Method of Polychromatic Hilbert Diagnostics of Phase and Temperature Perturbations of Axisymmetric Flames

Abstract

The work is aimed at solving the scientific and practical problem of non-disturbing diagnostics of the phase and temperature fields of reacting jets and flames. On the example of an axisymmetric hydrogen-diffusion flame and a hot air flow from a candle flame, a method was developed that is adequate to the problem being solved, based on Hilbert polychromatic visualization of phase optical density fields, measuring the temperature profile in selected areas of the medium under study, pixel-by-pixel processing of RAW images recorded by a photographic matrix in RGB channels. The visualized Hilbert structures carry information about the phase optical density perturbations induced by the temperature field. The phase structure of the probing light field in the axial symmetry approximation of the flame under investigation is analyzed using the Abel transform. Iterative selection of radial temperature profiles, adapted Bezier curves, is performed with the subsequent calculation of the spatial structure of the refractive index and phase function. The reconstruction of the temperature field by the example of the study of a hydrogen-air flame is carried out taking into account the diversity of the partial optical properties of the gas mixture in a model consistent with the Gladstone-Dale dispersion formula. The influence of disturbances in the air surrounding the flame on its axial symmetry is discussed. The criterion for the reliability of the research results is a comparison of the hilbertograms obtained in the experiment and the hilbertograms reconstructed from phase structures induced by temperature fields.