Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane–air flame

The experiments were conducted after the PAC system had reached the steady-state regime, during which the thermal and chemical properties of the system remain stable with continuous energy deposition by NRP discharges. The minimum time from initiating the discharge to reach the steady-state regime was determined to be 50 ms in our experiments.

The temporal evolution of H and O-atom fluorescence in the temporal vicinity of the discharge occurrence was studied using the 1D spectral imaging system described in the previous section. Figure 2 shows the spatially and spectrally resolved fs-TALIF signal of H atoms, recorded at the time of maximum plasma luminescence intensity for two different cases, namely off-resonant laser excitation with a wavelength of 207 nm (a), and resonant laser excitation at 205 nm (b). Both panels exhibit a broadband spectral continuum covering essentially the entire spectral range from 646 to 665 nm. This broadband emission is the plasma luminescence instead of laser-induced fluorescence. From a comparison with a simulation in PGOPHER [15] it can be concluded that the observed plasma luminescence is mainly N₂ fluorescence in the B³Π_g–A³Σ_u ⁺ band. The signal ranging from 650 to 655 nm is the v' = 7 → v'' = 4 band and the signal in the range 657–663 nm is the v' = 6 → v'' = 3 band [7, 8]. In addition to the broadband signal, figure 2(a) also exhibit a signal around 656 nm present at a vertical position of ∼3 mm, corresponding to the location of the flame tip. This signal corresponds to the Balmer-α line (3D → 2P) of H atoms [16, 17]. This signal is not observable from the flame without discharge, suggesting that the concentration of H atoms in the 3D state is negligible in the base flame and the discharge generates excited H atoms (H*) in the flame tip. Rusterholtz et al [8] observed H-atom Balmer-α fluorescence in an NRP pin-to-pin discharge in atmospheric air. In their experiment the voltage pulses had a duration of 10 ns, an amplitude of 5.7 kV, a PRF of 10 kHz, and the separation between the electrodes was 4 mm. The origin of the H atoms was attributed to electron-impact dissociation of water vapour present in the laboratory air. In the methane-air flame, the main precursors of H atoms are H₂O, CH* and OH* molecules [18], which is also confirmed in our experiments. Similarly, in our case we assume that the generation of H* atoms is from electron-impact dissociation of H₂O, CH* and OH* molecules via processes [19]

$$\begin{align*}&{\text{e}} + {{\text{H}}_{\text{2}}}{\text{O}} \to {\text{e }} + {{\text{H}}^{\text{*}}} + {\text{ OH}},\\[6pt] &{\text{e} + \text{C}}{{\text{H}}^*} \to {\text{C}} + {{\text{H}}^{\text{*}}}{ + \text{ e}},\\[6pt] &{\text{e}+ \text{O}}{{\text{H}}^*} \to {\text{e}+ \text{O}} + {{\text{H}}^*}.\end{align*}$$

Zoom In Zoom Out Reset image size

In the electron-impact dissociation of H₂O molecule, the contribution of OH* is neglected because the cross section of OH* production is about ten times less than OH in the ground state [20]. A comparison between panels (a), which corresponds to the laser wavelength tuned off-resonance, and (b), corresponding to the laser wavelength tuned on resonance, shows that the most obvious difference is that a higher signal intensity appears in the flame tip when the laser is tuned on resonance (b), while the intensity and shape of the plasma luminescence remain the same. The higher signal intensity at 656 nm in the flame tip with the laser wavelength tuned on resonance is due to the laser-induced fluorescence from the ground state H atoms present in the flame tip.

