Ignition and flame-holding of H2 and CH4 in high temperature airflow by a plasma torch

The effect of airflow temperature on ignition characteristics of a plasma torch was experimentally investigated. Various combinations of fuels (CH4 and H2) and plasma jets (PJs) (N2, O2, and N2/H2) were tested for a wide range of airflow temperature from 300K to 700K and the PJ power required for ignition was investigated. Ignition by the PJ occurred more easily in a high temperature airflow than in an atmospheric temperature airflow. The main reason for this was considered to be the increase in the reactivity of the fuel at high temperature rather than the effect of radicals in the PJ, because there was no difference in the spectroscopic measurement of the PJ between Tair = 300K and 700K. The addition of H2 to the N2 feedstock was also effective for ignition enhancement of both fuels (H2 and CH4). In particular, the H2 30%/N2 70% PJ was able to ignite both fuels even at atmospheric temperature and the lowest electric power input for the stable operation. One of the reasons for this advantage of the H2 30%/N2 70% PJ was the heat release from the diffusion flame of the H2 included in the feedstock with the airflow after injection. Moreover, the conditions around the local ignition site such as the local fuel concentration or the size of the contact area of the PJ and the fuel jet were found to be important factors for the success of ignition.