Combustion characteristics of microgravity multi-jet flames aboard the Chinese Space Station

To better understand flame interaction and the variation in flame shape and radiation in spacecraft fire scenarios, this study presents a detailed experimental investigation of multi-jet flame combustion conducted aboard the Chinese Space Station under microgravity conditions to examine the effects of velocity and dilution on combustion characteristics. In-situ optical diagnostics, including flame luminosity and chemiluminescence imaging, were employed to characterize flame structure, soot distributions, and the distribution of excited radicals. Results indicate that microgravity fundamentally alters combustion behavior by suppressing buoyancy and enhancing diffusion-dominated transport. Pure methane flames exhibit reduced luminosity due to lower temperatures, N 2 -diluted flames show enhanced brightness driven by flow-structure confinement and localized soot accumulation under microgravity. Quantitative analysis reveals that flame lengths increase monotonically with fuel velocity but exceed theoretical predictions due to muti-jet interactions. With a 60% increase in fuel flow rate, the flame heights of pure methane, methane diluted with 50% N 2 , and the 12.5% O 2 enriched mixture increased by 82%, 49%, and 57%, respectively. These divergent growth rates suggest that the flame's axial development is highly sensitive to both the dilution effect and the chemical reactivity of the mixture. Furthermore, the flame aspect ratio increases approximately linearly with the fuel velocity and is positively correlated with the radiative fraction, indicating a coupling between flame elongation and radiative heat transfer. The distributions of OH* and CH* emissions further elucidate the heat-release characteristics and jet-merging behavior, which are controlled by fuel properties and the interplay between momentum and diffusion. The study finds that nitrogen addition delays flame merging, whereas oxygen enrichment promotes it. These findings advance the fundamental understanding of soot formation, heat-release distribution, radiative transfer, and flames interaction in diffusion-dominated multi-jet combustion, while providing a valuable experimental benchmark for predictive model development and for the future design of multi-element combustion systems operating under microgravity or partial-gravity conditions. Novelty and significance statement The novelty of this research lies in the use of a three-jet array not as a simple extension of a single-jet burner, but as a controlled configuration for resolving flames interactions under sustained microgravity. The suppression of buoyancy aboard the Chinese Space Station enables the respective effects of jet momentum, molecular diffusion, chemical kinetics, soot transport, and radiation to be distinguished more clearly than under normal-gravity conditions. The measurements demonstrate that interactions among adjacent jets restrict oxygen entrainment, resulting in departures from conventional single jet flame length and flame width scaling, while fuel composition governs the height and extent of flame merging. In addition, a quantitative correlation is established between the flame aspect ratio and the radiative fraction. The significance of this study lies in providing new benchmark data and physical scaling relationships for interacting diffusion flames under reduced-gravity conditions. The findings are relevant to multi-element combustion systems for space-based power generation and propulsion, as well as to the fundamental understanding of flame interaction and radiative hazards in spacecraft fires.