Stabilisation limits of turbulent premixed flames by nanosecond repetitively pulsed discharges

Abstract

The mechanism of flame stabilisation using nanosecond repetitively pulsed (NRP) plasma discharges in a turbulent, premixed methane–air flame at high velocities was investigated, focusing on the lean extinction limits. In contrast to the majority of the existing studies that considered NRP discharges as an assistance to conventional stabilisers, here, no other flame-holding method is used. High-speed (10 kHz) OH* chemiluminescence showed that the plasma discharges produce individual OH*-pockets that merge together at a high enough frequency to form a continuous flame sheet. Increasing the discharge repetition frequency from 5 kHz to 10 kHz improves flame stability, but no change in flame structure and stability was observed when the frequency was increased beyond 10 kHz. Change in the plasma energy level in the range studied had little effect on the flame structure. The lean extinction limit was quantified at various flow velocities, equivalence ratios, discharge frequencies and energy levels. It was observed that the trend of the extinction equivalence ratio with bulk velocity was similar to that of a conventional bluff body stabilised flame and that plasma-only stabilised flame was equally effective at certain operating conditions. An effort to correlate the stabilisation limits by a conventional Damköhler number Da was made but was not satisfactory due to the presence of a characteristic flameholder lengthscale present in the Da expression. A modified Da was proposed to take the spark frequency effects into account, but this was not successful either. In contrast, the spread of the extinction data was smaller when a critical Karlovitz number was used, hence offering a way to extrapolate the present data to other conditions. The experiments demonstrate that NRP discharges can be used as an alternative stabilisation method for high-speed turbulent premixed flames.