Turbulent flame propagation enhancement by application of dielectric barrier discharge to fuel-air mixtures

The promotional effect of non-thermal plasma (NTP) on lean flame propagation in turbulent fuel-air mixtures in a reciprocating engine-like high-temperature and -pressure environment was investigated. A turbulent flow was created by installing a perforated plate with oblique holes in the combustion chamber of a rapid compression and expansion machine (RCEM). Ignition was conducted by conventional spark plug. The NTP was generated by a dielectric-barrier discharge (DBD) device installed in the combustion chamber near the spark plug. A portion of the lean fuel-air mixture (ϕ = 0.5) in the chamber passed through the NTP and diffused throughout the chamber before spark ignition. To elucidate whether the effect persisted even when the plasma-affected volume was diffused by the flow, two types of experiments with temporal delay were conducted. The fuels evaluated were n-heptane as a representative fuel with a strong low-temperature oxidation reaction, i-octane as a representative fuel with a weak low-temperature reaction, and a primary reference fuel consisting of a mixture of these two fuels. Temporal growth of the flame was observed using a high-speed camera with an image intensifier. The evolution of in-cylinder pressure was also monitored and the characteristic time of the mass fraction burned was evaluated accordingly. It was found that flame propagation was promoted by DBD for n-heptane-containing mixtures at a certain initial temperature while the i-octane-air mixture did not exhibit such enhancement. The results obtained suggest that long-lived intermediate chemical species formed by the plasma diffuse into the cylinder, affecting flame propagation through promotion of a low-temperature oxidation reaction. In addition to these findings, the effect of NTP on burning periods were evaluated.