Macroscopic and microscopic imaging of aluminum-HTPB in a counter flow system

Evaluation of solid-fuels for air breathing applications is inherently complicated due to the interplay of fluid mechanics, heat and mass transport to the burning surface. Counter-flow systems offer a quasi- 1-D geometry which significantly decreases the complexity while still maintaining the key operational parameter space of a non-premixed fuel oxidizer system. Hydroxyl-terminated polybutadiene (HTPB) is the most common binder in a solid propellant, but there is interest in increasing energy density though the addition of metallic fuels. In this study we incorporate nano-Aluminum (nAl) and micron-Aluminum (μAl) into HTPB at various loadings. High-speed macroscopic and microscopic imaging systems were used to characterize the near surface Al burning as a function of oxygen flow and pre-heat. The two imaging techniques revealed the morphology and size of the burning Al agglomerations. The regression rate of the fuel grain increased linearly with increase in the oxidizer flow rate. With the observation of high-speed macroscopic and microscopic imaging, combined with color pyrometry, we found that the agglomeration size of Al particles decreased while their temperature increased as the flow rate of the oxidizer increased. This indicates a more complete reaction of the Al particles with more O2, confirmed by XRD and SEM characterizations.