Effects of hydrogen addition on the propagation and autoignition of methane/oxygen/inert mixtures under engine-relevant conditions

Abstract

The confined spherical expanding flame method was used to measure laminar flame speeds and ignition delay times of blends of methane and hydrogen at engine-relevant conditions. Laminar flame speeds were measured for hydrogen addition to methane of up to 40 % on a molar basis, for pressures ranging from 8 to 30 atm, and for unburned mixture temperatures ranging from 420 to 530 K. It was found that the laminar flame speed increases with the hydrogen addition, as expected, and that the trend of the data is consistent with predictions resulting from using recently developed kinetic models. For properly chosen conditions, autoignition of the end gas was induced during the compression stage of the outward flame propagation, and ignition delay times were determined using the rapid changes in the pressure-time derivatives. The autoignition experiments were carried out for blends of methane, n-pentane, and hydrogen. The use of n-pentane was essential in order to achieve autoignition within the time scales available in the existing spherical facility. In the presence of n-pentane, a two-stage ignition behavior was observed, and the addition of hydrogen was found to reduce the ignition delay times. All data were modeled, and insight was provided into the mechanisms controlling flame propagation and the two stages of ignition. These results are expected to be of direct relevance to ongoing efforts in hydrogen utilization toward the decarbonization of the transportation sector.