Zilong Zhang

Against the backdrop of the global transition toward clean and low-carbon energy systems, hydrogen has emerged as a promising alternative to fossil fuels owing to its carbon-free characteristics and broad cross-sector applicability. However, the high diffusivity and wide flammability range of hydrogen pose significant safety challenges for its large-scale deployment. Conventional detection methods are generally limited to point-based data acquisition and struggle to capture the transient flow-field characteristics associated with hydrogen diffusion as well as combustion or explosion processes. This review aims to systematically clarify the exclusive technical advantages of schlieren visualization technology for hydrogen research, summarize its application progress in hydrogen and hydrogen mixture diffusion distribution and combustion/explosion flow-field testing, and propose future optimization directions and application expansion paths. Schlieren visualization, based on optical refraction principles, has evolved from a traditional experimental technique into a comprehensive system adapted to diverse scenarios, including high-speed schlieren, Z-type schlieren, background-oriented schlieren (BOS), and color schlieren. Owing to its non-intrusive nature, high spatiotemporal resolution and full-field visualization capability, schlieren technology can directly observe the fundamental diffusion behavior of hydrogen jets and capture distinctive flow features throughout all stages of hydrogen mixture combustion and explosion. It effectively overcomes the limitations of conventional detection methods and has become an indispensable tool in hydrogen energy safety research. Future research should focus on improving technical performance, strengthening interdisciplinary integration with machine learning and digital twin technologies, and expanding application scenarios to multi-field coupling systems, so as to support the safe and efficient development of the hydrogen industry and contribute to global carbon peaking and carbon neutrality goals.

Increasing the proportion of clean energy within the energy structure is a crucial strategy for achieving energy transformation. Hydrogen and ammonia, as leaders in clean energy technologies, have garnered significant global attention. The combination of hydrogen and ammonia has emerged as a novel form of energy storage, transportation, and conversion; however, the safety aspects of their application process warrant closer attention. Research on hydrogen safety has been conducted extensively, with particular focus on the leakage, diffusion, combustion, and explosion processes. Both theoretical research and engineering applications have advanced significantly. In particular, hydrogen detection technology, primarily based on electrical measurement, has matured considerably, while schlieren imaging-based flow field visualization technology is progressing steadily. In contrast, safety research concerning ammonia remains in its early stages. Research on the leakage and diffusion characteristics of ammonia predominantly focuses on liquid ammonia, with a strong emphasis on engineering applications. Studies on the combustion and explosion characteristics of ammonia primarily address flame parameters and the combustion development laws. Ammonia serves as an efficient hydrogen storage medium. The conversion process involving hydrogen and ammonia will occur simultaneously in both time and space. Current research has not adequately addressed the safety concerns associated with the application process of hydrogen–ammonia mixtures. Future research on the safety of hydrogen–ammonia application processes should focus on the diffusion characteristics and combustion and explosion behaviors, as well as the development of electrical measurement detection technologies and optical flow field visualization techniques for hydrogen–ammonia mixtures.