Soot, a byproduct of hydrocarbon combustion, is a significant contributor to global warming and air pollution, posing serious health risks. Understanding soot formation is crucial for mitigating its harmful effects. Current high-speed imaging techniques are limited to million-frames-per-second, insufficient to capture the dynamics of critical species and flame-laser interactions. Multiple laser pulses can alter soot nanoparticle properties, highlighting the need for single-pulse imaging. This research addresses this limitation by developing Laser-Sheet Compressed Ultrafast Photography (LS-CUP), a novel technique capable of billion-frames-per-second imaging. The introduction emphasizes the harmful effects of soot and PAHs, the need for improved understanding of soot formation mechanisms, and the limitations of existing high-speed imaging techniques in combustion diagnostics. It highlights the importance of single-pulse imaging to avoid altering the properties of soot nanoparticles and the need for a real-time, detailed two-dimensional (2D) view of soot formation at ultrafast timescales. The authors then introduce LS-CUP as a solution that combines planar imaging and compressed ultrafast photography to achieve billion-frames-per-second imaging speed, providing the means for detailed spatiotemporal analysis of soot formation.

The literature review section discusses existing research on soot formation in hydrocarbon flames, highlighting the complexities of the process and the limitations of current measurement techniques. It mentions previous work on the growth and clustering of soot precursors, particularly polycyclic aromatic hydrocarbons (PAHs), and the challenges of obtaining real-time, 2D information about the process. It emphasizes the need for simultaneous measurement of multiple key parameters (e.g., primary soot particle size, soot aggregate size, and temperature) to validate existing soot formation models. The limitations of existing high-speed imaging techniques in terms of frame rates, laser fluence, and spatial resolution are also discussed.

The core methodology involves the development and application of Laser-Sheet Compressed Ultrafast Photography (LS-CUP). LS-CUP combines laser-sheet illumination with compressed ultrafast photography (CUP) to achieve billion-frames-per-second (Gfps) imaging speed. A laminar kerosene flame is used as the experimental model, as kerosene is a commonly used fuel. Four optical signals are investigated: flame luminosity, elastic light scattering (ELS), laser-induced incandescence (LII), and laser-induced fluorescence (LIF). A dual-wavelength laser (532 nm and 1064 nm) is used, with 1064 nm for LII and 532 nm for LIF. The laser beam is shaped into a sheet to illuminate a 2D plane of the flame. The resulting signals are spatially encoded using a digital micromirror device (DMD) and then captured by a streak camera. The raw data are reconstructed using image processing algorithms to obtain time-resolved 2D images. Specific spectral filters are used to isolate the different optical signals of interest. The dual-channel capability of LS-CUP allows for simultaneous imaging of two different species. The methodology details the experimental setup, the principle of operation of LS-CUP, and the image reconstruction process. It includes descriptions of the laser system, the optical components, the flame generation, and the data analysis techniques. It also includes details about the specific experimental configurations used for obtaining LIF, LII, and ELS images.

The key findings of the study are presented through visualizations and quantitative data analysis. High-speed imaging reveals the time evolution of PAH LIF, showing an exponential decay with lifetimes in the range of 80-100 ns. Time-resolved LII images are used to determine the primary particle size distribution (5-25 nm) and track the cooling of soot particles. The combination of LII and ELS provides data on soot aggregate size and monomer number. The spatiotemporal maps generated by LS-CUP provide insights into the interactions between PAHs and soot particles in the flame. LS-CUP successfully captures the ultrafast dynamics of soot formation and its related processes. The combination of the different imaging modalities provides a comprehensive picture of the process. The results show that the LS-CUP system can achieve real-time, 2D imaging of ultrafast phenomena with high temporal and spatial resolution. The combination of LIF, LII, and ELS signals provides a more complete understanding of the soot formation process than is possible with previous techniques. The key findings include the successful demonstration of Gfps real-time planar imaging of multiple optical signals, the measurement of PAH-LIF decay times, determination of primary particle size distribution using one-color LII, the mapping of soot temperature using two-color LII, and the estimation of soot aggregate size and the number of monomers using ELS. The results demonstrate the potential of LS-CUP for advancing our understanding of complex physical and chemical processes in flames.

The results demonstrate the successful application of LS-CUP for real-time, single-pulse imaging of ultrafast processes in flames, surpassing the capabilities of existing techniques. The spatiotemporal maps obtained provide crucial insights into the dynamics of PAH formation and soot inception, growth, and oxidation. The simultaneous measurement of multiple signals allows for correlation between different species and processes. The study's findings provide strong experimental support for existing theoretical models of soot formation and suggest areas for future refinement. The technique's applicability extends beyond combustion to other fields involving ultrafast phenomena. The discussion section elaborates on the significance of the findings in relation to existing theories and models of soot formation. The authors discuss how the results address the limitations of previous studies and provide new insights into the complex processes involved in soot formation. They also emphasize the importance of the multi-modal imaging capability of LS-CUP in providing a more comprehensive understanding of the dynamics. The authors also explore the broader implications of their work and discuss the potential applications of LS-CUP in other areas of research.

LS-CUP represents a significant advancement in ultrafast imaging technology, offering unprecedented capabilities for studying rapid dynamic phenomena. Its ability to capture real-time, 2D images of multiple species at billion-frames-per-second resolution provides a powerful tool for advancing our understanding of combustion and other fields. Future work could focus on extending the technique to three-dimensional imaging and applying it to different combustion systems and other ultrafast phenomena such as hot plasma, sonoluminescence, and nuclear fusion.

While LS-CUP offers significant advantages, it also has some limitations. The current setup focuses on laminar flames; applying it to turbulent flames will require further development. The analysis relies on established models for interpreting LII and LIF signals, and potential improvements in these models could enhance the accuracy of the results. The spatial resolution is limited by the laser sheet thickness and the optical system; improvements in these areas could provide finer details. The computational cost of data reconstruction is high; exploring more efficient algorithms would be beneficial.