Abstract
To control the ignition and stabilization location of the second stage flame in a sequential combustor, nanosecond repetitively pulsed discharges (NRPD) were generated between three cylindrical electrodes. The NRPD were obtained by repetitively applying high voltage pulses to the central electrode. At operating conditions where the sequential flame was nearly quenched or weakly anchored, it was possible to re-ignite it and to control its location by adjusting the voltage (V) amplitude (5-10 kV) and repetition frequency (f) (1-100 kHz) of NRPD. This plasma enhanced auto-ignition was achieved with an acceptable increase of NOx emissions. Similar flame stabilization locations can be achieved by blending propane into the fuel stream, from which one could deduce that the auto-ignition delay was reduced by a factor of ten with NRPD. Direct images of the discharge indicated that applied NRPD corresponded to one the following modes depending on V and f: glow, transition and spark. With the electrodes placed downstream of the fuel injection, quenching effect of the cold fuel on NRPD generation was observed, while when the electrodes were positioned upstream of the fuel injection no ignition event could be observed. High speed imaging of OH* chemiluminescence revealed that spatially homogeneous and temporally continuous auto-ignition was much more efficiently triggered by NRPD in spark mode than NRPD in glow mode. High energy efficiency of NRPD was validated by measuring the plasma energy deposition. The result showed that NRPD with a power consumption about 100 W were sufficient to control a 50 kW sequential combustor. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.