Actuation of a lean-premixed flame by diffuse non-equilibrium nanosecond-pulsed plasma at atmospheric pressure

Abstract

This study investigates the effect of diffuse non-equilibrium nanosecond-pulsed plasma at atmospheric pressure on a lean-premixed CH4-air flame (ϕ = 0.65, P ∼ 0.3 kW). The domain of diffuse plasma existence is explored for both the case of the cold flow (no flame) and the case where a flame is stabilized downstream. The dynamics of plasma propagation and the flame displacement, following a high-voltage pulse, were measured using intensified charge-coupled device imaging. The energy of the plasma was measured using electrical probes and measurements of the second positive system of nitrogen were used to determine the rotational temperature and vibrational populations in the plasma. The effect of plasma on a flame was investigated by varying the pulse repetition frequency gradually from 1 to 7 kHz. Time-resolved imaging of the plasma emission shows that the primary streamer travels at higher velocities with increased pulsing frequency and with the presence of a flame ignited downstream of the discharge. Time-resolved imaging of the flame, following a high-voltage pulse, shows that the flame moves upstream into the unburned methane-air mixture with increased pulsing frequency. As the flame is displaced upstream, the nature of the discharge also changes, whereby less energy is coupled to the gas volume. Spectroscopic results reveal that the region in which the flame stabilizes is that of highest vibrational excitation and lowest rotational temperature. This actuation method is evidence of low-temperature chemical flame enhancement and potential control of a lean-premixed laminar flame at atmospheric pressure.

Energy efficiency of a nanosecond repetitively pulsed discharge for methane reforming

“…The plasma source is capable of operating in the diffuse and filamentary regime as reported in previous studies with similar conditions [26,[32][33][34]. Qualitatively, for high values of T i , the discharge operates in a hybrid regime with a well defined filament in the center of the plasma and a volume of diffuse glow.…”

“…On the contrary, higher ν causes a less-stable diffuse regime and the discharge typically turns into a bright spark. Evans et al [34] reported that there is a limit in the amount of discharge events per volume of gas treated before the discharge transits to a spark. As ν increases, more discharge events happen for the same gas volume when the flow rate is kept constant, thus explaining this behavior.…”

“…For this reason, N 2 is injected as a tracer gas in both the CH 4 and CH 4 -CO 2 experiments, and the emission from the second positive system of N 2 (C 3 Π u -B 3 Π g ) is measured. Experimental spectra are compared to modeled N 2 (C 3 Π u -B 3 Π g ) emission spectra using an in-house code and benchmarked with the commercial software SPECAIR [34]. The modeled spectra are dependent on the vibrational temperature (T v ) and T r of N 2 .…”

Non-equilibrium plasma for ignition and combustion enhancement

“…The thermal effect is restricted by limiting the current in the discharge channel, either by a dielectric barrier or by ending the pulse. However, due to the challenges of temperature measurements in plasma-assisted combustion environment, a small thermal effect cannot be completely ruled out [154,192].…”

Nanosecond Pulsed Discharge Dynamics During Passage of a Transient Laminar Flame

“…Stange et al [19] observed that an AC dielectric barrier discharge (DBD) in the microdischarge mode applied to the fuel flow in a coaxial air-propane nonpremixed flame configuration would cause a flame speed increase, but the authors did not provide details into the potential mechanisms enabling the interaction. More recent work by Elkholy et al [20] and Evans et al [21] apply nanosecond pulsed plasma upstream of lean, premixed methane air flames, with the former using a DBD configuration and the latter a pin-to-pin configuration, and also observe flame speed increase. Typical per-pulse energies used were O(100µJ) and the pulse repetition frequencies up to 10kHz, for actuation on flames with power O(100W ).…”