Hydroxyl-Terminated Polybutadiene Reacting Boundary Layer Examined by Tomographic Laser Absorption Imaging

Hydroxyl-terminated polybutadiene (HTPB) is a common fuel and binder for solid rocket propellants and hybrid combustion systems, yet predictive performance of these systems is challenging due to the complex multi-phase combustion phenomena. A lack of experimental data in the heterogeneous reaction zone near the burning surface of HTPB limits the ability to advance modeling efforts. This work uses laser absorption imaging (LAI) as a temporally- and spatially-resolved non-intrusive diagnostic method to examine the reacting boundary layer of HTPB. A tunable 4.98 μm continuous-wave quantum-cascade laser, targeting two carbon monoxide (CO) rovibrational transitions, is expanded into a 20 mm wide sheet that crosses the exit plane of cylindrical HTPB grains burning in an oxygen cross-flow. A mid-wave infrared camera recording at 73 kHz, with a window of 320×4 pixels, measures the light attenuation from molecular absorption inside the reaction zone, delivering simultaneous temperature and CO mole fraction data across 1280 lines-of-sight covering the entire axisymmetric flowfield. A Tikhonov-regularized onion-peeling inversion converts projected absorbance into radially-resolved species and temperature profiles. Planar measurements for a range of fuel grain lengths are stitched together into a composite 2D image that describes the evolution of thermochemical properties in the reaction layer. The resulting data set provides a benchmark against which numerical models can be compared, while the diagnostic approach provides a framework for future investigations into the combustion physics of solid fuels with heterogeneous behavior.