The first microgravity combustion experiments were conducted in 2017 aboard the Japanese Experiment Module Kibo on ISS, titled Elucidation of Flame Spread and Group Combustion Excitation Mechanism of Randomly Distributed Droplet Clouds (Group Combustion). The objective of this experiment is to study the effects of droplet interaction, radiative heat loss from a flame, and droplet motion during flame spread. This report describes some recent accomplishments from the Group Combustion experiments with droplet-cloud elements to study the effects of droplet interaction and randomly distributed droplet clouds with about 100 droplets to study the macroscopic characteristics of flame spread and local flame spread characteristics. Two types of anomalous behavior observed near the group-combustion-excitation limit are also reported, which had not been expected from parabolic flight experiments and percolation calculations to express the group-combustion excitation through flame spread over droplets.

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The Group Combustion experiment was performed on the orbit from September 2016 to July 2017, as the first combustion experiment in Kibo. For the on-orbit experiment, the Group Combustion Experiment Module (GCEM) was developed by JAXA. Main components of the GCEM was launched by the HTV-5 in August 2015. Through the preparation of the experiment, there were several technical challenges as well as unexpected difficulties including the loss of the camera units of the experimental apparatus by the launch failure of SpX-7 mission. In addition, the on-orbit experiment faced multiple technical issues during the operation. The user integration (UI) team and the investigators conquered such issues one by one, with close cooperation and ingenuity. As a result, acquisition of experimental data that are beyond expectation was achieved. In this review, various efforts for development of the experimental apparatus as well as the efforts on preparation and execution of the on-orbit experiment are introduced.

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Liquid-fuel combustion experiments titled “Elucidation of Flame Spread and Group Combustion Excitation Mechanism of Randomly Distributed Droplet Clouds (Group Combustion)” were conducted in 2017 as the first of the combustion experiments in the Japanese Experimental Module “Kibo” aboard the International Space Station (ISS). This paper reports temperature-field analysis by the TFP method based on visible light emissions for the flame spread of droplet-cloud elements composed of four droplets with two or three interactive droplets, which was conducted as part of the “Group Combustion” experiments. The results show that the position of the interactive droplets affects the flame spread around the droplets, and the flame-spread limit is extended by the interactive effect. If a burning droplet pre-heats a droplet existing outside the flame-spread limit, the pre-vaporization also extends the flame-spread limit around two interactive droplets.

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In “KIBO” of the International Space Station, microgravity combustion experiments of a fuel droplet array were performed to investigate the interaction between flame-spread and droplet motion. The fuel droplet array consisted of 2-4 fixed droplets and movable droplets (up to 16 droplets) and was suspended on a straight SiC fiber, which enabled the movable droplets to move in the fiber direction. The initial droplet diameter of a droplet array was 1.0 mm for all tests. As a liquid fuel, n decane was employed. The first fixed-droplet was ignited by a hot wire to initiate flame-spread along a fuel droplet array. Sequential direct images and backlit images of the spreading flame and the droplet array were taken alternatively during flame-spread. A movable droplet was accelerated by the spreading flame and moved away from the neighboring burning droplet. Termination of flame-spread due to droplet motion and droplet coalescence was observed. Flame spread behavior was categorized into four typical patterns. Parameters which determine flame-spread pattern was discussed.

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By integrating all degrees of freedom of the electric field and small ions with an ensemble averaging, we construct thermodynamic quantities for colloid dispersions. The unoccupied volume is accepted as the proper thermodynamic variable to describe the chemical and thermal equilibrium of the small ions in the region outside of all the particles. Using the electric potential of the dispersion which satisfies a set of equations derived by linearizing the inhomogeneous Poisson-Boltzmann equation, we calculate the electric part of the internal energy and obtain that of the Helmholtz free energy by applying the Legendre transformation with respect to temperature. The electric part of the Gibbs free energy can be deduced from that of the Helmholtz free energy by the two means of the total sum of chemical potentials and the Legendre transformation with respect to the unoccupied volume. Thus the thermodynamic theory has been successfully formulated for the colloid dispersion. The pair potential derived from the Gibbs free energy possesses a medium-range strong repulsive part and a long-range weak attractive tail being additionally affected by the fraction of the unoccupied volume.

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The Soret-Facet Mission was conducted in microgravity to measure the Soret coefficient ST using a two-wavelength Mach-Zehnder Interferometer (2-MZI). Interferometer analysis generally uses procedures like fringe tracing (thinning method) that impede processing speed and accuracy. In the present study, an automated phase analysis using spatio-temporal images was developed based on the simple conversion of intensity 𝐼(𝑡) into phase 𝜙(𝑡). This automatically analyzes steps, starting with obtaining the spatio-temporal images of moving interference fringes to calculating phase change Δ𝜙(𝑡). It dramatically improved processing speed and larger analyzing areas compared to the thinning method. Applying filters and a threshold to the fringes using suitable conditions—which the method determined automatically and quickly—was confirmed to be efficient for the correct unwrapping of phase.

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