Laser-Driven X-Rays and Fluorescence Imaging Capture Details of Atomizing Sprays

A team at Lund University observed and quantified an atomizing spray by recording x-ray absorption while using two-photon laser-induced fluorescence imaging. High-contrast fluorescence images provided fine details of the spray structure and minimized blur from multiple light scattering, while x-ray radiographs quantified how the liquid was distributed. Sprays like those used for liquid fuel combustion in vehicle, ship, and plane engines can be difficult to visualize with normal light because they consist of thousands of small droplets that scatter light in all directions.

The amount of liquid present in a spray can be measured by detecting the amount of x-ray radiation transmitted through it. However, this approach usually requires x-rays generated by large synchrotrons, which are available at only a few facilities around the world. The researchers overcame this barrier by developing a tabletop laser-plasma accelerator designed to produce x-rays tailored for high-resolution, time-resolved x-ray imaging.

In the laser-plasma accelerator, x-rays are generated by focusing an intense femtosecond (fs) laser pulse into a gas or a preformed plasma. The researchers also used these fs laser pulses to perform two-photon fluorescence imaging.

Researchers from Lund University developed an imaging method that provides an unprecedented view of sprays such as the ones used for liquid fuel combustion. Pictured (from the left) are doctoral student Kristoffer Svendsen, postdoctoral researcher Diego Guénot, group leader at the Division of Combustion Physics Edouard Berrocal, group leader at the Division of Atomic Physics Olle Lundh, and doctoral student Jonas Björklund Svensson. Courtesy of Edouard Berrocal, Lund University.

“Even though they are much smaller than a synchrotron, the new laser accelerators produce x-rays in the right energy range to be absorbed by liquids and can deliver it in femtosecond pulses that essentially freeze the spray motion for imaging,” researcher Olle Lundh said. “Also, the x-ray flux is high enough to produce a good signal over a wide area.”

The researchers used an 800 mJ laser pulse of 38 fs duration to generate an x-ray beam that allowed projection radiography of water jets generated by an automotive port fuel injector. A fraction of the laser pulse was used to form a light sheet and to induce two-photon fluorescence in a dye added to the water. The resulting high-contrast fluorescence images provided details of the spray structure, with reduced blur from scattering, while the integrated liquid mass was extracted from the x-ray radiography.

“Two-photon imaging of a relatively large area requires higher energy, ultrashort laser pulses,” researcher Edouard Berrocal said. “The fact that we used an intense femtosecond laser beam to generate x-rays meant we could simultaneously perform x-ray and two-photon fluorescence imaging.” According to Berrocal, use of these two imaging modalities at the same time with a relatively large viewed area has not been done before.

To test their technique, the researchers generated x-rays and placed a spray in front of the x-ray camera. When they found that the spray could be clearly visualized this way, the researchers modified the setup to add the two-photon fluorescence imaging. Using the combined technique to image water jets created by an automotive fuel injector produced a higher measurement sensitivity than has been achieved with the large synchrotron x-ray sources, according to the team.

“This imaging approach will make studying sprays much easier for both academic and industry researchers because they will be able to perform studies, not only at the handful of synchrotron facilities, but also at various laser plasma accelerator laboratories over the world,” researcher Diego Guénot said.

The researchers plan to expand the technique to obtain 3D images of sprays and study how they evolve over time. They also want to apply it to more challenging sprays such as biodiesel or ethanol direct-injection sprays, as well as to spray systems used for gas turbines. In the future, the new approach could be used for characterizing sprays and could help scientists better understand the physics of liquid atomization.

The research was published in Optica, a publication of The Optical Society (OSA) (www.doi.org/10.1364/OPTICA.378063).