Laminar flame speed and autoignition characteristics of surrogate jet fuel blended with hydrogen

Abstract

Hydrogen combustion has emerged as one promising option toward the achievement of carbon-neutral in aviation. In this study, the effects of hydrogen addition on laminar flame speeds, autoignition, and the coupling of autoignition and flame propagation for surrogate jet fuel n-dodecane are numerically investigated at representative engine conditions to elucidate the potential challenges for flame stabilization and the autoignition risks in combustor design. Results show that the normalized flame speed increases almost linearly with hydrogen addition for fuel-lean conditions, while for fuel-rich conditions it increases nonlinearly and can be up to 20. This poses great challenges for avoiding flameholding and flashback, particularly for fuel-rich mixtures. Results further show that flame speed enhancement due to the increased flame temperature can be neglected under fuel-lean conditions, but not for fuel-rich mixtures. For the dependence of ignition delay time on temperature, there exists a unique intersection between pure n-dodecane/air and H2/air mixtures. Near the intersection temperature, there exists subtle kinetic coupling of the two fuels, leading to different H2 roles, e.g., accelerator or inhibitor, for the autoignition process of n-dodecane/H2/air mixtures. With this intersection temperature, the diagram for autoignition risks is constructed, which demonstrates that H2 acts as an inhibitor under subsonic cruise conditions while either an inhibitor or an accelerator under supersonic cruise conditions depending on the combustor inlet temperature and the amount of hydrogen addition. With the potential coupling of autoignition and flame propagation, the 1-D autoignition-assisted flame calculations show that hydrogen addition can alleviate or even eliminate the two-stage ignition characteristics for pure n-dodecane/air flames. For n-dodecane blended with hydrogen, the autoignition-assisted flame propagation speed, as well as the global transition from flame propagation to autoignition, can still be described by an analytic scaling parameterized by the ignition Damkӧhler number.