Combustion, or burning, is a rapid, self-sustaining chemical reaction that releases a significant amount of heat. Examples of common combustion processes are burning candles, forest fires, log fires, the burning of natural gas in home furnaces, and the burning of gasoline in internal combustion engines. For combustion to occur, three things must normally be present: a fuel, an oxidizer, and an ignition stimulus. Fuels can be solid, liquid, or gas. Examples of solid fuels include filter paper, wood, and coal. Liquid fuels include gasoline and kerosene. Propane and hydrogen are examples of gaseous fuels. Oxidizers can be solid (such as ammonium perchlorate, which is used in Space Shuttle booster rockets), liquid (like hydrogen peroxide), or gaseous (like oxygen). Air, which contains oxygen, is a particularly common oxidizer. An electrical spark is an example of an ignition stimulus.
Combustion is a key element in many of modern society's critical technologies. Electric power production, home heating, ground transportation, spacecraft and aircraft propulsion, and materials processing are all examples in which combustion is used to convert chemical energy to thermal energy. Although combustion, which accounts for approximately 85 percent of the world's energy usage, is vital to our current way of life, it poses great challenges to maintaining a healthy environment. Improved understanding of combustion will help us deal better with the problems of pollutants, atmospheric change and global warming, unwanted fires and explosions, and the incineration of hazardous wastes. Despite vigorous scientific examination for over a century, researchers still lack full understanding of many fundamental combustion processes.
Some objectives of microgravity combustion science research are to enhance our understanding of the fundamental combustion phenomena that are affected by gravity, to use research results to advance combustion science and technology on Earth, and to address issues of fire safety in space. NASA microgravity combustion science research combines the results of experiments conducted in ground-based microgravity facilities and orbiting laboratories and studies how flames ignite, spread, and extinguish (go out) under microgravity conditions.
Research in microgravity permits a new range of combustion experiments in which buoyancy-induced flows and sedimentation are virtually eliminated. The effects of gravitational forces often impede combustion studies performed on Earth. For example, combustion generally produces hot gas (due to the energy released in the reaction), which is less dense that the cooler gases around it. In Earth's gravity, the hot gas is pushed up by the denser surrounding gases. As the hot gas rises, it creates buoyancy-induced flow that promotes the mixing of the unburned fuel, oxidizer, and combustion products.
The ability to significantly reduce gravity-driven flows in microgravity helps scientists in several ways. One advantage is that the "quieter" and more symmetric microgravity environment makes the experiments easier to model (describe mathematically), thus providing a better arena for testing theories. In addition, eliminating buoyancy-induced flows allows scientists to study phenomena that are obscured by the effects of gravity, such as the underlying mechanisms of fuel and heat transport during combustion processes. Because buoyancy effects are nearly eliminated in microgravity, experiments of longer duration and larger scale are possible, and more detailed observation and examination of important combustion processes can occur.
Scientists often desire an even mixture of the component parts of fuels so that models developed for their experiments can use simplified sets of equations to represent the processes that occur. Sedimentation affects combustion experiments involving particles or droplets because, as the components of greater density sink in a gas or liquid, their movement relative to the other particles creates an asymmetrical flow around the dropping particles. This can complicate the interpretation of experimental results. On Earth, scientists must resort to mechanical supports, levitators, and stirring devices to keep fuels mixed, while fluids in microgravity stay more evenly mixed without sticking together, colliding, or dispersing unevenly.
To date, combustion science researchers have demonstrated major differences in the structures of various types of flames burning under microgravity conditions and under 1 g conditions. In addition to the practical implications of these results in combustion efficiency, pollutant control, and flammability, these studies establish that better understanding of the individual processes involved in the overall combustion process can be obtained by comparing results from microgravity and Earth gravity tests. One clear example of the advantage of these comparison tests is in the area of fire safety. Most smoke detectors have been designed to detect soot particles in the air, but the sizes of soot particles produced in 1 g are different from those produced in microgravity environments. This means that smoke-detecting equipment must be redesigned for use on spacecraft to ensure the safety of equipment and crew.
Comparisons of research in microgravity and in 1 g have also led to improvements in combustion technology on Earth that may reduce pollutants and improve fuel efficiency. Technological advances include a system that measures the composition of gas emissions from factory smoke stacks so that they can be monitored. In addition, a monitor for ammonia, which is one gas that poses dangers to air quality, is already being produced and is available for industrial use. Engineers have also designed a device that allows natural gas appliances to operate more efficiently while simultaneously reducing air pollution. This may be used in home furnaces, industrial processing furnaces, and water heaters in the future. Another new technology is the use of advanced optical diagnostics and lasers to better define the processes of soot formation so that soot-control strategies can be developed. Devices have also been developed to measure percentages of soot in exhausts from all types of engines and combustors, including those in automobiles and airplanes.
