Quantitative, three-dimensional imaging of aluminum drop combustion in solid propellant plumes via digital in-line holography

Abstract

Burning aluminized propellants eject reacting molten aluminum drops with a broad size distribution. Prior to this work, in situ measurement of the drop size statistics and other quantitative flow properties was complicated by the narrow depth-of-focus of microscopic videography. Here, digital in-line holography (DIH) is demonstrated for quantitative volumetric imaging of the propellant plume. For the first time, to the best of our knowledge, in-focus features, including burning surfaces, drop morphologies, and reaction zones, are automatically measured through a depth spanning many millimeters. By quantifying all drops within the line of sight, DIH provides an order of magnitude increase in the effective data rate compared to traditional imaging. This enables rapid quantification of the drop size distribution with limited experimental repetition.

Characterizing pyrotechnic igniter output with high-speed schlieren imaging

“…Considering the initial particle sizes of the pyrotechnic constituents (ranging from 13-22 µm), it is reasonable to assume that if discrete particles are visible in the images, then these must consist of clumps of the initial pressed powder bed that have been expelled from the charge cavity due to the growth of a pressure gradient within the bed or perhaps from the occurrence of transition to the deconsolidated burning regime within the charge cavity. An area of future work would be to increase the magnification or to couple in-line digital holography [26] for improved measurement of the particulate size distribution.…”

“…Traditional gas-dynamic diagnostic methods of schlieren, shadowgraphy, and holography have not been effectively applied to pyrotechnic EED outputs in any previous studies. High-speed schlieren imaging of shock waves from explosive micro-detonators fired into dynamic witness plates [25] and digital in-line holography of particle burning characteristics in propellant exhaust plumes [26] are the closest applications.…”

Optical measurements in multiphase combustion

“…In recent efforts, techniques such as digital inline holography have provided detailed characterization of the aluminum size distribution during propellant strand burns [23]. Unlike videography techniques, digital in-line holography (DIH) overcomes focal depth issues and provides statistical analysis from a small number of experiments by capturing individual particle sizes and trajectories [51,49,23,91].…”

“…However, the depth of focus uncertainty is large due to the limited aperture and the comparatively large pixel size on the sensor. For the limited aperture size, the depth of focus can be estimated by [68,49]:…”

“…In this chapter, all the holography results use the hybrid refinement method proposed by Gao et al, and Guildenbecher et al [39,38,51,49]. This hybrid method, uses the minimum amplitude and maximum edge sharpness to locate edges.…”