Soot formation in high-pressure combustion: Status and challenges

Combustion of hydrocarbon fuels lacks accurate and detailed reaction mechanisms, and the majority of flame research is focused on atmospheric conditions, with little progress made in high-pressure environments. How-ever, the high-pressure environment dominates the actual fuel combustion, such as diesel engines, automobile engines, and aircraft engines. Thus, understanding and mastering the combustion characteristics and soot for-mation mechanisms in high-pressure environments can significantly improve energy efficiency and reduce pollutant emissions, contributing to the goal of "carbon peaking and carbon neutralization". This review presents a comprehensive overview of cutting-edge soot research in high-pressure combustion conditions, while also analyzing and summarizing the existing research issues. Specifically, the review discusses the soot formation mechanism in laminar diffusion flames and related diagnostic techniques. The analysis then delves deeper into the study of soot in high-pressure environments, categorizing and introducing various fuels. The analysis reveals that as pressure increases, soot particle size, concentration, and flame temperature all increase. Furthermore, there exists a certain relationship between the soot volume fraction (fv) and the pressure (p), i.e.,fv = Cpn. Recent simulation studies on diffusion flame under high pressure are discussed, along with various factors related to flame stability and the soot formation path. Finally, the challenges in the current high-pressure combustion research are identified, and an outlook for the future is put forward.