Shock-tube laminar flame speed measurements of ammonia/airgon mixtures at temperatures up to 771K

Laminar flame speed measurements of lean (φ = 0.75), stoichiometric (φ = 1.0), and rich (φ = 1.25) mixtures of ammonia in “airgon” (79% Argon, 21% O₂) were conducted at room temperature (295 K) and various pressures in a static setup, and for the first time at temperatures above 500 K behind reflected shock waves. Constant-pressure static experiments were performed in the cylindrical, Constrained Reaction Volume (CRV) section of the Imaging Shock-tube (IST) to study flame propagation and estimate minimum ignition energies. Ammonia/air experiments were also conducted at room temperature to assess the effect of the diluent gas. The shock-tube flame speed method allowed access to the temperature range of 477 K–771 K at atmospheric pressure. Simultaneous side-wall schlieren and end-wall OH* chemiluminescence diagnostics permitted time-resolved tracking of the propagation of outwardly expanding flames spark-ignited by an Nd:YAG laser system. An area-averaged linear-curvature model was employed to compute unstretched laminar flame speeds. Measurements were compared with 1-D, planar flame speed simulations based on different chemical kinetic models for ammonia oxidation, including those by Glarborg et al., Okafor et al., Zhang et al., Chen et al., Otomo et al., and Shrestha et al. These models agree well with experimental results and each other at room temperature, but clearly diverge at higher temperatures. Experimental results agree well with the models of Okafor et al. and Zhang et al. at high temperatures. Flame speed sensitivity analyses were performed and key reactions, as predicted by the models, were identified.