Overview

In the United States, combustion processes contribute to about 85% of delivered energy and are an integral part in many industrial fabrication processes. Combustion produces greenhouse gases and soot, which contributes to global warming and causes significant health concerns. The Combustion Science Discipline conducts idealized experiments on the International Space Station where the removal of gravity enables researchers to study details of the combustion processes that cannot be readily studied on the ground. The Space Station environment also provides an important test-bed for studying spacecraft fire risk.

Combustion occurs when fuel and oxygen react to produce carbon dioxide, water and heat. For the foreseeable future, the overwhelming majority of delivered energy in terrestrial applications will be from combustion or other chemically reacting systems. These energy uses cover the range from electric power and transportation to processes directly tied to the delivered material (e.g., glass and steel manufacture). These processes produce some of the most important environmental hazards currently facing humanity (global climate change, acid gas pollution, mercury contamination from coal, and wild-land fires).

Despite being the subject of active research for over 80 years, combustion processes remain one of the most poorly controlled phenomena that have a significant impact on human health, comfort and safety. This is because the simplest combustor (e.g., kitchen stove) remains beyond our detailed numerical modeling capabilities. Typically, the combustion process involves a large number of chemical species (hundreds) and reactions (even thousands). It is these species and reactions that determine flammability limits (combustor operating ranges) and pollutant emissions. Much of combustion research involves developing a comprehensive and predictive quantitative understanding of this complex process. This understanding usually occurs in smaller, bench-scale experiments that are amenable to detailed study and modeling. In these bench-scale experiments, however, buoyant forces, created by gravity and the large temperature differential between the flame and the ambient gas, dominate and frequently obscure the physical and chemical phenomena of interest, and hence, complicate analyses. In the absence of gravity, buoyancy can be suppressed, and the analyses can be reduced to much simpler one-dimensional systems.

To understand the strong influence of gravity on flames, one can consider the density gradients in flames. Typical flame temperatures are on the order of 2,300 degrees Kelvin, whereas ambient temperatures are approximately 300 degrees Kelvin. This produces an eight-fold density change over the scale of a centimeter. The resultant density gradient induces a strong velocity field that dominates all but the highest flow fields. This flow field causes the flame to lift and point upward. The resultant flow instabilities cause the flicker typical of flames on Earth. Figure 1 contains a gallery of 1-g to 0-g comparison flames.

By varying or eliminating the effects of gravity, we can extract fundamental data that are important for understanding combustion systems. This approach has been implemented to some extent in existing terrestrial reduced-gravity platforms, but the experimental time scales and sizes have been limited. Long-duration

Current Combustion Experiments

Archived Combustion Experiments

Combustion Science Facilities/Areas on the ISS