Combustion research using microgravity environment was initiated by the droplet combustion experiments by Kumagai et al. in 1950s. The experiments clearly demonstrated that combustion phenomena on earth are strongly affected by buoyant convection. Also, it triggered to accelerate combustion research by using large-scale drop towers/shafts, in world-wide. Various excellent research results have been obtained through those experiments. Based on the accumulation of such research by short-duration microgravity experiments, the first on-orbit Japanese combustion experiment, called “Group Combustion”, was performed in the Japanese Experiment Module “Kibo” aboard the International Space Station (ISS) in 2017. Also, the solid materials flammability experiment, called “FLARE”, has been conducted in Kibo since 2022. The Solid Combustion Experiment Module (SCEM) was developed by JAXA for the FLARE experiment. The SCEM has superior features to conduct various types of combustion experiments. At present, 2 experiments have been continuing their preparation toward the flight experiments. Also, early-phase considerations of the flight experiments have been performed for another 3 experiments. Most of such experiments, to be performed in future, plan to utilize the SCEM with new experimental inserts and some devices as necessary. As a future development of the combustion research in Kibo, JAXA plans to establish a new Kibo utilization platform on combustion research field. It is supposed that the SCEM is utilized as the core infrastructure to accommodate future combustion experiments. The combustion research platform is expected to shorten the preparation duration on the selected experiments with standardization of the experiments to be performed. In addition, the platform is expected to increase the users of the on-orbit experiments including those from industry.

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Since Kumagai and Isoda conducted the first combustion experiment in microgravity in 1950s, droplet-combustion experiments have been evolving. Recently, a wide variety of multiple-droplet-combustion experiments has been conducted in space-based microgravity environments, such as “Group Combustion” experiments with randomly distributed droplet clouds, droplet-cloud elements, droplet-cluster array, and movable droplet arrays conducted aboard the Japanese Experiment Module “Kibo” on the ISS, and “Phoenix-2” experiments with droplet arrays using TEXUS sounding rocket. This paper describes some key techniques in droplet-combustion experiments, such as tethering droplets in space using fine SiC fibers and droplet formation on the SiC fibers by supplying liquid fuel through fine glass tubes. As recent experiments using the techniques, we report some results from “Group Combustion” experiments and the outline of “Phoenix-2.”

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FLARE, an international research project on fire safety, was initiated in 2012, and long-term on-orbit combustion tests have been conducted using SCEM on the ISS/Kibo. The objectives of the FLARE project are to investigate flame spread behavior over solid materials in microgravity and to establish a flammability evaluation method for providing an index for materials used in reduced gravity environments. This article presents an overview and history of the FLARE project, followed by recent on-orbit experimental results. Finally, the successive project, FLARE-2, is introduced as the future direction of the FLARE initiative.

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Flammability limit is a concentration limit for sustaining steady combustion. Measuring an intrinsic flammability limit, which is independent of experimental apparatus, has been a fundamental interest in combustion science for over a century. This report reviews pivotable studies and presents a comprehensive viewpoint on flammability limits. A series of microgravity experiments and numerical simulations with counterflow premixed flame have revealed that the factors like Lewis number, flame stretch, and radiation heat loss significantly influence near-limit combustion dynamics, influencing the flammability limit. Extensive researches have also been conducted on the flame ball, a steady spherical flame observed in a quiescent premixture at leaner condition than the flammability limit of conventional flames under microgravity. The final goal is to unify the intrinsic flammability limit that comprehending propagating flame and flame ball, which also covers recent observations on several peculiar combustion behaviors using low speed counterflow field under microgravity.

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The present paper provides an overview of our ongoing research on aluminum jet flames and summarizes our previous findings. In our prior studies, we identified the flammability limits of aluminum jet flames and examined their combustion behavior. Additionally, we employed machine learning techniques to classify and identify the boundaries between steady-state combustion, the transition from steady-state to unsteady-state combustion, and unsteady-state combustion itself. Multiple simulations using Support Vector Machines (SVM) yielded a high average accuracy of 99.4 %, demonstrating the effectiveness of the combined clustering and SVM approach. By applying the results obtained from machine learning to experimental parameters, we predicted combustion behavior under various conditions. While the predictions for steady-state combustion matched the experimental results well, the boundaries between transitional and unsteady-state combustion remained unclear. Nevertheless, the application of machine learning proved highly beneficial in understanding the combustion behavior of aluminum jet flames. These insights are expected to contribute to the development of stable combustion technology for aluminum jet flames and, ultimately, to the realization of the aluminum energy cycle.

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