Effects of hydrogen addition on laminar dilute n-dodecane spray flames

With the increasing demand for carbon-neutral propulsion technology, hydrogen addition appears to be a viable short-to-medium-term solution. This study investigates the dynamics of overall lean, laminar dilute n-dodecane spray flames in premixed hydrogen/air mixture by simulating one-dimensional dilute spray flames over a wide range of hydrogen addition and n-dodecane droplet diameter. The characteristic residence time and evaporation time are analyzed to reveal the pre-evaporation mode, and examine the distinct evaporation characteristics throughout the flames. The flame structure and flame propagation speed are then examined, with the impact of subtle fuel chemistry coupling at low-temperature-chemistry regime being revealed. At atmospheric pressure conditions, the flame speed monotonically increases with hydrogen addition due to the enhanced reactivity. However, it has a nonmonotonic trend with initial droplet diameter due to the subtle coupling of evaporation and chemical kinetics. More interestingly, at engine-relevant conditions, the flame propagation speeds firstly decrease and then increase with hydrogen addition, this is because small hydrogen addition inhibits the low-temperature reaction pathway that could offset the effects of enhanced high-temperature reactivity on flame propagation. In addition, the flame propagation speeds are less sensitive to the droplet diameter unless the droplets are sufficiently large and the long evaporation process reduces the effective local equivalence ratio of the overall fuel-lean mixtures.