Influence Of Microgravity On Laminar Jet Diffusion Flame’s Image Parameters And Combustion Characteristics

Fire research under microgravity condition is an important aspect in fire science. Microgravity environment is often referred to the ambients as International Space Station and Space shuttle in orbits where the gravity level is lower than normal gravity. These space facilities typically have low air currents up to0.3m/s that are induced by the heating and ventilation systems, and such a low-speed air stream can make a microgravity flame stronger. In quiescent and microgravity environments, flames are often symmetrical in shape and nearly invisible, flames are often cooler and less radiant and soot-particle sizes are greater than normal gravity, under low rates of imposed air flow, flames intensify and become bright and sooty, flame characteristics used for fire detection in normal gravity have been studied widely, but these differences in fire characteristics compared to those in normal gravity strongly influence the detector to sense the incipient fires.Because it is difficult to put out fires and organize rescue after the fire happens and the political and economic value of these environments are huge, it is crucial to put more attention on the fire safety problem in microgravity environment. So the fire detection methods based on researches of normal fires cannot be directly used to solve problem of microgravity fire detection, it is important and necessary to study the difference between normal gravity and microgravity flames under still and coflowing air conditions.Laminar jet diffusion flames are of interest because they provide ideal flame systems that are far more tractable for theoretical and experimental studies than practical turbulent diffusion flames. Hence, experiments were carried out in the Drop Tower Facility of National Microgravity Laboratory of China, and observations based on short-duration experiments under microgravity of the characteristics (flame shape, flame color, flame temperature, flame radiation and flame oscillation fruquency) of laminar jet diffusion flames burning in still and coflowing air conditions are described. So the aim of this paper is to study the variation of image parameters and combustion characteristics for laminar jet diffusion flames under normal gravity and microgravity environments.We introduced integrated flame shape (maximum flame diameter and ratio of flame length to maximum flame diameter) and residence time formulas, integrated analysis solutions of the maximum flame diameter and Lf/Wmax for microgravity laminar diffusion flames in both still air and coflowing air were obtained, which predict well with the present and previous studies. Also integrated analysis solutions of residence time in gas jet flames are presented considering the buoyancy and air-velocity effects, which were given to analyze how the radiation changed with residence time varied both in normal gravity and microgravity.The experimental results obtained show that the characteristics of microgravity laminar jet diffusion flames were significantly affected by air-flow velocities for the absence of buoyancy. Under microgravity, with increasing coflowing air velocity, the mixing rate of fuel and air increased and the flame sheet moved closer towards the nozzle, maximum flame diameter decreased with air flow velocity increasing; in contrast, the air velocity had little effect on flame shapes in normal gravity.The experimental results show that normal gravity flames are brighter than microgravity flames, and the long residence time for microgravity flames result in enhanced soot formation and increased radiative loss, which lead to local extinction and low temperature at flame tip and soot changes color into dull red. Flame radiation fractions for microgravity flames are larger than those in normal gravity both for C2H4and CH4, and coflow air velocity has a much more pronounced effect on radiation fractions for microgravity flames compared to those in normal gravity, radiation fractions for ethylene-fueled flames in microgravity are large leading to local extinction at flame tip, and the analysis solution of flame radiation fraction was obtained.Compared to normal gravity flames, the phenomenon of flame oscillation can’t be obtained for diffusion flames in microgravity, and for normal gravity flames, the flame oscillation frequency can be suppressed when air coflow velocity is large enough (critical air coflow velocity), the relation of flame oscillation frequency and air coflow velocity is obtained, and the relation of critical air coflow velocity and initial fuel flow velocity is also studied.