Laser camera films combustion at 12 billion images per second

28 Feb 2023

Caltech and University of Gothenburg see new details of temperature and dynamics.

A better understanding of how hydrocarbons burn is an important topic in environmental science, with nanoparticles of unburnt soot being active agents in climate change and detrimental to human health.

Various advanced high-speed camera technologies have been used to image hydrocarbon flames, including the use of laser illumination, but most have been limited to million-frames-per-second rates; not enough to provide a complete picture.

A project involving the Caltech lab of Lihong Wang and the University of Gothenburg has now developed a laser camera platform able to capture images at billion-frames-per-second rates, offering new insights into the creation of soot from precursor species and the dynamics of combustion.

Reported in Light: Science & Applications, the technology could also help to answer questions in other topics of advanced physics, such as the nature of hot plasma, sonoluminescence and nuclear fusion.

"The formation of soot particles from precursor gaseous polycyclic aromatic hydrocarbons (PAHs) has remained a mystery in combustion science as well as in astronomy," said the project in its published paper. "A real-time, detailed 2D view of soot formation is still not available. There is a need for simultaneous measurement of key parameters such as primary soot particle size, soot aggregate size, and temperature to validate the soot formation theory and models."

The project builds on previous work at Caltech studying compressed ultrafast photography (CUP), in which an encoding and subsequent reconstruction of the illuminating light signals can capture transient optical events at high frame rates using a streak camera.

In the new study, CUP is combined with planar imaging based on laser sheet illumination, in which interactions between laser and matter are "optically sectioned" and confined to particular spatial areas, to create laser-sheet CUP (LS-CUP). This can allow imaging rates to hit the billion-frames-per-second mark.

More pictures taken, more details of combustion

In trials using a kerosene flame, the LS-CUP platform employed two wavelengths capable of creating laser-induced incandescence from soot particles, 1064 and 532 nanometers, at nanosecond pulse rates. A light sheet 0.4 millimeters thick was created using a cylindrical lens and directed onto the flame.

Results allowed the project to map particle size distributions by observing the scattered light, and to follow the development and decay of the PAH species of interest through laser-induced fluorescence and incandescence. The effective frame rate was calculated to be 12.5 billion images per second, said by the project to be at least a thousand times faster than today's best laser equipment, with the project analyzing 200-frame sequences of gathered data.

"The more pictures taken, the more precisely we can follow the course of events," commented Gothenburg's Yogeshwar Nath Mishra. "Before, problems arose when the camera was limited to a few million images per second. Producing two-dimensional pictures of different types of combustion has required repeated laser pulses, which impacts the combustion temperature when the laser adds energy."

The new method could become a versatile tool, able to study both active laser-induced phenomena and allow passive imaging of flame luminosity and species chemiluminescence, providing a range of valuable data about the dynamics of soot production for engineers and climate scientists.

"LS-CUP represents a generic and indispensable tool that combines a portfolio of ultrafast combustion diagnostic techniques," commented the team in its published paper.