A shock tube study of chaperon efficiencies for the NH3 + M → NH2 + H + M reaction during ammonia pyrolysis

Abstract

Ammonia (NH₃) pyrolysis behind reflected shock waves was studied using laser absorption spectroscopy. Experimental measurements of the rate coefficient for the

forward reaction were obtained by targeting the NH radical. The mole fraction of the NH radical is uniquely sensitive to the rate coefficient of the

reaction under dilute conditions since it is slower than other NH producing reactions. The NH₃ mole fraction prior to the arrival of the shock wave was measured using a scanned-wavelength infrared laser absorption diagnostic at $10.35\mu m$. A fixed-wavelength ultraviolet laser absorption diagnostic at 336.0998 nm was used to obtain the time history of NH after the passage of the reflected shock. Chemical kinetic model rates were tuned to obtain the best fit to the experimental results. Experiments in a temperature range of 1900 K to 2300 K were conducted with ammonia diluted in argon (Ar), nitrogen (N₂) and hydrogen (H₂) at a pressure of 1 atm. Results of experiments with ammonia diluted in Ar agree with current chemical kinetic models for ammonia pyrolysis. The chaperon (third-body) efficiencies of N₂ and H₂ relative to Ar were obtained. When using a linear mixture rule, N₂ and H₂ are seen to have chaperon efficiencies of 5.2 and 4.4 relative to Ar. These are the first direct measurements of chaperon efficiencies of N₂ and H₂ relative to Ar for this reaction at high temperatures.