Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article A numerical study of localized swirling injection of oxidizer for homogeneous combustion with oxygen enrichment Kumar Aanjaneya a, ⁎ , Yawei Chen b , Weiyu Cao c , Claus Borgnakke a , Arvind Atreya a a Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, United States b Simerics, Inc., Novi, MI, United States c Ansys, Inc., Lebanon, NH, United States ARTICLE INFO Keywords: Furnace Homogeneous combustion Industrial Swirl NOx ABSTRACT Conceptually similar to MILD, FLOX, CDC, HiTAC, etc., Homogeneous Combustion (HC) continues to be pursued as an appealing technique towards minimizing NOx emissions. HC combustors are generally driven by high- momentum inlet jets which enable intense dilution of the reactants. This work numerically studies the effec- tiveness of localized swirling injection in enhancing reactant dilution for an HC combustor running with en- riched oxidizers ( > X 21% O2 ). Even though localized and low intensity, swirling injection is found to have far- field effects (over ~75 diameters). Effects of swirling injection on the flow field (near and far field) and on NOx emissions are explained. While swirl does help in reducing NOx, there exists an optimal swirl intensity beyond which NOx emissions increase. A mutual competition is seen between swirl assisted and entrainment driven dilution; and at higher swirl intensities, the reduction in the latter overwhelms the gains accrued by the former (in terms of NOx emissions). Along expected trends, thermal NOx is deduced to be the dominant pathway of NOx formation for oxy-enriched cases. Damköhler numbers in the reaction zone are low even for the highest oxygen content tested here ( = X 40% O2 ). Volumetric standard deviation of Heat Release Rate (HRR) is seen to perform well as a measure of the tendency of transition to the conventional mode of combustion. 1. Introduction Homogeneous Combustion (HC), which is conceptually similar to MILD [1], FLOX [2–4], CDC [5–7], HiTAC [8–10], EEC [11,12] etc., continues to be investigated as a promising technique to minimize pollutant emissions (specifically NOx) from combustion systems. As compared to conventional systems, HC systems simultaneously de- monstrate reduced NOx emissions and enhanced efficiencies. Main- taining the Damköhler number (Da) to be of the order of unity helps establish HC [13]. Damköhler number (Eq. (1)) is the ratio of the re- action rate and the mixing rate (or the inverse ratio of the respective time scales). For a general nth order chemical reaction: = = Da kC flow chem T n flow 0 1 (1) In the endeavor to utilize HC for applications of industrial heating, the biggest hurdles arise due to the usage of enriched oxidizers ( > X 21% O2 ). Increased reactant concentrations in enriched oxidizer systems cause an increase in reaction rates, thereby resulting in a shift towards the Burke- Schumann limit ( Da ) as chem becomes much smaller than flow . In this limit, the reaction proceeds in the conventional flaming mode which exacerbates NOx formation. Also, for a given rate of chemical heat input and equivalence ratio ( ), oxidizer enrichment leads to a reduction in the net momentum being injected via the oxidizer nozzle; which causes further disruptions in the establishment of HC [1,14]. HC combustors are generally driven by high-momentum reactant inlet jets (also true for the setup being studied in this work). In a par- allel-jet furnace with the Strong-Jet/Weak-Jet (SJWJ [15]) configura- tion, two mechanisms which foster the dilution of the weak (fuel) jet have been expounded on in previous work [16]. It was also found that the strong (oxidizer) jet undergoes dilution via only one mechanism i.e. entrainment. The present work is an attempt to facilitate a secondary mode of dilution for the oxidizer jet by modifying nozzle geometry. Such a secondary mechanism of dilution may attain more significance as HC is extended to higher levels of oxygen enrichment. The effect of geometry on HC has been extensively reported on in literature. Tu et al. [17] have worked on optimizing the furnace chamber shape; and report that compared to a cuboidal chamber, a chamber shaped like a frustum of a pyramid is better to enhance re- circulation and reduce NOx. Work has also been done on chamber https://doi.org/10.1016/j.fuel.2020.118773 Received 24 February 2020; Received in revised form 7 June 2020; Accepted 18 July 2020 ⁎ Corresponding author. E-mail address: kumaraa@umich.edu (K. Aanjaneya). Fuel 283 (2021) 118773 0016-2361/ © 2020 Elsevier Ltd. All rights reserved. T