Combustion Physics
Laminar
Soot Processes (LSP)
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, temperatures 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 as infrared "heat". Without the heat radiated by 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. 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 objectives for this experiment are to observe truly nonbuoyant flames, which are only possible in microgravity; to determine laminar smoke points (the conditions under which flames begin to produce excess soot that is not consumed by the flame); to test simple concepts of modeling soot in nonbuoyant flames; and to obtain information that will help researchers to evaluate soot formation processes in flames using non-premixed gaseous fuels. If the soot modeling theories that Professor Faeth is testing prove viable, he says that they "will vastly simplify problems of predicting outcomes of unwanted fires or practical combustion processes." (Practical combustion processes are those that use combustion for a practical purpose, like industrial furnaces or aircraft propulsion systems.) Even if the theories prove to be nonviable, Faeth will be able to collect data about the rate of soot formation, the location and concentrations of soot in flames, and the structure of soot particles that will add to the knowledge base of this particular aspect of combustion, which in turn will eventually lead to the development of improved combustion technologies.
This experiment will be conducted using the Combustion Module on Spacelab. NASA/Lewis Research Center maintains in-depth information on LSP.
Structure
of Flame Balls at Low Lewis-number (SOFBALL)
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. This experiment will be conducted using the Combustion Module on Spacelab.
During the MSL­1 experiment, different dilute gaseous mixtures will be used to produce flame balls and to analyze their size, stability, heat release, and other properties. The gaseous mixtures are made up of hydrogen and air (a little hydrogen in a lot of air), hydrogen and oxygen highly diluted with carbon dioxide, and hydrogen and oxygen highly diluted with sulfur hexaflouride. Gaseous mixtures like these are said to have a low Lewis number, meaning that the ratio of heat diffusion to mass diffusion of the limiting ingredient (in this case, hydrogen) is low. The key to producing flame balls seems to lie in igniting these mixtures in a microgravity environment. Microgravity is necessary to this process because the buoyancy forces present on Earth would disturb the flame balls and rip them apart.
Read more on flame balls from Lewis Research Center..
The colorized 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). ]
Droplet
Combustion Experiment
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 gasses to learn more about the physics of combustion in the simplest burning configuration, a sphere. 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. The drop tower experiment, unfortunately, does not provide enough time in free fall to use initially large droplets that are needed for extended observations.
DCE will be conducted in the specially designed Droplet Combustion Apparatus, an enclosed chamber in which single droplets of heptane will be burned in an atmosphere composed of a mixture of helium and oxygen. The internal apparatus is shown at left (click for a larger image). The droplet is formed by injecting the heptane through two injectors on opposite sides of the test platform within the chamber. The injectors are retracted after the drop is formed. The drop is then ignited by two hot-wire igniters that are brought near the droplet from opposite sides to begin combustion with minimum disturbance to the droplet. The burning droplet will be observed and recorded using video cameras and high-resolution photographs. From these recordings, the investigators hope to obtain data about the physical and chemical processes that take place in droplet combustion, including conditions under which the flames extinguish, the chemistry of the combustion reaction, and the production of pollutants such as nitrogen oxides and soot particles.
Fiber-supported
droplet combustion
To burn bigger droplets, even in microgravity, we need to have some kind of supporting mechanism for the fuel, otherwise 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 Combustion (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 allo us to learn more about the role of convection in burning by introducing a controlled air-flow into the burning environment during the experiment. The FSDC-2 experiment was developed at NASA's Lewis Research Center.
