RESEARCH ARTICLE Investigation of the thermal behavior effect of the surrounding material environment on the swirling flame structure Mohamed Ali Thamri 1 | Jamil Zinoubi 1,2 | Soufien Gannouni 1 | Taoufik Naffouti 1,2 1 Faculty of Sciences of Tunis, Department of Physics, Laboratory of Energizing and Thermal and Mass Transfer, University of Tunis El- Manar, Tunis, Tunisia 2 Department of Physics, Preparatory Institute of the Engineers Studies of El-Manar, University of Tunis El-Manar, Tunis, Tunisia Correspondence Soufien Gannouni, Faculty of Sciences of Tunis, Department of Physics, Laboratory of Energizing and Thermal and Mass Transfer, University of Tunis El-Manar, 2092 El-Manar, Tunis, Tunisia. Email: gannounisoufien@gmail.com Summary This paper is focusing on the numerical simulation of a swirling flame, resulting from the interaction of multiple fires, evolving in a free and unlimited environment. A typi- cal system, formed by a central fire source surrounded by four heat sources, is used. Since the thermal characteristic of the surrounding sources is the main engine for the rotation of flame, a detailed study is performed by varying the heating flux of these sources. This study shows that an increase of the heating flux of surrounding sources has as a result an intensification of the penetrating air puffs through the openings between the surrounding four heat sources. These puffs tangentially drive the central flame, thus producing a marked improvement on the angular momentum. Moreover, this study shows that the flame height is strongly affected by the flame rotation. Moreover, two different aspects of the flame height evolution are observed from the flow visualization and the thermal and dynamic fields for the different cases studied. KEYWORDS FDS, flame height, flames interaction, swirl number, swirling flame 1 | INTRODUCTION The rotating or swirling flows is a very large subject, related to a large number of applications. The generic phenomena in these rotating flows are related to the centrifugal and Coriolis forces. These flows have a positive or negative effect. For example, the agitation of coffee in a cup promotes the mixing, the wake whirls of planes cause a vor- tex burst, volcanic eruptions, and tornadoes in the geophysical field, as well as fire flames, pose risks to human life and the environment. 1 In reactive systems, this phenomenon finds many industrial applications—in gas turbines, rotary and centrifugal machines, piston engines, certain furnaces, and pumping of high melting temperature liquid metals. 2 Thus, the effect of rotation on the behavior of these flows must be well understood in order to develop possible tech- niques to control it. It is for this reason that the swirling flows remain an important subject in the combustion community because they pro- mote flame stabilization and are commonly used to effectively mix the fuel with air. Therefore, several experimental and numerical studies have been conducted. However, in the case of fire, as a result of their interaction with the surrounding environment, a rotating flame is formed, which can be described by the combination of a cylinder and a cone. 3 This creates an additional complexity and strongly non-linear phenomena generally, resulting in an extremely turbulent flow. A flame is “swirl” when it has a rotational movement relative to its main direction of evolution. This type of flow can be characterized by a dimensionless number called swirl number, “s,” which expresses the ratio of the angular momentum to that of the momentum in the propagation direction. 4 This parameter quantizes the importance of rotational movement relative to advection. Depending on the value of this number, a flow is qualified of weak swirl (S < 0, 6), or double helix (0, 6 < S < 1, 3), or strong swirl (S > 1, 3). In addition, the flame evolu- tion is characterized by the existence of three different zones: persis- tent or continuous flame zone, intermittent flame zone, and buoyant thermal plume zone. 5 During the vertical flame propagation, the velocity and the axial temperature follow different evolutions by pass- ing from one zone to another. 5-7 A swirling flame can be created by a fire source interacting with a rotating flow. 8 The latter has a direct impact on the flame length that affects the heat-released rate into the environment, which can cause risks such as in the event of an internal fire. 9 In this context, several Received: 24 June 2019 Revised: 9 July 2020 Accepted: 10 August 2020 DOI: 10.1002/fam.2906 Fire and Materials. 2020;1–9. wileyonlinelibrary.com/journal/fam © 2020 John Wiley & Sons Ltd 1