Quantitative studies of instabilities of confined spherically expanding flames: Application to flame propagation and autoignition of natural gas blends with hydrogen at engine-relevant conditions

Abstract

The availability of accurate fundamental combustion properties at engine-relevant conditions is of paramount importance in testing kinetic and transport models so that they can be used in large-scale simulations of practical devices. However, a literature search reveals that flame data can be scarce at engine-relevant conditions, and those that are available may be of limited value due to large uncertainties associated with experiments at high temperatures and pressures that can render the flames unstable. The main contribution of this study was the introduction of one-dimensional and multi-dimensional direct numerical simulations to quantify the onset of flame instabilities at conditions used in measuring laminar flame speeds. This is a deviation from the traditional approaches that use theoretical predictions and/or circumstantial evidence to vet and interpret experimental data. To that end, the confined spherically expanding flame method was used to measure laminar flame speeds for reacting mixtures of hydrogen-natural gas blends, for pressures ranging from 8 to 30 atm, and for unburned mixture temperatures ranging from 420 to 530 K. This approach proved to be successful as the predicted and experimental results were consistent with each other, and the reported data herein were measured for conditions that the direct numerical simulations revealed that they resulted in stable flames throughout all stages of propagation. As expected, the laminar flame speed increases with hydrogen addition, and this increase was found to be slightly more notable compared to recently published data under similar conditions for hydrogen-methane flames. Additionally, under suitably chosen conditions, autoignition in the end gas was induced during the compression stage of stable flame propagation, and ignition delay times with remarkably low repeatability scatter were measured through the rapid changes in the pressure-time derivatives. The autoignition studies included n-pentane as an additive to achieve autoignition within the available times in the existing constant-volume spherical facility. For blends of hydrogen with natural gas and n-pentane, a two-stage ignition behavior was observed, and the hydrogen addition resulted in the reduction of ignition delay times. All reported data were modeled, offering insights into the mechanisms governing flame propagation and autoignition. The results of the present study are expected to contribute to ongoing endeavors related to hydrogen utilization and decarbonization in the transportation sector.