Studies of C₂H₆ air and C₃H₈/ air plasma assisted combustion kinetics in a nanosecond discharge

The paper presents the studies of ethane and propane/air plasma assisted combustion at a pressure of 60 torr and temperature 300K. O atoms in the plasma have been measured as a function of time after a single discharge pulse using TALIF (Two photon laser induced fluorescence) at 60 torr and temperature of 300 K for these mixtures at various equivalence ratios. A plasma chemistry model of hydrocarbons has been developed. This is done by combining a reduced mechanism of the latest low temperature hydrocarbon mechanism with plasma air chemistry along with plasma and flame NO formation chemistry. The reaction rates of excited nitrogen species with hydrocarbons are not know accurately and have not been included. O atoms measured in air are compared with the new mechanism predictions. The O atom measurements compare well with the mechanism predictions. The O atoms decay slightly faster in the case of ethane than predicted. For the case of propane, the O atoms decay much faster than predicted by the mechanism. A better mechanism for low temperature hydrocarbon combustion is required for the plasma. The O atoms begin chain reactions giving rise to OH and H radicals. H₂O is formed during these chain reactions. But soon all the radicals decay in ∼1.5msec. After the end of chain reactions, species such as CH₂O, CH₄, C₂H₄, H₂O₂, O₃ begin to accumulate. CO and CO ₂ are formed only at the end of these chain reactions through slow oxidation. The mechanism has been used to study ignition by a single discharge for air/fuel mixtures at high temperatures in the range 500-700K and pressures in the range 300-500 torr. An initial mole fraction of O atoms (∼5 x 10 ^-5) has been added without a nanosecond discharge and the mixture is observed in time. It is found that for higher pressures of ∼ 400torr, 700K, there is a two stage ignition in ∼28msec for stoichiometric air while it is 43msec for same mixture at 300 torr showing that there is a dependence on initial pressure and temperature to be studied to take advantage of the nanosecond discharge.