Premixed Gas Flames
In premixed gas flame research, the fuel and oxidizer gases are completely mixed prior to ignition. Scientists are interested in flame speed (the rate at which the flame zone travels away from the ignition source and into the unreacted mixture) as a function of both the type of fuel and oxidizer used and the oxidizer-to-fuel ratio. With sufficiently high or low ratios, the flame does not move into the unreacted mixture; these critical ratios are referred to as lower and upper flammability limits and are of considerable interest in terms of both safety and fundamental science. Gravity can strongly affect both flame speed and flammability limits, chiefly through buoyancy effects. Scientists in this area are also researching gravity's effects on the stability, extinction, structure, and shape of premixed gas flames.
Gaseous Diffusion Flames
In this area of research, the fuel and oxidizer gases are initially separate. They tend to diffuse into each other and will react at their interface upon ignition. The structure of these flames under microgravity conditions is quite different than on Earth because of buoyancy-induced flows caused by Earth's gravity. Scientists study flammability limits, burning rates, and how diffusion flame structure affects soot formation. Within this area, results of studies of the behavior of gas-jet flames in a microgravity environment, both in transition and in turbulent flows, are being used to develop models with potential applications in creating effective strategies to control soot formation in many practical applications.
Liquid Fuel Droplets and Sprays
In this research area, scientists study the combustion of individual liquid fuel droplets suspended in an oxidizing gas (air, for example). For these experiments, investigators commonly use fuels such as heptane, kerosene, and methanol. Gravity hinders fundamental studies of droplet combustion on Earth due to flows induced by high-density droplets that sink and buoyancy-induced upward acceleration of hot combustion products relative to the surrounding gas. These flows cause drops to burn unevenly, making it difficult for scientists to draw meaningful conclusions from their experiments. This area of study also includes the investigation of the combustion of sprays and ordered arrays of fuel droplets in a microgravity environment for an improved understanding of interactions between individual burning droplets in sprays. Knowledge of spray combustion processes resulting from these studies should lead to major improvements in the design of combustors using liquid fuels.
Fuel Particles and Dust Clouds
This area is particularly important in terms of fire safety because clouds of coal dust have the potential to cause mine explosions and grain-dust clouds can cause silos and grain elevators to explode. It is particularly difficult to study the fundamental combustion characteristics of fuel-dust clouds under normal gravity because initially well-dispersed dust clouds quickly settle due to density differences between the particles and the surrounding gas. Because particles stick together and collide during the sedimentation process, they form non-uniform fuel-air ratios throughout the cloud. In microgravity, fuel-dust clouds remain evenly mixed, allowing scientists to study them with much greater experimental control with a goal of mitigating coal mine and grain elevator hazards.
An important factor in fire safety is inhibiting the spread of flames along both solid and liquid surfaces. Flame spread involves the reaction between an oxidizer gas and a condensed-phase fuel or the vapor produced by the "cooking" of such a fuel. Research has revealed major differences in ignition and flame-spreading characteristics of liquid and solid fuels under microgravity and normal gravity conditions. Material flammability tests in 1 g, which are strongly affected by buoyancy-induced flows, do not match results obtained in microgravity. It is therefore useful to study both flame spread and material flammability characteristics in microgravity to ensure fire safety in environments with various levels of gravity. The knowledge gained from these studies may also lead to better understanding of dangerous combustion reactions on Earth. Microgravity experiments eliminate complexities associated with buoyancy effects, providing a more fundamental scenario for the development of flame-spreading theories.
Smoldering combustion is a relatively slow, nonflaming combustion process involving an oxidizer gas and a porous solid fuel. Well-known examples of smoldering combustion are "burning" cigarettes and cigars. Smoldering combustion can also occur on much larger scales with fuels such as polyurethane foam. When a porous fuel smolders for a long period of time, it can create a large volume of gasified fuels, which are ready to react suddenly if a breeze or some other oxidizer flow occurs. This incites the fuel to make the transition to full-fledged combustion, often leading to disastrous fires (like those involving mattresses or sofa cushions). Since heat is generated slowly in this process, the rate of combustion is quite sensitive to heat exchange; therefore, buoyancy effects are particularly important. Accordingly, smoldering combustion is expected to behave quite differently in the absence of gravity.
Combustion synthesis, a relatively new area of research, involves creating new materials through a combustion process and is closely tied to work in materials science. One area of particular interest is referred to as self-deflagrating high-temperature synthesis. This occurs when two materials--usually two solids--are mixed together, are reactive with one another, and create a reaction that gives off a large amount of heat. Once the reaction is started, the flame will propagate through a pressed mixture of these particles, resulting in a new material. Much of the initial research in this groundbreaking area involves changing variables such as composition, pressure, and preheat temperature. Manipulating these factors leads to interesting variations in the properties of materials created through the synthesis process.
Flame processes are also being used to create fullerenes and nanoparticles. Fullerenes, a new form of carbon, are expensive to produce at this time and cannot be produced in large quantities, but scientists predict more uses for them will be developed as they become more readily available. Nanoparticles (super-small particles) are also of great interest to materials scientists due to the changes in the microstructure of compacted materials that can be produced by sintering, which results in improved properties of the final products. These nanoparticles can thus be used to form better pressed composite materials.