Effects of hydrogen addition on flame structures in counterflow n-dodecane/air spray flames

On the path to a carbon-neutral aviation industry, hydrogen drop-in technology could be a viable transition solution that could take advantage of existing infrastructure. The effect of hydrogen addition on the surrogate n-dodecane spray flame dynamics is investigated in a counterflow flame configuration under engine-related operating conditions, with the H₂ASp-A configuration representing the concept of premixed H₂/air mixture with n-dodecane droplets opposing an air flow, and ASp-H₂A representing the droplets introduced in an air flow opposing premixed H₂/air mixture. Results indicate that the n-dodecane/air spray flame with varying pressure can lead to significant differences in the behavior of multiple solutions, with some single solution behavior transitioning into multiple solutions due to the competition between endothermic evaporation and exothermic chemical reactions. Hydrogen addition on the spray side in H₂ASp-A configuration can induce some single solution behavior transitioning into multiple solutions, whereas the addition on the non-spray side in H₂ASp-A has little impact on multiple solution behavior. In addition, Hydrogen addition under both configurations increases flame speed and temperature, but H₂ASp-A and ASp-H₂A exhibit different flame dynamics. In ASp-H₂A, a lean hydrogen/air premixed reaction zone could be formed at low strain rate that significantly enhance the temperature on the non-spray side, while in H₂ASp-A, it causes non-monotonic speed variation based on autoignition-assisted flame propagation and the balance between hydrogen’s inhibition on low temperature chemistry and enhancing effects on reactions. At the same time, the increased droplet diameter can mitigate the rise in flame speed due to hydrogen addition in both ASp-H₂A and H₂ASp-A, and hydrogen addition in the ASp-H₂A configuration has a better effect on increasing the flame temperature compared to the H₂ASp-A. In terms of extinction, hydrogen addition and reduced initial droplet diameter enhance the spray flame resistance to strain rate in both configurations, with H₂ASp-A performing better at lower hydrogen levels and ASp-H₂A at higher levels.