Combustion Under Pressure - A New Understanding Revealed

Status Report From: NASA Office of Biological and Physical Research (OBPR)
Posted: Saturday, November 10, 2001

Written by the Microgravity News staff at Hampton University for the Spring 2001 Newsletter.

Automobiles, jet aircrafts, and even rockets all have one thing in common: they are powered by internal combustion engines operated under high pressures, in the range of 5-100 atmospheres (atm). (By comparison, normal atmospheric pressure that we experience at sea level is only 1 atm.) Combustion under high pressures is thermodynamically more efficient; that is, more of the heat energy produced by the combustion reaction is converted to desired mechanical energy. Furthermore, because of the intensified burning, it also enables the reaction to take place under more fuel-lean conditions, in which there is more oxygen than chemically required to consume the fuel. These unique attributes lead to improved fuel efficiency, reduced emissions of combustion- generated pollutants, and reduced production of carbon dioxide, which is a major contributor to global warming.

However, most of what is known about the combustion processes within internal combustion engines comes from experiments conducted at 1 atm, where flames are relatively easy to control and observe. When pressure increases, as microgravity Principal Investigator Chung Law, of Princeton University, explains, so does the degree of difficulty in conducting well-controlled experiments and consequently obtaining useful scientific data.

schematic diagram of the combustion chamber system
An ingenious design makes what used to be a mystery quite clear for combustion researchers. This apparatus, designed by Law, allows high-pressure combustion reactions to be observed for the first time. Inert gas in the outer chamber keeps the fire in check, never allowing it to get out of control or reach the optical glass, through which a high-speed digital camera records the reaction.

An Impossible Mission

Can researchers tell what will happen to a flame at high pressure from experiments conducted under normal atmospheric pressure conditions? According to Law, such extrapolations are highly unlikely to lead in the right direction. "The basis for extrapolation, namely data obtained around 1 atm, is just too limited for any reliable prediction of what could be happening with a flame at 50 or even 100 times the normal pressure." There's just no substitute for conducting experiments under high-pressure conditions.

But in order to conduct well-controlled, high-temperature, high-pressure combustion experiments in the past, researchers often had to sacrifice the ability to observe the combustion processes. "High-pressure experiments have been frequently done in what we call 'bombs' - totally enclosed, windowless systems," says Law. After igniting a fuel mixture inside such a combustion chamber, researchers would take measurements of the pressure increase caused by the burning of the fuel. "From that," Law explains, "you would speculate what has happened inside the bomb based on some assumed combustion processes." While some valuable data, such as the fuel consumption rate, have been obtained from conducting these kinds of experiments, the lack of visual observation could render the studies mostly qualitative and quite unsatisfactory.

Of critical importance, then, is actually observing the flame during the combustion process. But combustion chambers that allow the flame to be visually observable through special optical windows are vulnerable to the buildup of temperature and pressure inside the chamber. After ignition, a flame will continue to grow until it engulfs the combustion chamber. At the end of combustion, the combustion products not only have a very high temperature, frequently in excess of 2,000 kelvin, but the chamber pressure is also several times that of the starting pressure, which is already quite high. Optical glass thin enough to allow the flame to be observed without distortion cannot withstand this enormous pressure and temperature buildup. However, making the glass thicker would compromise its optical quality.

A Picture Is Worth a Thousand Words

Challenged by the need to unambiguously study the effects of pressure on flame propagation, Law and his research associates, Stephen Tse and Delin Zhu, devised an apparatus that would allow them to obtain images of the flame as it propagates, while maintaining the chamber pressure constant at its initial value, which can be as high as 60 atm. The apparatus comprises two chambers, one inside the other, with aligned optical windows. A sleeve connecting the two chambers can be opened and closed. After evacuating both chambers, and with the sleeve closed, researchers pump the combustible gas under study into the inside chamber and an inert gas into the outer chamber. After the pressures inside the two chambers are equalized, the sleeve is opened. The inert gas and the combustible gas come into contact, but with very little mixing.

images of fuel drops burning on a wire
Combustion theory gets an update when flames in high-pressure combustion reactions reveal their wrinkles. At 1 ATM, the flame surface remains smooth as it propogates outward, but at even slightly increased pressures (5 atm), the flame develops a bumpy appearance. Modeling of flames in internal combustion engines will benefit from this new revelation.

