Digital In-Line Holography Helps Researchers ‘See’ into Fiery Fuels

Accidents, such as trucks crashing on a highway or rockets failing on a launch pad, can create catastrophic fires. It’s important to understand how burning droplets of fuel are generated and behave in such cases. Sandia researchers developed 3-D measurement techniques based on digital in-line holography (DIH). Sandia advanced DIH techniques with new algorithms to mine critical information from recorded holograms and new applications in tough fire environments, said Daniel Guildenbecher (in Sandia’s Diagnostic Science & Engineering Dept.). Measuring the size/velocity of thousands of particles allows researchers to better understand particle formation/transport. “We live in a 3-D world, and if you think of traditional imaging, it’s 2-D,” he said. “This technique is one of the few that can give you a 3-D measurement of a flow such as a fire.”

DIH passes a laser through a particle field. The interaction between the laser and the particles creates diffraction patterns, which a camera records. Then 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. That gives the position of particles as they were in 3-D space.

In a propellant fire, large molten aluminum drops form at the burning surface. They’re lofted into the environment and can severely damage anything they fall on. Refocused digital holograms provide a clear picture of the burning particles. By measuring the size and velocity of thousands of such particles, researchers can better understand how the particles are formed and transported in this flow. “Fundamental understanding of particle formation and transport is necessary to develop next-generation [computer] models which predict this scenario,” Guildenbecher said. “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 DIH method uses nanosecond laser pulses to freeze the particle motion and kilohertz imaging to track droplets’ size and velocity. Recording and quantifying all droplets in a 3-D volume—the digital hologram—lets researchers quickly measure thousands of individual drops, allowing them to accurately quantify size and velocity. In addition, measuring particle shape enables them to differentiate spherical drops from other particulates in the flow.

Results of the successful demonstration were published in a 2013 paper in Optics Letters. The team published additional papers in such publications as Applied Optics, Optics Express, and Experiments in Fluids and presented their work at numerous conferences over the past three years.

Read the Sandia news release.