasma discharge had a significant impact on the flammability limits, with φ = 0.45 at flame temperature about 1500 K.

**Figure 14.** Flammability limits with different blends of hydrogen without or with NSPD.

Combining XH2 = 0.2 and NSPD increased the flammability limit to φ = 0.35 at 1350 K, allowing self-sustained combustion at lower flame temperatures and reduced NOx emissions. The improved flammability limits reduce fuel consumption due to the enhanced reactivity and chemical effects of H2 and thermal and kinetic effects of NSPD.
