Combustion Physics
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
Soot
The importance of soot to a fire is evident in the most simple of everyday examples. If you've ever built a fire in the fireplace, you know that the fire is brighter and feels hotter when you are 2-3 feet away, than does the flame on a gas stove. In fact, the temperature in a blue flame on a gas stove are considerably hotter than a typical wood fire. This is because soot radiates a large fraction of the flame energy as visible light and infrared "heat". Without the heat radiated by the soot, almost all of the heat from the fire would go up the chimney. Improperly controlled fires lead to release of excessive soot and the associated carbon monoxide. This carbon-monoxide emission associated with soot is the primary source of fire fatalities. To better understand the role of soot in combustion, we need to study flames that are free of effects induced by gravity. The Laminar Soot Process Experiment (LSP) on MSL-1 is designed to perform this study and will be executed in the Combustion Module Facility-1 (CM-1)of Spacelab..
The Laminar Soot Process Experiment is being performed on MSL-1 for four specific reasons:
- To build on previous experiments, and learn more about the properties of soot generated in special kinds of flames.
- To measure the concentration of soot, as well as the structure of soot produced, by different fuels, and changes in the combustion parameters.
- To develop and examine our theories on how soot forms, collects, and radiates energy.
- To examine the viability of relationships for the amount of soot produced and the temperature of flames in microgravity.
Great
Balls of Fire!!!
The SOFBALL (Structure of Flame Balls at Low Lewis-number) experiment will determine if you can actually build a stable, stationary, spherical ball of flame. If so, the experiment also will be able to determine what is the mechanism that allows the stable flame, as well as how various mixture properties, such as fuel/oxidizer concentrations and temperature, affect the flame-ball's stability and existence. The flameballs we hope to get from the SOFBALL experiment will be the first-ever balls of flame in space.
We simply don't understand the mechanisms of flame extinction (what makes the fire go out) or stability (what keeps it going) in pre-mixed gases used for combustion. These stationary spherical flame balls are the simplest case to study and learn from. By studying these flames, we'll have a better understanding of near-limit combustion, thereby leading to improvements in engine efficiency, reduced emissions, and fire-safety.
Dr. Paul Ronney, principal investigator on SOFBALL, wants to study flame balls because they are "the simplest possible flame one could envision," making them good tests of theoretical models of combustion. "Basically, we have three different models of hydrogen and oxygen chemistry that we're putting into our numerical simulations. And they all give the right flame speeds for hydrogen-air mixtures," under conditions different from those that produce flame balls. Ronney has discovered, however, that for weakly combustible mixtures that produce flame balls, the results from numerical predictions using these models differ greatly from the experiments conducted with flame balls in drop towers and reduced-gravity aircraft. These disparities may be the result of the less than optimum levels of microgravity provided by these facilities, but they may also point to inaccuracies in the theories. Ronney is trying to determine which, if any, of the models currently in use is accurate.
The picture at left shows low-gravity approx 1mm flame balls [ref: Ronney, P. D., Whaling, K. N., Abbud-Madrid, A., Gatto, J. L., Pisowicz, V. L., "Stationary Premixed Flames in Spherical and Cylindrical Geometries," AIAA J. 32, 569- 577 (1994). ]
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 case of spherically symmetric (like a bowling-ball) burning. This reduces the complex geometry of a fire to a case that is essentially one-dimensional (i.e. the radius of the sphere), making things much easier to study and understand.
This is the purpose of the Droplet Combustion Experiment (DCE). DCE will use various fuels - in drops ranging from 1 mm (0.04 inches) to 5 mm (0.2 inches) - and mixtures of oxydizers and inert gases to learn more about the physics of combustion in the simplest burning configuration, a sphere. This experiment cannot be completed on the ground, drop towers do not provide enough time in free fall.
The colorized picture at left shows a 3mm droplet of heptane at the start of a 3 second descent in the NASA/Lewis Research Center Drop Tower. The sequence of pictures shows the droplet shrinking.
Burning
Bigger Droplets
To burn bigger droplets, even in microgravity, we need to have some kind of supporting mechanism for the fuel, otheriwse the burning drop may move around, hit the walls of the container, or move out of the camera's field of view. The Fiber Supported Droplet Conbustion (FSDC) experiment, which will be performed in the glovebox, allows us to study the burning of fuels such as n-heptane, n-decane, methanol, ethanol, methanol/water mixtures, and heptane/hexadecane mixtures in droplets as large as 6 mm (nearly 1/4 inch). In addition, FSDC will learn more about the role of convection in burning by introducing a controlled air-flow into the burning environment during the experiment.
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
