Multi-speciation and ignition delay time measurements of ammonia oxidation behind reflected shock waves

Abstract

Ammonia (NH3) oxidation was studied using emission and laser absorption diagnostics in a shock tube. Experiments were carried out using stoichiometric mixtures of NH3 and O2 highly dilute in Ar seeded with CO2 at post-reflected-shock conditions from 1700–2500K near 2atm. Five optical diagnostics in the infrared (IR), visible, and ultraviolet (UV) were implemented to measure NH3 (before and after shock-heating), NO, electronically excited amino radicals (NH2∗), electronically excited hydroxyl radicals (OH∗), and temperature during oxidation. To mitigate mixture uncertainty due to NH3 adsorption, a scanned-wavelength laser absorption diagnostic near 10.4μm was developed to measure initial NH3 concentration. A two-color, fixed-wavelength laser absorption diagnostic near 225 nm was developed to measure quantitative NO and NH3 time-histories. A detailed spectroscopic model of the NO A2Σ+←X2Π system was used to provide NO absorption cross-sections at both wavelengths, and NH3 absorption cross-sections were measured at the conditions of interest. Emission from NH2∗ near 600 nm was collected to provide qualitative insight into radical formation prior to ignition, and emission from OH∗ near 308 nm was used to determine ignition delay times (IDTs). A two-color, fixed-wavelength laser absorption diagnostic at 4.17μm and 4.19μm was developed to measure temperature using small quantities of seeded CO2. Measured time-histories are compared against predictions from two kinetic models. Experimental results and model predictions show several discrepancies, both in terms of timescales and absolute quantities. Sensitivity analysis shows that the reactions H + O2⇌ O + OH, NH3 + M ⇌ NH2 + H + M, and NH3 + H ⇌ NH2 + H2 are dominant across all conditions, and that NH3 pyrolysis reactions in general drive much of the chemistry. Direct, low-scatter rate constant measurements of the NH3 + M ⇌ NH2 + H + M reaction and additional multi-speciation measurements of NH3 pyrolysis and oxidation at combustion-relevant temperatures and pressures are recommended.Novelty and significance statementThe novelty of this work is exemplified most strongly by the use of UV laser absorption diagnostics to measure NH3 and NO during high-temperature oxidation of NH3. Simultaneous measurements of temperature via CO2 laser absorption at two wavelengths and light emission from NH2∗ and OH∗ are also novel. Simultaneous, time-resolved, quantitative measurements of NH3, NO, and temperature and qualitative measurements of NH2∗ and OH∗ light emission are of significant utility as a model validation-suited data set. Additionally, the data presented herein and the associated findings provide evidence of current chemical kinetic model deficiencies pertaining to NH3 chemistry and suggest avenues for model improvement.