To study the temporal dynamics of the H-atom concentration, we recorded emission spectra of the flame while varying the time delay between the discharge onset and the triggering signal of the spectrometer detection. Particular focus was put on the flame tip region through which the discharge channel penetrates, and therefore the signal present from 2 to 5 mm along the vertical axis was integrated. This area is displayed in the black box in figure 2(b). Figures 3(a) and (b) show spectrally resolved signals versus the delay time. Panel (a), in figure 2, corresponds to the laser wavelength off resonance while panel (b) corresponds to the laser wavelength on the H-atom resonance. These two images are acquired at time zero which is defined as the time at which the maximum emission from the discharge is observed. With the laser tuned off resonance, the broadband luminescence signal purely originates from the discharge-induced plasma and thus it can be treated as an indicator of the evolution of the plasma. In figure 3(a) it can be seen that the plasma luminescence lasts for about 15 ns. The decay time of the plasma emission is close to that in air reported in [8]. In the present case the plasma emission decays mainly due to fast ns-timescale quenching through collisions with naturally present species, primarily the major species, i.e. N₂, O₂, CO₂, H₂O and CH₄, but also with trace atoms like N and O. In addition, species created by the discharge, such as CH₃, CH₂, and CH also affect the quenching rate of the plasma emission, but the overall contribution of the collisional quenching is marginal. With the laser tuned on resonance, i.e. figure 3(b), it is evident that there is a distinct signal due to H-atom fluorescence during the entire investigated temporal range (about 100 ns). The H-atom signal is fairly constant throughout the whole temporal window except during the discharge period at around the time zero, where it overlaps with the plasma luminescence.

Zoom In Zoom Out Reset image size

Instead of studying the fluorescence from the flame tip region spectrally resolved, the spectral resolution was sacrificed in exchange for spatial resolution. The signal in a 2 nm wide spectral region around the 656 nm wavelength, displayed in the black box in figure 2(b), was integrated while varying the delay time. The result of this study is shown in figures 3(c) and (d), where panels (c) and (d) correspond to the laser tuned off and on resonance, respectively. From this result, it is clear that the spatial range of the flame is from 2 to 6 mm. Over the temporal range investigated, the position of the flame tip keeps virtually unaltered and stable.

In order to extract the enhancement of H-atoms due to NRP discharge the signal recorded with the laser tuned off resonance was subtracted from the signal recorded with the laser tuned on resonance. The data in figure 4 shows such a subtracted signal versus delay during a discharge cycle of 100 µs. The signal remains fairly constant across the entire discharge cycle, which shows that the H concentration level remains rather constant after each individual discharge. The red dashed line corresponds to the H atom fluorescence signal recorded without discharge, i.e. it reflects the natural H atom concentration in the base flame. Comparing this fluorescence level with the black data points suggests that the H atom concentration roughly doubles when the discharge is applied. The strong signal variation around the discharge peak was probably the result of subtracting two large variables.

Zoom In Zoom Out Reset image size

Our previous reports on temporal dynamics of H-atom production due to plasma forcing in a flame, could not resolve the first 50 ns directly after plasma discharge [14]. The spectroscopic method presented in this work, however, grants access to this temporal domain. As mentioned before, H* atoms are formed during this short period in the flame tip via electron-impact dissociation processes. The natural question is that whether H atoms in ground state are formed or not. The result in figure 4 answers the question, i.e. the amount of ground-state H atoms, produced during the 15 ns period by electron-impact dissociation, are below the detection limit of these measurements. Based on measurements in the flame without plasma forcing and chemical kinetics simulations, the detection limit could be estimated to 300 ppm of the natural occurrence of H-atoms. The increase of H atoms concentration by discharge compare to the base flame is solely a result of averaged electric energy deposited.

In this section, we present and discuss the results obtained for atomic oxygen. Atomic oxygen plays an important role in the kinetics of hydrocarbon oxidation. As it can be produced in large quantities by NRP discharges, it is a key species to study to fully understand PAC. Oxygen atoms were excited from the ground state (2p³P) to the excited (3p³P) state through two-photon absorption using 226 nm fs laser pulses. The 845 nm fluorescence emitted upon relaxation to the intermediate 3s³S state was recorded [21]. Preliminary 2D fs-TALIF imaging of O-atoms recorded under same experimental conditions (10 kHz, 8.5 kV) have been reported in our previous work [22], where significant enhancement of O-atom fluorescence and thus concentration due to NRP discharge was observed. To study the temporal dynamics of this enhancement, 2D fs-TALIF images at different times within the discharge cycle of 100 μs were recorded using the same method as in [14]. A few examples are shown in figure 5. Two features can be noticed: (a) the flame shape, flame tip position, strength of O-atom fluorescence and its distribution remain basically unchanged during the cycle; (b) O-atom fluorescence appears in the discharge channel that connects the flame tip and the bottom electrode, and its lifetime lasts for approximately 4 μs.

