Flame Structure Studies of Neat and HN₃-Doped H₂/O₂/Ar Flames by Laser-Induced Fluorescence, Mass Spectrometry, and Modeling

Abstract

Compounds rich in nitrogen are attracting much scientific interest because of their high energy content. Given this interest, it is desirable to be able to describe accurately the combustion mechanism and kinetics of HN₃, the smallest, high-nitrogen compound. We report a combined experimental and modeling study of neat and HN₃-doped H₂/O₂/Ar flames. We employed thin-wire thermometry and hydroxyl (OH) laser-induced fluorescence (LIF) to measure the flame temperatures and molecular beam-mass spectrometry, LIF, or both, to measure the species concentrations. We assembled a detailed chemical mechanism containing 24 species and over 100 reactions and tested it by comparing our experimental profiles HN₃, H₂, O₂, H₂O, N₂, NO, NH, and OH to those predicted by the PREMIX flame code. Our model predicts well the species profiles, except for HN₃ and NO. Rate and sensitivity analyses reveal that the HN₃ + OH = N₃ + H₂O reaction is important in HN₃ consumption and NO production, and we provide a revised rate expression for this reaction that is consistent with our experimental results.

Keywords:

• Combustion
• Flame
• Hydrazoic acid (HN₃)
• Kinetic modeling
• Kinetics
• Laser-induced fluorescence
• Molecular-beam mass spectrometry
• PREMIX flame code

ACKNOWLEDGMENTS

We thank Dr. R. Pesce-Rodriguez of the Army Research Laboratory (ARL) for the gas chromatography–mass spectrometric analysis of HN₃ and Dr. A. Kotlar of ARL for the use of his computer program for determining the LIF flame temperatures. Also, we thank Dr. W. Anderson of ARL for useful discussions regarding the chemical mechanism and for reviewing the manuscript. Support from the Ordnance Environmental Program (R. Sausa) and National Research Council Postdoctoral Research Program (M. Grams) is gratefully appreciated.

Notes

Rate expressions are in the form k = AT^Be^<−E/RT>, where A and E have units of cm/mol-sec-K and cal/mol, respectively.

Rate expressions are in the form k = AT^Be^<−E/RT>, where A and E have units of cm/mol-sec-K and cal/mol, respectively.

*The expression given is from Knyazev and Korobeinichev (2010), who fit the data of Henon and Bohr (2000) in the 800–2000 K temperature range.

*Sensitivity coefficients are normalized logarithmically to the maximum NO mole fraction and then scaled to the N + NO = N₂ + O reaction.

The total rate integral is 1.46 × 10⁻⁷ mol/cm²-s for the neat flame and 1.27 × 10⁻⁷ mol/cm²-s for the doped flame.

This article is not subject to U.S. copyright laws.

M. P. Grams is a National Research Council Postdoctoral Research Associate.