Stereoscopic high-speed imaging of iron microexplosions and nanoparticle-release.

In this work, the combustion behavior of seeded iron particles (d50 = 70 µm) in a laminar diffusion flame was studied in a modified Mckenna flat-flame burner. Two high speed cameras in stereo configuration allowed 3D position and 3D velocity measurements of burning iron particles as well as 3D evaluation of particle microexplosions. Microexplosive processes are important since it can affect both combustion stability and formation of product components. The observed microexplosions happened before particle extinction resulting in change of trajectories, velocities, radiation intensities and fragmentation into smaller particles. It was observed for the first time that fragments of these microexplosions tend to produce planar structures. A frequent release phenomenon was observed during the iron particle combustion using magnified thermal radiation imaging and high-speed shadowgraphy. This release phenomenon was indirectly confirmed with scanning electron microscopy of combust products, revealing multiple cracked particle shells and hollow structures. Black body radiation characteristics was observed indicating the release being in condensed phase and emission spectroscopy identified FeO as intermediate species during combustion. The observed release is believed to mainly consist of iron-oxide nanoparticles formed in the homogenous reaction between vapor iron and oxidizers.

Spatially resolved, single-ended two-dimensional visualization of gas flow phenomena using structured illumination.

A method for 3D mapping of scattering particle concentration in a gaseous medium based on the backscattered light in a single direction has been demonstrated. The technique is originally developed for microscopy but now implemented on larger-scale samples. The technique used is known as structured illumination, where a sinusoidal grid pattern is projected onto the medium, thus marking the in-focus plane. This makes it possible to discriminate against light originating from the out-of-focus parts of the sample, which usually makes it difficult to detect inner structures of the medium. In this study a flow of nitrogen was introduced into a flow of water droplets, with the aim to optically select only the plane where nitrogen was present. The results indicate that the technique could be used to study, e.g., combustion devices with limited optical access.