Combustion is the chemical reaction that occurs between substances when materials burn. The flame of a lit match, for example, consumes the matchstick as fuel and produces light, heat, carbon dioxide, and water vapor. On Earth, these combustion products rise due to buoyancy, resulting in an elongated, upward flame. 

But, in the microgravity environment of the orbiting space station, the loss of gravity-driven phenomena like buoyancy and sedimentation prevent combustion products from rising up and drifting away. Instead, combustion products tend to diffuse evenly around the fuel source. In space, flames form spheres.

 It may sound hazardous to study combustion on a spacecraft; the environment inside the space station is pressurized, oxygenated, and sealed. If a fire breaks out, leaving the spacecraft is not an easy option. To study combustion safely, astronauts use facilities like the Combustion Integrated Rack (CIR) and the Microgravity Science Glovebox to contain their experiments. 

Combustion research in microgravity has led to some incredible discoveries. On orbit, flames tend to use less oxygen and burn at lower temperatures than flames on Earth. And, under certain conditions, fuel droplets continue to ‘burn’ even after the flame appears extinguished, a phenomenon called ‘cool flames.’ Understanding how flames behave in space is more than just cool physics; it’s also critical for improving fire suppression methods for spaceflight.

Studying combustion in microgravity can also be highly beneficial for Earth industries. Currently, about 80% of the world’s energy still comes from fossil fuels like oil, coal, and natural gas. When these fuels burn, they release carbon residue particles, or soot. Soot is a form of air pollution which is created when hydrocarbon fuels don’t fully burn. On Earth, buoyancy-driven convection spreads these toxic particles through the air, causing environmental damage and negative health effects.

As the world transitions to renewable energy, microgravity research can aid this transition by improving combustion processes on Earth. Because fires in microgravity form spheres, the soot particles remain in the flame longer instead of rising up and dispersing in the air. As a result, all or almost all the soot particles are consumed in combustion. Modeling the behavior of these particulates in microgravity can help improve the efficiency and reduce the soot pollution from fossil-fuel driven powerplants and combustion engines.