Measurement of the 3D Rayleigh index field via time-resolved CH* computed tomography

Rayleigh index is a critical parameter to quantify the mechanism that leads to thermoacoustic instabilities. However, due to the limitations of measuring instruments and the challenges of methodologies, it is still difficult to measure this parameter in a temporally and spatially resolved manner. This work introduced a method that can calculate the time-resolved global and local distributions of Rayleigh index based on the time-resolved measurement of three-dimensional (3D) heat release rate and pressure. A high-speed camera equipped with an intensifier and a customized fiber bundle was employed to continuously capture the CH* chemiluminescence projections from nine views, based on which the time-resolved, 3D heat release rates were obtained via computed tomography of chemiluminescence (CTC) technique. For the present combustor, it is shown that both the flame structure, and global and local Rayleigh index exhibited large-scaled periodic variations throughout the thermoacoustic cycle, demonstrating the necessity of performing temporal and spatial measurements of the flame structure and Rayleigh index, which are of paramount importance in the investigations of combustion instability.