Shape and Sooting Properties of Methane Laminar-jet Diffusion Flames in Microgravity

Shape and sooting properties are fundamental characteristics of hydrocarbon fuel diffusion flames. The study of laminar diffusion flame behavior under microgravity conditions provides a crucial approach for elucidating the physical and chemical mechanisms of diffusion combustion and for establishing turbulent diffusion combustion models. On-orbit microgravity experiments were conducted on coaxial coflow methane laminar jet diffusion flames using the combustion science experiment cabinet aboard the Chinese Space Station, with a focus on analyzing the influence of coflow conditions on flame morphological features and soot characteristics under microgravity. The main experimental conditions included: coflow gases with a high-oxygen concentration, air, N₂-diluted air, and Ar-diluted air; a coflow velocity to jet velocity ratio of less than 0.5; methane flow rates producing both far-field and near-field jet flames; and ambient temperature and pressure conditions. The results indicate that a simplified model based on jet flow field similarity theory can effectively predict the shape of microgravity flames in the far-field region of the jet. The coflow composition affects the flame shape by altering the combustion stoichiometry. The near-field flame length is independent of the coflow velocity but inversely proportional to the stoichiometric mixture fraction Z_st, while the maximum flame diameter is proportional to the square root of the inverse of Z_st. When air is diluted with inert gases in the coflow, the soot formation-dominated zone in the jet diffusion flame decreases, whereas the soot oxidation-dominated zone expands. The soot content in the flame decreases with increasing volume fraction of inert gas, with the dilution effect and thermal effect on soot formation characterized by the inert gas volume fraction and flame temperature, respectively.