Application of the tomographic BOS technique to a H2-air premixed flame

Background Oriented Schlieren (BOS) is a line of sight optical (LoS) technique used for visualizing and measuring density gradients in transparent media by exploiting the fluid density and refractive index relationship. This study applies BOS to a premixed axis-symmetric hydrogen flame, aiming to quantitatively estimate its density and temperature distributions using the ARAP (Arbitrary Ray Axisymmetric Projection) deconvolution algorithm. The BOS technique consists in acquiring images of a background with and without the flame; light rays passing through the flame are deflected due to the heterogeneous refractive index field. Cross-correlation algorithms are then employed to quantify these deflections. The ARAP transform is preferred to Abel-based methods because it models light rays as converging to a single point according to the pinhole camera model. Assuming constant composition, density gradients are evaluated, and considering ideal gas behaviour and constant pressure, the temperature distribution is estimated. The study explores the influence of parameters like regularization and integration steps for the ARAP transform. Consistency analysis involves evaluating temperature trends at various flame heights and comparing them with thermocouple measurements. Additionally, a preliminary estimation of tomographic BOS uncertainties is provided.