Whether suspended in midair or burning their way through a ball of plastic, flames have consumed the attention of combustion researchers in experiments for years, both on Earth and in space. Further refining our understanding of the structure and performance of flames is the focus of two experiments on the International Space Station -- one with gases and one with solids.
The Structure and Liftoff in Combustion Experiment, or SLICE, examines co-flow, laminar, diffusion flames -- where the fuel and oxidizer are not premixed. This investigation focuses on the flames that lift away from the burner, floating in midair.
The Burning and Suppression of Solids, or BASS study, looks at flames from a variety of burning materials with different shapes. Researchers use this investigation to assess the effectiveness of nitrogen in suppressing microgravity fires.
For both experiments, the goal is to understand the basic structure of flames and fires. With the results of SLICE and BASS, researchers can refine computational models and put them to practical use on Earth and in space.
"You'd like to be able to sit down at your computer and design an automobile engine or an airplane engine or a boiler to be as efficient as possible, producing as few pollutants as possible" said Marshall Long, SLICE principal investigator and Yale University professor. "We're trying to do experiments that will provide a better understanding of the range of conditions that models can predict."
To produce better models, scientists need reliable data that is free of extraneous influences. On Earth, flame studies are affected by gravity-induced instability. Earthly buoyancy -- which makes hot gasses rise -- causes flickering, even when the air is still. If researchers can reduce that buoyancy to near zero, they can study a broader range of distinct flame characteristics that can be masked by Earthly conditions. Microgravity flames also can be soot free or high in soot depending on flow conditions, whereas flames on Earth often show little variation.
Gravity influences flame structure on Earth, including temperature and molecular distribution, size, shape, etc. The environment of space, however, causes changes in the transport of both heat and molecules for flames, although the fundamental chemical reactions are unchanged. "Microgravity is interesting because things behave differently. If there is no gravity, hot air doesn't rise anymore," said Long. "That's why the space station is the ideal place to do these experiments."
While short-duration bursts of microgravity can be achieved in drop facilities, for about 5 seconds, or in microgravity aircraft, 20 seconds, experiments and the resulting data are limited by time and g-jitter—oscillations in the apparent gravity caused by aircraft vibrations. In space, however, researchers have the benefit of longer tests without disturbances.
"When you do experiments in microgravity, you can develop models without the complicating effects of buoyancy," said Paul Ferkul, BASS principal investigator, NASA's Glenn Research Center, Cleveland. "Gravity, from a modeling point of view, is a very difficult thing to account for."
Using the space station as a lab, the SLICE study researchers look at non-premixed flames, meaning that the fuel and oxidizer initially are separate. That is the kind of combustion that occurs in diesel engines, gas turbines, and boilers. SLICE used methane and ethylene, sometimes diluted with nitrogen, as fuel. When flow conditions were right, the flame detached from its source and moved downstream. A flame that is not anchored to the burner can be modeled more accurately.
"When the flame moves downstream, sometimes it just blows out, like blowing out a candle on a cake," said SLICE Project Scientist and Co-investigator Dennis Stocker, NASA Glenn. "But in other conditions you can establish a stable lifted flame. It seems to be just floating in midair above the burner."
The BASS investigation further explores flame behavior by looking at the burning characteristics and extinction of a wide variety of solid fuels in microgravity. It tests whether materials with adequate ventilation in microgravity burn as well as or better than the same material in normal gravity with identical pressure, oxygen concentration, temperature, etc.
Results from this study will contribute to the combustion computational models used in the design of fire detection and suppression systems both in space and on the ground.
On Earth, the best way to put out a fire is to aim the extinguisher at the base of the flame. In microgravity, trying to use the same extinguishing techniques might actually make the fire worse by providing oxygen to the flame.
During BASS operations, the crew burns a variety of solid materials in different shapes under different flow conditions in the Microgravity Science Glovebox. The geometries under study are either spherical or flat. The flat fuel is as thin as a sheet of paper or fabric about the size of a 4-inch long piece of tape, or 2 by 10 centimeters. Spherical samples are three-quarter-of-an-inch (2-centimeter) diameter balls of plastic.
"We're trying to build up a database of different solids burning in zero gravity at different conditions and different geometries," Ferkul said.
One of the fuel sources is polymethylmethacrylate, or PMMA, better known by the brand names such as acrylic and Plexiglas. BASS experiments also burn two other plastics and a cotton and fiberglass custom fabric designed specifically to study flame spread. BASS uses nitrogen as an extinguishing agent.
Knowledge from space station flame studies like SLICE and BASS will contribute not only to better models, but also to practical applications such as reducing pollution. Even small gains in combustion efficiencies can lead to significant improvements.
"If you add up coal, petroleum, natural gas and so on, the U.S. is burning on the order of a trillion dollars a year in fuel," Stocker said. "While SLICE doesn’t apply to all types of combustion, given the amount of fuel burned, even small improvements in fuel efficiency could lead to billions of dollars in fuel savings as well as reduced emissions here on Earth."
And that, for these scientists is one of the important things. While the basic science is significant, by doing research in space, they can improve life on Earth.