Regression of solid polymer fuel strands in opposed-flow combustion with gaseous oxidizer

The combustion of solid fuels is a complex feedback loop, coupling the decomposition of the solid fuel into volatile gases with the gas-phase combustion which is responsible for the heat flux that drives decomposition. This study aims to explore the combustion of a solid fuel, hydroxyl-terminated polybutadiene (HTPB), with different mixtures of oxygen and nitrogen in an opposed-flow burner (OFB) configuration to better understand these coupled processes. An experimental OFB setup is described, which utilizes a nichrome wire and linear variable differential transformer (LVDT) to capture regression rate and shadowgraph imaging to measure flame thickness. Experimental measurements are compared with results from a complimentary one-dimensional opposed-flow combustion model with a pyrolyzing solid fuel boundary condition that conserves mass, species, and energy at the solid-gas interface. The oxidizer mass flux, ratio of oxygen to nitrogen, and separation distance of the fuel and oxidizer are varied to understand their influence on the combustion process and subsequently their effect on the regression rate. In numerical results, fuel regression rate increases when oxygen mole fraction or mass flux increase, or when separation distance decreases. Experimental regression rates and flame thicknesses are compared to simulated results. Though the actual values do not agree exactly, numerical and experimental results are reasonably close and present similar trends. These results demonstrate the utility of simple optical diagnostics in measuring OFB flames and provide a starting point for future opposed-flow combustion model improvements.