A quantitative evaluation method of 3D flame curvature from reconstructed flame structure

Abstract

Three-dimensional (3D) imaging techniques, such as computed tomography of chemiluminescence and volumetric laser-induced fluorescence, are advanced combustion diagnostic tools that can provide rich insights on the spatial distribution of combustion intermediates. Besides the flame structure that is directly reconstructed, some deep and underlying information such as the geometric features are quite useful for a complete understanding of combustion mechanisms and, therefore, is desired to be further explored. However, to the best of our knowledge, due to the concave and convex variations on flame surface and the mathematical challenges, almost no attempts have been made to experimentally calculate the 3D geometric features of a turbulent swirl flame. In this work, we propose a point-extracted method that can effectively extract a point cloud from the reconstructed turbulent swirl flame. In addition, the two principal curvatures, mean curvature and the Gaussian curvature were evaluated with the so-called triangle-mesh-based method (TMB). One main contribution of this work is to perform a comparison between TMB and direct numerical simulation for curvature evaluation. The quantities obtained by this method are expected to be valuable for the validation of combustion modeling and for the investigation of combustion mechanisms. The 3D curvatures including principle curvatures, mean curvature and Gaussian curvature were experimentally evaluated in a turbulent swirl flame for the first time. The Gaussian curvatures can be related to three typical kinds of curved surfaces, i.e. elliptical, cylindrical and saddle surfaces. For the swirl flame used in this work, the cylindrical surface is regarded as a “base flame surface”, based on which some humps called flame wrinkles would form at some positions. At the tip of each hump, the flame will present an elliptical cup-like surface, where the Gaussian curvatures are relatively large. The transition region from cylindrical surface to elliptical surface mostly manifests a saddle surface.