Phase contrast imaging in combustion: Visualizing melt dynamics in metal fire experiments

Reaction in powdered energetics involves a multitude of timescales born out of combustion physics. With high self-illumination, an external source of illumination is generally not used in bulk combustion of powder or pelletized energetics. Melt phase visualization is useful for understanding multiphase reaction dynamics but is inaccessible in common imaging setups for studying solid reactive material. The imaging setup in this study is based on concepts from Fourier optics, utilizing standard imaging optical components. The technique uses jetted particles as fast-radiating sources to illuminate adjoining slower melt dynamics, providing a unique solution to multiphase visualization in a multiscale solid-state reactive system. Only a narrow bandwidth of the emitted thermal light is used for imaging. In the Fourier filtering scheme, using a combination of intensity and spatial filtering of the reference beam, the contrast of a multiphase body is enhanced in the scene. Qualitative imaging results from the setup show features of shadowgraphy as the dominant pathway for contrast development. Direction asymmetry in the transfer function and cutoff with angular frequencies or blurring indicates a scope of improvement with the usage of thin lenses, instead of stock components. This work demonstrates that phase contrast imaging in combustion studies of powdered energetics provides a viable method for visualizing melt phase dynamics.