Zoom In Zoom Out Reset image size

The temporal dynamics of O-atom fluorescence in a discharge cycle was studied using the spectroscopic method previously discussed. Figure 6(a) shows the spatially- and spectrally-resolved results of fs-TALIF of O-atoms recorded at the time when the plasma luminescence is the strongest, for two different cases, i.e. off- and on-resonant laser excitation. With laser tuned to off-resonance, a broadband plasma luminescence covering the entire 17 nm wide window is observed. Interestingly, there is no observation of 845 nm O-atom fluorescence with laser off-resonance, which is different from the case of atomic hydrogen in figure 2. These results suggest that the discharge-induced plasma significantly enhances the ground-state O-atom concentration, but the kinetic energies of the electrons are not high enough to excite the O-atoms from the ground-state to the 3p³P state. With laser on-resonance, strong 845 nm fluorescence from the O-atom group is detected.

Zoom In Zoom Out Reset image size

Without the interference of plasma-induced O-atom 845 nm signal, the subtraction of net fluorescence enhancement becomes simpler. The subtracted signal, together with the integrated fluorescence over the full image using fs-TALIF technique, are plotted as a function of time in figure 6(b). The red dashed line represents the O-atom fluorescence signal recorded using spectroscopic technique without discharge. Comparing it with the red square data points indicates that the average O-atom concentration is quadrupled under plasma forcing. As can be seen, the results obtained by both techniques show a fairly constant signal level over the whole cycle, i.e. similar behavior as for the H-atom signal shown in figure 4. The stable behavior of the H and O-atom concentrations during a discharge cycle shows that the plasma-assisted-flame system is insensitive to individual discharge pulse forcing once a steady state has been established. Our results further validates the assumption that the steady-state plasma-assisted-flame system does not respond to each discharge pulse forcing but is only affected by the average electrical power deposited by the plasma [23, 24]. In terms of practical applications, the stability of NRP discharges is beneficial in, for example, combustion instability control [25].

Interestingly, O-atom fluorescence can be detected in the narrow discharge channel that connects the flame tip and the bottom electrode in the fresh unburned gases. Figure 7(a) shows the detailed image of the channel recorded at 150 ns after the plasma emission peak. The channel is approximately 1.6 mm long and 0.2 mm wide. Using 2D imaging method, we studied the decay of O-atom fluorescence signal in the channel. Compared to 2D imaging method, the spectroscopic method shows a lower sensitivity and it is not capable to detect the presence of O-atom in the discharge channel. The decay of plasma emission around 845 nm wavelength in the channel was measured using the spectroscopic method, and we compare the decays of O-atom fluorescence and plasma emission in figure 7(b). One can see that O-atom is detectable within 10 μs and its concentration shows a much slower decay compared to the fast decay of plasma emission. Exponential fitting to the data suggests a decay constant of about 2.0 ± 0.3 μs whereas plasma emission shows a decay constant of only approximately 5 ns.

Zoom In Zoom Out Reset image size

Note that Stancu et al reported that the O-atom density produced by NRP discharges in atmospheric air quickly builds up in a few tens of ns and then decays in about 25 μs [6, 18]. These results were obtained for 10 ns high-voltage pulses applied at 10 kHz, but the discharges were generated in pure air, preheated at 1000 K. In the present study, the conditions in the inter-electrode area are somewhat complex, with layers of cold methane-air mixture, flame, and burned gases at about 1900 K. It is expected that these very different initial conditions will induce different discharge characteristics. However, the difference in O-atom lifetime in the post discharge chemistry, for NRP discharges in air and in methane–air mixture, suggests that O-atoms reacts in a very fast manner with methane. This O-atom lifetime measured for NRP discharges in a methane–air mixture could be valuable for the validation of chemical models in PAC.