In-situ thermochemical analysis of hybrid rocket fuel oxidation via laser absorption tomography of $$\text {CO}$$, $$\text {CO}_{2}$$, and $$\text {H}_{2}\text {O}$$

Abstract

A laser absorption tomography (LAT) technique was developed for investigating the thermochemical structure of a solid fuel oxidation layer in a hybrid rocket geometry. The measurement strategy utilizes tunable infrared lasers to target rovibrational transitions of three major combustion species: carbon monoxide ( $$\text {CO}$$ ), carbon dioxide ( $${\text{CO}}_{2}$$ ), and water ( $$\text {H}_{2}\text {O}$$ ). Species-specific molecular absorption was measured using a quantum cascade laser (QCL) near 4.98 $$\mu$$ m for $$\text {CO}$$ , an interband cascade laser (ICL) near 4.19 $$\mu$$ m for $$\text {CO}_{2}$$ , and a diode laser near 2.48 $$\mu$$ m for $$\text {H}_{2}\text {O}$$ . Spectrally- and spatially-resolved absorption data was collected by translating collinear laser beams across the exit plane of a fuel cylinder with an oxidizer core flow at various fuel-grain lengths. Under an assumption of azimuthal symmetry, Tikhonov-regularized Abel inversion was performed to yield radially-resolved absorption coefficient, from which a two-line method was used to infer temperature and species mole fraction. Planar measurements at different axial distances were compiled to form two-dimensional images, spatially-resolving the thermochemical structure downstream of the oxygen injector. The method is demonstrated to visualize the oxidation of two fuels, poly(methyl methacrylate) (PMMA) and high-density polyethylene (HDPE), with two injector geometries, highlighting the capability to discern variations in hybrid rocket motor design.