Autoignition of ammonia-hydrogen mixtures under engine relevant conditions - second law analysis

Abstract

Hydrogen addition can enhance ammonia reactivity and improve its ignition characteristics. This study investigates the fundamental mechanisms governing ignition improvement in ammonia and hydrogen mixtures by quantifying exergy destruction during autoignition at a pressure of 5 MPa, temperature of 900 K, and equivalence ratios of 0.5 to 1. Exergy destruction of each elementary reaction is quantified using species production rates and Gibbs free energy changes. Results show that hydrogen addition significantly increases instantaneous exergy destruction due to enhanced radical generation and chain-branching reactions. However, the total exergy destruction decreases by 19% and 30% for hydrogen addition levels of 40% and 60%, respectively, primarily due to the shorter ignition delay time, which reduces the duration available for irreversible processes. This is further reflected in decreased exergy destruction from NHX oxidation and NOX chemistry, while exergy destruction from radical growth reactions increases due to enhanced H and OH production.