Ignition of premixed hydrocarbon-air flows by repetitively pulsed, nanosecond pulse duration plasma

The paper presents results of plasma assisted combustion experiments in premixed hydrocarbon-air flows excited by a low-temperature transverse repetitively pulsed discharge plasma. The experiments have been conducted in methane-air and ethylene-air flows in a wide range of equivalence ratios, flow velocities, and pressures. The plasma was generated by a sequence of high-voltage (similar to 10 kV), short pulse duration (similar to 50 ns), high repetition rate (up to 50 kHz) pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation. On the other hand, the extremely low duty cycle of the repetitively pulsed discharge, similar to 1/500, greatly improves the discharge stability and helps sustaining diffuse and uniform nonequilibrium plasma. Generating this repetitively pulsed plasma in premixed hydrocarbon-air flows results in ignition and flameholding, occurring at low plasma temperatures, 140-300 degrees C, inferred from the nitrogen second positive band system spectra. At these conditions, the reacted fuel fraction, measured by the FTIR absorption spectroscopy, is up to 80%. The experiments demonstrate significant methane and ethylene conversion into CO, CO2 and H2O even at the conditions when there is no flame detected in the test section. At these conditions, fuel oxidation occurs due to plasma chemical reactions, without ignition. This provides additional evidence for the nonthermal fuel oxidation triggered by plasma-generated radicals. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.