Proper Orthogonal Decomposition of the Unperturbed Blue Whirl

The blue whirl (BW) is a recently discovered flame characterized by its blue color and minimal soot production. It was initially observed in laboratory-scale fire whirl (FW) experiments involving the combustion of liquid hydrocarbons on water (Xiao et al., PNAS, 2016); these BWs had diameters of 2-2.5 cm, heights of 6-8 cm, and were slightly lifted above water surface. Recent advancements in three-dimensional (3D) numerical simulations (Chung et al., Science Advances, 2020; Zhang et al., Computers and Fluids, 2018) have shed light on the flame structure. Now we conducted 3D direct numerical simulations of BWs, extending over 16 seconds of physical time and requiring an extremely refined computational grid. In addition, we used Proper Orthogonal Decomposition (POD) method to identify the primary flow structures and study flame and flow stability and dynamics.