Determining the electric field in a 10 ns pulsed plasma in fuel-air mixtures using EFISH

Abstract

Transient plasma ignition (TPI) utilizes non-equilibrium plasmas, produced by nanosecond high-voltage pulses, to improve lean-fuel combustion performance and reduce emission. It is known that the relatively high reduced electric field ( E / N ) in TPI plays an important role in generating energetic electrons and facilitating energy-efficient radical productions, resulting in reliable ignition for lean combustion. Determining the reduced electric field in the discharge is hence important for the understanding of the TPI process and ultimately allowing for the control of the plasma chemistry. This study reports spatiotemporally resolved measurements of the electric field ( E ) in a 10 ns pulsed plasma that is produced by a pin-to-pin electrode configuration in extremely lean CH 4 /air mixtures, with the equivalent ratio (ϕ ) ranging from 0.4 to 0.8, at 1 bar using the electric field induced second harmonic generation technique. At a relatively low voltage such as 8 kV, a streamer discharge is typically initiated between the electrodes. The peak electric field was found to be 45 kV cm −1 , corresponding to an E / N of 184 Td, for an 8 kV pulse in a lean (ϕ = 0.4) CH 4 /air mixture at 1 bar. Increasing the pulse voltage to be ⩾ 13 kV under a single pulse operation would cause the discharge to transition to a spark, which was found necessary to achieve reliable ignition for combustion. The peak E was measured to be 66 kV cm −1 or an E / N of 405 Td, for a 13 kV pulse. Interestingly, the E reaches its maximum during the rising phase of the voltage pulse and drops quickly as the current increasing. The radially symmetric electric field was found highest near the high voltage pin, followed by that near the ground pin, and lowest near the midplane. The effect of the pulse repetition frequency on E / N was studied by employing two-pulse sequences with various pulse-to-pulse delays ranging from 10 to 1000 µ s. Measurements of the electric field during the 2nd voltage pulse revealed that the transient spark occurred at a lower voltage with a lower peak E ( i.e. 40–48 kV cm −1 ), but corresponding to a greatly enhanced E / N due to the elevation in the gas temperature of the discharge. In addition, the impact of ϕ on E was found insignificant, implying that the measurements obtained at the extreme lean conditions can be applied to stoichiometric or richer fuel-air mixtures.