Flame spread over charring and non-charring materials in microgravity on ISS/Kibo

As part of the FLARE project, long-duration microgravity experiments were conducted on two additional materials—non-charring PMMA and charring NOMEX HT90-40—using the Solid Combustion Experimental Module (SCEM) onboard the ISS/Kibo module. Flammability maps were obtained under opposed-flow velocities, including quiescent conditions, extending previous studies performed with filter paper. For PMMA, both the limiting oxygen concentration (LOC) and the minimum LOC (MLOC) agreed well with predictions from a simplified two-dimensional scaling model, confirming its applicability to thermally thin, non-charring materials. In contrast, NOMEX exhibited robust three-dimensional spherical flames once the two-dimensional thermal balance broke down, even at moderate flow velocities. Under these conditions, the flame radius R f decreased with decreasing opposed-flow velocity, and extinction occurred when R f reached a critical value. To quantify this behavior, the preheat-zone length L g of three-dimensional flames was modeled as a function of R f and the Reynolds number Re , and incorporated into the thermal balance to derive a limiting oxygen concentration for three-dimensional flames. The resulting expression reproduced the observed relationships among R f , V g , and L g , and correctly predicted the extinction behavior. Applying the same formulation to filter paper and PMMA further demonstrated that the critical flame radius provides a unified criterion for the transition and extinction of three-dimensional flames across different material classes. These findings demonstrate that both the two-dimensional and three-dimensional flammability limits of charring and non-charring materials can be predicted within a unified experimental–modeling framework, and they provide essential guidance for advancing microgravity fire-safety modeling. Novelty and significance statement: The novelty of this work lies in establishing a unified, physics-based framework for predicting flame-spread limits of both charring and non-charring thermally thin materials in microgravity. First, long-duration ISS experiments demonstrated that the limiting oxygen concentration (LOC) and minimum LOC of PMMA are accurately captured by a simplified two-dimensional model, confirming that extinction is governed by the breakdown of two-dimensional thermal balance. A second and central contribution is the quantitative characterization of three-dimensional spherical flames observed in NOMEX beyond the two-dimensional limit. By modeling the preheat-zone length L g as a function of flame radius R f and Reynolds number Re and incorporating this into the thermal balance, an explicit LOC criterion for three-dimensional flames was derived. Applying the same formulation to filter paper and PMMA showed that the critical flame radius provides a consistent extinction threshold across materials, offering a unified experimental–modeling basis for predicting flame-spread limits in microgravity.