Direct numerical simulations of laboratory-scale NH3/air jet flames: Analysis of flame structure, flame stabilization and NO emission characteristics

Abstract

In the present study, three-dimensional direct numerical simulations (DNS) of experimental ammonia/air premixed jet flames with different turbulent intensities were performed. The DNS results were first compared to the measurements with good agreements. Based on the DNS data, the turbulent flame structure, stabilization mechanism and NO emission characteristics of the flames were investigated. It was found that the flame with higher turbulent intensity exhibits a higher degree of wrinkling and an increased flame surface area. In addition, turbulent eddies can enter into the reaction zone and disrupt the distributions of NH and temperature more strongly for the flame with higher turbulent intensity. It was shown that the heat release rate of the turbulent flame can be approximated by the results of strained laminar flames to some extent. Enhanced heat release rates were observed in the regions of negative curvature near the reactant side and in the regions of positive curvature near the product side, which is due to the local enhancement of radicals such as NH and NH${}_2$ that contribute significantly to the heat release. To understand the flame stabilization mechanism of the turbulent flames, corresponding one-dimensional unstrained and strained unsteady laminar flames were simulated. It was found that auto-ignition initially occurs and the reaction front transitions into a propagating front following the ignition process for both the unstrained and strained laminar flames. The ignition characteristics of the turbulent flames are largely consistent with those of the laminar flames. The study also revealed the NO formation characteristics. NO is consumed in the reaction zone and produced in the product side. The maximum NO mass fraction increases with increasing axial distance. Analysis of NO pathway suggests that this phenomenon is due to the enhanced NO production in the downstream regions, which is related to the accumulation of radicals such as OH, O and H.

Novelty and significance

This research reports the first direct numerical simulations of laboratory-scale ammonia/air turbulent premixed jet flames with varying turbulent intensities. The novelty of this research is that the flame structure, flame stabilization and NO emission characteristics of ammonia/air jet flames are explored using detailed DNS data, which are crucial for improved understanding of ammonia combustion. Furthermore, the present work provides high-fidelity DNS data of turbulent ammonia combustion for the development of combustion models.