A DNS study of differential diffusion effects on the ignition of a turbulent NH3/H2/N2-air mixing layer

Abstract

his study investigates how differential diffusion influences auto-ignition of an NH3 /H2 /N2 -air scalar mixing layer in homogeneous turbulence using two-dimensional direct numerical simulations. To resolve ambiguity in distinguishing fuel-side nitrogen from N2 in air, a tagged NH3 species tracking approach is introduced, enabling consistent definitions of the mixture fraction and differential diffusion parameter, 𝑍HN. Under turbulent conditions, ignition kernels predominantly emerge near the isoline of the most reactive mixture fraction, where 𝑍HN is elevated. The spatial correlation between 𝑍HN and local heat release rate (HRR) is strong, whereas that between the scalar dissipation rate, 𝜒, and HRR is weak, demonstrating that 𝑍HN serves as a superior local indicator of auto-ignition. With increasing turbulent Reynolds number (𝑅𝑒t), enhanced differential diffusion of H2 accelerates ignition, reducing the ignition delay time. However, at high 𝑅𝑒t , excessive turbulent mixing intensifies heat and radical losses, leading to transient kernel extinction and reduced cumulative heat release despite elevated 𝑍HN. These findings reveal a turbulence-induced crossover from differential-diffusiondriven ignition advancement to heat/radical-loss-driven suppression, providing new insights for understanding nonpremixed NH3 /H2 turbulent combustion.