Combustion Physics - Why? Studying Gas Jet Flames Studying The Simplest Case Building on our Previous Experience A Simple Return on Investment Calculation

Combustion physics is the science of burning. This area of research is guided principally within NASA at the NASA/Lewis Research Center in Cleveland, Ohio. In the absence of gravity, combustion takes place in a very different manner than in a 1-g environment like we have here on Earth. Gravity plays a role in why flames "shoot upwards", smoke rises, and large air circulation currents are established. These effects can mask many of the physical processes that occur, preventing us from understanding what exactly is happening. Despite the fact that combustion is central to life in the 20th and 21st century - it powers our automobiles, generates our electricity, and heats our homes, cooks our food on the back-yard grill, and can add many pollutants to our atmosphere - we have much to learn about the physics of combustion.

Studying Gas Jet Flames

Gas jet flames of one sort or another are common in everyday life. Whether we use a gas stove in our kitchen, a bunsen burner in our laboratory, a butane lighter, or a propane torch, burning jets of gas represent one of our most common implementations of combustion. Although we use these tools often, we still only have a limited understanding of the detailed physics and processes that occur in these commonplace flames.

The Enclosed Laminar Flames (ELF) experiment, to be performed in the middeck glovebox aboard USMP-4 will provide insight into how various air flow velocities can influence the stability of the flame. On the ground we are familiar with how the wind can cause, for example, a candle flame to flicker, become highly unstable, and even produce increased amounts of soot. Studying the influence of these air currents on the ground is extremely difficult because of the presence of additional convection currents that are driven by gravity. By going into space, we can study the perturbing effects of air currents on the flames without the additional complication of convection-driven air flow at the same time.

Studying The Simplest Case

If you want to study something complicated, it often helps to break the system down in to simpler components. Unfortunately with combustion, gravity is the thing that adds most of the complications in one way or another. However, on the space-shuttle, most of the effects of gravity are removed, and in the case of combustion, we can study the simplest cases of burning, whether they are spherically symmetric drops of fuel such as was the case on MSL-1's Droplet Combustion Experiment, or the highly stable, smoothly flowing jets of gas as part of the Enclosed Laminar Flames experiment that will be studied on USMP-4. Reducing the complex interaction of gravity-driven perturbations to fire can greatly enhance our ability to study and understand a highly complicated series of interacting processes.

Building on our Previous Experience

Combustion experiments stole much of the show on the most recent MSL-1 Microgravity Science Laboratory flight in July, with "fires in space" getting most of the news coverage during the mission. All the combustion experiments achieved more than their planned runs as they burned virtually every drop or whiff of fuel available to them. Each experiment took different, complementary approaches to studying the basics of combustion.

The Structures of Flameballs at Low Lewis numbers (SOFBALL; in the Combustion Module (CM-1) made the biggest news with the tiniest fires, flame balls about the size of a pinhead and glowing with 1/50th the energy of a birthday candle (pictured at right). To everyone's joy, the flameballs, generated by electric sparks, burned - motionless in their chamber - for 500 seconds when the experiment was designed to blow them out.

These are believed to be the weakest fires ever stoked, and should lead to clues on how to design engines that burn with leaner fuel-air mixtures and thus produce less pollution. SOFBALL had 15 tests planned; 26 were completed for a total of three hours of burn time that will be studied for years, says alternate payload specialist Paul Ronney, who is also the principal investigator. SOFBALL results will affect fire safety on Earth as well as aboard spacecraft.

The Laminar Soot Processes (LSP) experiment, working somewhat like a Bunsen burner in space, produced flames twice as large as those formed on Earth and which appeared as steady as freeze-frames on TV. The laminar (smooth flow) flames formed soot, a pollutant, sooner than expected. Scientists also saw flames extinguished by energy radiating from soot, a new phenomenon that will alter studies for years to come. The results should also resolve a controversial hypotheses that will simplify how flames are modeled. Of 14 planned tests, 19 were conducted. Follow-on experiments to study laminar jet flames are planned with the Enclosed Laminar Flames experiment aboard USMP-4.

The Droplet Combustion Experiment (DCE), with its own facility, released droplets of hydrocarbon fuel (less than 1/6 ounce for the entire flight) into a chamber then ignited it with heated wires. This provided "one dimensional" models of how the flame moves inward and exhaust products move outward. Each droplet is really three-dimensional, but droplets pull themselves into spheres, one dimension can describe the whole droplet, making modeling easier. DCE achieved 56 tests, 21 more than the planned 35.

Fiber Supported Droplet Combustion (FSDC-2; in the glovebox) ignited larger drops held in place on a fireproof thread (the drops were large enough that the thread is a minor factor). The payload crew became so adept at these experiments that they performed 73 tests beyond the 52 planned, including the first-ever experiments with two droplets next to each other. The drops were forced apart by their exhaust products, then pulled together as they depleted the fuel vapor between themselves. This was named the "Thomas Twin Effect" in honor of mission specialist Don Thomas who performed the experiment.

A Simple Return on Investment Calculation

Do you know the potential for return on investment in Combustion Research?:

In any area of the economy where a huge amount of money is spent, even the most modest improvements in efficiency can mean savings of very large amounts of money. One goal of the combustion research within NASA's microgravity science program is to generate knowledge that may eventually lead to more efficient combustion, and therefore a saving of fuel.

In the first half of 1996, the United States imported an average of 9,285,000 barrels of oil per day.* In addition, to this imported oil, we used about 18,000,000 barrels of domestic oil per day.* On September 9, 1996, a barrel of OPEC crude oil cost $20.34.* One can therefore estimate the yearly expenditure on crude oil as nearly $200 billion.

A mere 1 percent increase in fuel efficiency, like taking your gas mileage from 25 miles per gallon to 25.25 miles per gallon, would translate into a savings to America of nearly 100 million barrels of oil a year (roughly $5.5 million per day), repaying more than the cost of the entire mission every year.

*Data Obtained from the American Petroleum Institute Web-Site, http://www.api.org/news/factglnc.htm

Did You Know That:

The cost of a space shuttle mission is about $2 per person in America. In 1992, Americans spent over $114 billion dollars at the pump, or about $456 per person.** So, the same 1 percent increase in efficiency above, which each American taxpayer would pay $2 to obtain, would save them $4.56 each year.

**Data obtained from 1992 Census of Retail Trade published by the Census Bureau 

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last updated October 29,1997

Some text adapted from the Microgravity Science Newsletter - Winter 1996

Author: Dr. John Horack
Curator: Linda Porter
NASA Official: Dr. Greg Wilson