Transport-chemistry interactions identified for hydrogen autoignition in shear flows

In modern high-speed propulsion systems, hydrogen is typically used as a fuel because of its high specific energy content. These systems often operate by injecting a hydrogen jet in an oxidizer at elevated temperatures to achieve autoignition. Autoignition is a complex process preceding steady combustion, and therefore demands detailed analysis of the underlying flow, transport and chemistry for systematic design and development.

New research reported in Physics of Fluids describes the autoignition characteristics of nitrogen-diluted hydrogen jets in an oxidizing environment of heated air. The authors performed numerical simulations that assumed laminar flow for two distinct configurations of the fuel jet. The key objective of this work was to identify the dominant chain reactions of the autoignition sequence in these complex flows, tracking the reaction rate histories using a Lagrangian particle tracking approach.

Chemical reactions begin with radicals generated at favorable locations as the injected fuel interacts with an oxidizer. The initiation of autoignition is found in regions where atomic hydrogen and hydroperoxyl (HO₂) radicals are present, highlighting the importance of these radicals in the ignition process. The concentration of atomic hydrogen is further increased through multiple reaction pathways, spreading the initiated ignition until the flame stabilizes.

These results provide fundamental insights into the transport-chemistry interactions of hydrogen autoignition and unravel the influence of flow-induced mixing characteristics on the ignition delay response. These simulations are a step toward understanding flows that are representative of those found in high-speed combustion, further leading to engine improvements where autoignition can be improved through the proper design of the fuel-oxidizer flow interactions, levels of fuel dilution, and thermo-chemical conditions.

Source: “Autoignition of hydrogen in shear flows,” by Abhijit Kalbhor, Swetaprovo Chaudhuri, and Lazar Chitilappilly, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5026400 .