The team was led by Forman Williams, a professor of mechanical and aerospace engineering at the University of California, San Diego.

Spherically symmetrical combustion of liquid fuel droplets has been considered for decades in combustion research, the authors note. An advantage of spherical symmetry is that only one spatial dimension enters the description of the combustion process, enabling one-dimensional, time-dependent conservation equations to apply. This facilitates both computational and theoretical descriptions of the problem, and thus enhances understanding of experimental results. Multi-dimensional situations make the problem more difficult, uncertain and inaccurate, they observed.

Normal gravity destroys the spherical symmetry of the combustion process; microgravity experiments enable achieving spherical symmetry. Until the ability to perform space-based experiments arrived, however, microgravity work was limited to drop-towers and aircrafts flying parabolic trajectories—conditions notably lacking in the ability to provide longer observation times.

Space experiments provide the opportunity to investigate phenomena such as departures from quasi-steady combustion, transition from diffusive to radiative extinction, liquid-phase transport, influence of convection on flame dynamics, effect of sooting, multi-component droplets and flammability limits.

—Dietrich et al.

During the experiments, researchers ignited large droplets of methanol (as a representative alcohol fuel) and heptane (as a typical alkane fuel); initial droplet droplet diameters were between 1.5 and 5.0 mm, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 atm and ambient environments containing oxygen and nitrogen were diluted with both carbon dioxide and helium.

The experiments … 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 quasi-steady burning after visible flame extinction.

Verification 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 combustion.

—Dietrich et al.

The cool flames occurred in a wide range of environments, including air similar to the earth’s atmosphere and atmospheres diluted with nitrogen, carbon dioxide and helium. The resulting combustion reaction creates toxic products, such as carbon monoxide and formaldehyde, which in turn burn off.

Droplet and flame histories for a heptane droplet. The gap in the droplet history is when the droplet drifted out of the field of view of the camera. The thin vertical lines with letters correspond to the different times of the image sequencing shown in the paper. The thick dashed vertical line (coincident to when the flame diameter and standoff decrease quickly) is when visible flame extinction occurred. The chart shows that after visible flame extinction, droplet vaporization continues for an extended period before an abrupt plateau. The plateau in the droplet history slightly precedes the appearance of a very large vapor cloud in the color camera view. The linear vaporization behavior in this ‘cool flame’ region occurs as a result of low-temperature chemical reactions that somehow are initiated by the visible hot-flame extinction … These droplets exhibited strong vaporization and appreciable radiometer signals after the early disappearance of visible flames, indicating continued chemical heat release from flames that could not be imaged with the available instrumentation. — Dietrich et al.

Researchers believe that the cool flames are the result of elementary chemical reactions that do not have the time to develop around burning fuel droplets on earth, where they can only exist for a very short period of time. When droplets of fuel burn on earth, buoyancy limits the amount of time gases can hang around in the high temperature zone around the droplets. So there isn’t enough time for the droplets’ chemistry to support the cool flames. But in microgravity, there is no buoyancy, so there is enough time for the gases to stay around the droplets and for that chemistry to develop.

The challenge for future applications is to get the right mix of fuels to generate this cool flame combustion here on earth. To investigate this question, NASA is planning a new series of experiments tentatively called Cool Flame Investigation, starting next winter and continuing for about a year.

Researchers emphasized that the research is only possible on the ISS, where scientists have access to a microgravity environment that provides a sufficient amount of test time for cool flames to occur.

All the experiments take place in the Multiuser Droplet Combustion Apparatus that can generate and ignite droplets from different fuels in different atmospheric conditions. The chamber is crammed with sensors and equipped with video cameras that record experiments. The chamber is inside an experimental facility called the Combustion Integrated Rack, which is roughly the size of a 5.5-foot bookcase and weighs close to 560 lbs (254 kg) and which records the data and transmits it to ground. The Combustion Integrated Rack is located in the Destiny module of the ISS.

The experiments are run by remote control from NASA’s John Glenn Research Center in Cleveland. Results were analyzed by a team of scientists from UC San Diego, the University of Connecticut, NASA, Princeton, the University of South Carolina, UC Davis, and Cornell.

Resources

• Daniel L. Dietrich, Vedha Nayagam, Michael C. Hicks, Paul V. Ferkul, Frederick L. Dryer, Tanvir Farouk, Benjamin D. Shaw, Hyun Kyu Suh, Mun Y. Choi, Yu Cheng Liu, C. Thomas Avedisian, Forman A. Williams (2014) “Droplet Combustion Experiments Aboard the International Space Station” Microgravity Science and Technology doi: 10.1007/s12217-014-9372-2