Droplet Combustion Experiments Aboard the International Space Station

This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 mm, ambient oxygen mole fractions between D. L. Dietrich ( ) · M. C. Hicks NASA John H. Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, USA e-mail: [email protected] V. Nayagam Case Western Reserve University, Cleveland, OH, USA P. V. Ferkul Universities Space Research Association, Cleveland, OH, USA F. L. Dryer Princeton University, Princeton, NJ, USA T. Farouk University of South Carolina, Columbia, SC, USA B. D. Shaw University of California, Davis, CA, USA H. K. Suh · M. Y. Choi Kongju National University, Chunan, South Korea H. K. Suh e-mail: [email protected] Y. C. Liu · C. T. Avedisian Cornell University, Ithaca, NY, USA F. A. Williams University of California, San Diego, CA, USA 0.1 and 0.4, ambient pressures between 0.7 and 3.0 atm and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 Hz. The results revealed that as the ambient oxygen mole fraction was reduced, the diffusiveextinction droplet diameter increased and the radiativeextinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygen/nitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O2/0.56 N2/0.30 CO2) and 0.17 (0.17 O2/0.63 N2/0.20 CO2) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inertgas suppressant (carbon dioxide) into a nominally air (1.0 atm) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasisteady burning after visible flame extinction. Verification 66 Microgravity Sci. Technol. (2014) 26:65–76 of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet