A new kind of flame: Observation of the discrete flame propagation regime in iron particle suspensions in microgravity

The rate of flame propagation is generally understood to directly reflect the burning rate of a combustible mixture, as it is proportional to the square root of the overall reaction rate in the flame. However, in the case of heterogeneous mixtures composed of spatially separated discrete heat sources, such as suspensions of solid-fuel particles, theory suggests that the flame propagation speed may become insensitive to the reaction rate. Such a flame propagation regime, known as the “discrete flame” in the literature, is predicted to occur in suspensions of fast-burning metal particles and (or) low-conductivity gas media where the flame propagation becomes dominated by the heat transfer between fast-burning heat sources. In an attempt to experimentally confirm the existence of the discrete flame regime, flame fronts propagating in transparent glass tubes through iron particle ( d 32 = 33 µm) suspensions in low-thermal-diffusivity oxygen/xenon mixtures were studied in a high-quality microgravity environment. The experiment was performed on board the European Space Agency MAXUS-9 sounding rocket that lifted off from the Esrange Space Center (Sweden) on April 7, 2017. In total, 41 combustion runs were completed in suspensions with 20% and 40% of oxygen content during the 12 min of microgravity. While the experimentally determined combustion time of a single iron particle differs by a factor of more than three in mixtures with 20% and 40% oxygen content, the flames in these mixtures were found to propagate with practically equal speed in accordance with the predictions of the discrete flame theory.