Mie Scattering Imaging of Metal Particle Dispersion in a Solid Fuel Ramjet Combustor

Metalized solid fuels offer significant performance advantages for solid fuel ramjets (SFRJs), but their utility depends critically on particle ignition behavior, combustion timescales, and transport through the highly non-uniform flame stabilized downstream of a flameholding feature. This study examines the dispersion, evolution, and flow-flame interaction of boron carbide (B4C) particles in a rectangular, optically accessible SFRJ combustor using 5-kHz Mie scattering and filtered chemiluminescence diagnostics under flight representative post-inlet conditions with unvitiated heated air (2.5 lb/s, 720 F) and an 87 psia chamber environment. Lorenz-Mie predictions show that surface-oxidized B4C exhibits scattering intensities orders of magnitude higher than soot under parallel polarized illumination at a 90 degree collection angle, enabling selective visualization of additive particles even within the strongly sooting near-wall region. High speed Mie scattering imaging resolves distinct particle transport pathways governed by backward facing step recirculation, hydrodynamic vortex shedding, and downstream redistribution driven by shear layer dynamics. Filtered chemiluminescence measurements indicate that metalized operation modifies the reacting shear layer, increases fluctuation intensity in hydrodynamically unstable regions, and shifts the peak heat release signature downstream relative to pure HTPB operation. Coefficient of variance analysis further reveals that the addition of B4C heightens temporal unsteadiness in the reacting shear layer and redeveloping boundary layer while main- taining similar flame stabilization behavior near the fuel surface. The combined Mie scattering and chemiluminescence diagnostics provide spatially and temporally resolved characterization of B4C particle clouds and their coupling to core SFRJ flow structures, offering new insight into particle entrainment, residence time modulation by recirculation and vortex transport, and the mechanisms that determine whether metalized additives actively contribute to combustion in practical SFRJ configurations. These results visualize the challenges of achieving effective parti- cle ignition and sustained energy release in high speed, shear dominated ramjet environments and highlight the importance of understanding metal particle transport physics when designing next generation SFRJ systems employing high energy density additives.