Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u'/S-L) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (S-T). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (S-T/S-L) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities. (C) 2021 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute.

Automated summary

This paper presents experimental studies on the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz number conditions. NH and OH radicals were measured using PLIF imaging and LDA measurements. The results show that the NH layer remains thin and continuous, the NH and OH layers overlap in a thin region, and the turbulent burning velocity is linearly related to the turbulent intensity.