Volumetric laser absorption imaging of temperature, CO and CO2 in laminar flames using 3D masked Tikhonov regularization

Abstract

Mid-infrared laser absorption imaging of flame temperature and species concentration is expanded to three dimensions with linear tomographic methods using Tikhonov regularization. A spatial convolution of two small-scale (< cm) laminar Bunsen-style flames, fueled by either ethylene or ethane, comprised the target flowfields. The flame doublets were alternately backlit with tunable radiation from a quantum cascade laser near 4.85 μm and an interband cascade laser near 4.19 μm to resolve rovibrational absorption transitions of carbon monoxide and carbon dioxide, respectively. 2D images were collected at 11 different projection angles, yielding an aggregate of 50,688 unique lines of sight capturing the scene with a pixel resolution of approximately 70 μm. A linear tomographic reconstruction of absorbance areas with 2D Tikhonov regularization was performed using various numbers of projection angles and compared to a reference image based on an Abel inversion of a single axisymmetric flame. Increasing the number of projection angles improved the tomographic reconstruction with respect to resolving the voids, such as the inner core of the flame, though 11 projection angles still resulted in under-predicted void depths and reconstruction artifacts outside of the flowfield. A 3D masked regularization method was then applied to further constrain the tomographic reconstruction in multiple dimensions and mitigate imaging artifacts. The introduction of the 3D regularization and mask were found to enhance spatial resolution of gradients within the flowfield and improve overall reconstruction accuracy, effectively reducing the requisite number of projection angles.