In a propellant fire, large molten aluminum drops form at the burning surface. The drops are lofted into the environment and can severely damage anything that they fall on. Liquid breakup must be understood to predict the scale and intensity of such fires.

Sandia National Laboratories researcher Daniel Guildenbecher (shown) and colleagues have developed 3D measurement techniques, based on digital in-line holography, to understand the generation and behavior of burning droplets of fuel. In his lab, Guildenbecher simulates a four-wave mixing cell to generate phase-conjugate light, one of the techniques used. (Image Credit: Randy Montoya)
To study the effects, Sandia National Laboratories researchers enhanced digital in-line holography (DIH) techniques with new algorithms. By providing a 3D measurement of a fire’s flow, the imaging method will help users “see” into catastrophic fires caused by events like transportation accidents and rocket failure.

The Sandia team passes a laser through fire while high-speed cameras record the diffraction patterns. Refocused digital holograms provide a clear picture of the burning particles. By measuring the size and velocity of thousands of such particles, the engineers can better understand how the particles are formed and transported in the flow.

The researchers use computers to solve diffraction integral equations, allowing them to take light recorded at the camera plane and refocus it back to the original planes of the particle locations. The refocused light gives the position of particles as they were in 3D space.

“Fundamental understanding of particle formation and transport is necessary to develop next-generation [computer] models which predict this scenario,” said Daniel Guildenbecher, a researcher in thermal/fluid experimental sciences. “Due to the corrosive environment, it’s very difficult to measure these phenomena using traditional instruments. You need to have advanced diagnostics and advanced modeling.”

Sandia’s digital in-line holography method uses nanosecond lasers to freeze the motion of particles and kilohertz imaging to track the droplets’ size and velocity. Recording and quantifying all droplets in a 3D volume — the digital hologram — lets researchers quickly measure the thousands of individual drops, allowing for accurate quantification of size and velocity. In addition, measuring particle shape enables engineers to differentiate spherical drops from other particulates in the flow.

Previous particle-field DIH focused largely on measuring spherical particles in controlled environments. Sandia’s new data-processing algorithms automatically measure complex particle structures in 3D space, quantifying their accuracy through laboratory experiments.

The researchers also have used digital in-line holography to quantify liquid breakup due to strong gas flows and surface impacts. The team measured complex, ring-shaped ligaments in 3D space, which provided new physical insight into how droplets form.

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