Measured and computed temperature profiles of coflow laminar diffusion flames of 65% methane and 35% nitrogen in both normal gravity and microgravity. See large image for a side-by-side comparison. (Yale University) The price of gas is skyrocketing, but thankfully so is a potential mitigation. The recent mission of the Space Shuttle Discovery, which returned home on March 9, 2011, included the delivery of fuel and equipment for the Structure and Liftoff In Combustion Experiment, or SLICE, to the International Space Station. The goal of SLICE is to use microgravity flame studies to learn more about how combustion works and ways to make it more efficient.
Flame studies on Earth are hampered by gravity-induced instability - picture a flickering candle flame - which complicates analysis. This is due to buoyancy, which is when less dense materials rise within a fluid of greater density - this time imagine a hot air balloon. Buoyancy is nearly absent in microgravity, making it possible to study a broader array of flame characteristics, such as the range of soot concentration and the flame temperature.
Project scientist and co-investigator Dennis Stocker explains the benefits of investigating flames in space, "It is possible to study spherical (i.e., one-dimensional) flames in microgravity, which is not possible in Earth gravity, but dramatically simplifies the otherwise complex analysis. Microgravity flames have large scales and long residence times, allowing for improved studies of the flame structure and soot, respectively…An understanding of lifting and lifted behavior is also valuable, because of the importance of flame stability and the potential benefits for combustion at fuel lean conditions where both optimum performance and low emissions can be achieved."
Here on Earth, knowledge from space station flame studies can contribute to reduced pollution. Stocker points out that even small gains in combustion efficiencies can lead to significant improvements, given the high use of fuels to warm homes, cook food, and fuel vehicles from cars to spacecraft. "While we benefit greatly from combustion, it is a significant source of greenhouse gasses and contributes to the global climate change. Furthermore, combustion-caused pollutants, like soot, harm our health and unwanted fire remains an important safety risk."
Scheduled to operate in late 2011 or early 2012, SLICE will study the combustion of gaseous fuels in the microgravity environment of the space station. These fuels include varied dilutions of methane and ethylene. Methane is the main component of natural gas and ethylene is an example of fuels with a higher soot production. These were selected because their combustion chemistry is well understood, making the flames easier to computationally model.
Astronauts will run the investigation while communicating with ground researchers at NASA Glenn Research Center and Yale University. The scientists will examine the characteristics of the flames, such as color, shape, and the lifted distance from the burner. The resulting data will contribute to practical improvements in areas such as turbulent combustion, ignition, and flame stability via improved flame computational models. Using these superior models, engineers could design more efficient combustion devices for use in everything from in-home stoves to power plants and vehicles.
SLICE is a precursor to a series of future studies, known as the Advanced Combustion via Microgravity Experiments or ACME. Information from previous flame investigations is used to optimize scientific returns from each subsequent study. For instance, the Enclosed Laminar Flames or ELF investigation, flown on the space shuttle mission STS-87 in 1997, provided the basis for the hardware used for SLICE. The aim of SLICE and the entire ACME project is to advance energy technology - something that may one day contribute to greater fuel efficiency via improved combustion and possibly less pain at the pump.