Application of Background-Oriented Schlieren to Counterflow Diffusion Flames for Refractive Index Gradient Identification

BOS (Background-Oriented Schlieren) is an optical diagnostic technique for measuring refractive index gradients associated with density variations.In this study, BOS was applied to a counterflow diffusion flame to quantify refractive index gradients.The displacement field was extracted using a Fourier transform-based demodulation method and converted into refractive index gradients, followed by an inverse Abel transformation.The reconstructed refractive index gradient profiles agree well with one-dimensional numerical solutions in both shape and peak location.The results reveal a distinct double-peak structure in the refractive index gradient, arising from density variations induced by mixing and heating.The peak-to-peak distance obtained from BOS agrees well with one-dimensional solutions across varying strain rates controlled by the nozzle spacing, demonstrating its capability to capture the thermal expansion zone and its thickness.