The combustible gas is then immediately ignited by a centrally located spark. The resulting spherical flame propagates outward until it meets the boundary of the inert gas and is extinguished. Since the volume of the inner chamber is much smaller than that of the outer chamber, there is negligible pressure buildup within both chambers during combustion. The entire process, from flame ignition to propagation and extinction, can be recorded on high-speed video. "The ability to do this kind of experiment puts us one step forward in understanding high-pressure combustion," says Law. Observing the images of the flame as it propagates turned out to be highly rewarding. Law was surprised to see that the flame has a strong propensity to develop wrinkles over its surface for high chamber pressures. This is shown in figure 1 for the flame propagation sequences in mixtures of hydrogen and air at two different pressures. At 1 atm, the flame surface remains smooth as it propagates outward upon ignition.

However, at even a moderately high pressure of 5 atm, wrinkles develop over the flame surface. The fact that the flame surface can become unstable and develop wrinkles is not surprising. Indeed, as early as the 1940s, Russian physicist Lev Landau predicted that the flame surface is always unstable. However, smooth flames such as those on a gas stove are the kind that is routinely observed, and the possible occurrence of wrinkled flames has been treated as more of an exception than the rule. What is surprising from Law's experimental observation is the strong propensity and prevalence of wrinkled flames at higher pressures. In hindsight, Law explains, this is reasonable because chemical reactions progress faster at higher pressures, yielding faster-burning flames that are more unstable. The recognition that flames prefer to propagate in the wrinkled mode at high pressures fundamentally alters the understanding of the burning processes within internal combustion engines. This is because the rate of fuel consumption increases with the flame's increasing area. Since the presence of wrinkles dramatically increases the flame's surface area, the flame actually burns much faster than previously realized.

Without seeing the flame, an investigator conducting high-pressure combustion experiments in closed vessels could easily be misled about the meaning of the fast rate of fuel consumption. If a smooth flame is assumed, then measuring the pressure increase inside the closed chamber could lead one to believe that a particular fuel has a very fast burning rate and to conclude that the chemistry of the combustion process must also be very fast. "It's not the chemistry so much as it's the morphology of the flame surface - the 'wrinkledness' of the flame - that causes the faster burning rate," says Law. "Without seeing the flame, you would be attributing the increased pressures to the wrong cause. If you wanted to improve the efficiency of the combustion process [by altering only the chemistry], you would be going in the wrong direction."

The discovery of the omnipresence of wrinkles at high pressures also promises to modify the understanding of the progress of chemical reactions in high-pressure combustion. In these instances, some of the reaction rates could have been assigned too high a value based on the higher burning rates measured. "This is a classic example of how errors in the interpretation of experimental results could propagate and consequently falsify fundamental physico-chemical data," Law cautions.

You Start With the Simplest

Law and his team at Princeton have begun their work on high-pressure combustion with hydrogen, the simplest of fuels. "All the other fuels - for example, methane, propane, benzene, and the alcohols - have hydrogen as a component," explains Law. "It's a building block. If you cannot describe what's happening in the case of hydrogen, you cannot proceed with studying these hydrocarbon fuels."

Law has conducted a large portion of his research at Earth's gravity, where the presence of buoyancy can have a significant influence on the propagation of weak flames, such as those associated with fuel-lean burning. Because of the slow flame propagation rate, buoyancy causes the hot combustion products to rise relative to the environment during flame propagation. This distorts the flame from the spherical shape it would have were gravity minimized. Such a distortion makes it much more difficult to analyze the experimental data and extract the fundamental information. Moreover, the effects of gravity are aggravated under high pressures. At higher pressures, the gas is even more buoyant because density is proportional to pressure. The higher the pressure, the greater the density differences between the hot gases and the cooler gases surrounding the flame. "The whole thing, then, calls for doing experiments in microgravity," Law concludes.

In NASA's 2.2-Second Drop Tower at Glenn Research Center in Cleveland, Ohio, Law is able to conduct his experiments on high-pressure burning without the disturbing influence of gravity. As the combustion chamber is released inside the drop tower and begins to fall, a spark plug inside the inner chamber is discharged, igniting the flame. The flame is spherical, and its propagation rate is well-defined and accurately measured from the imaged flame radii on the video. In the future, Law plans to introduce lasers to nonintrusively measure the temperature of the combustion reaction and to determine the composition of gases participating in the burning process. These measurements would allow even more insight into the chemistry of the combustion reactions. While the drop tower provides an excellent microgravity platform, it is limited when it comes to really slow-burning, weakly combustible mixtures, which are of relevance to the study of flame extinction phenomena. Experiments on such slow-burning fuel mixtures require much longer microgravity times in order to observe the burning process in its entirety. Eventually, these experiments may need to be conducted on the International Space Station, which provides continuous access to microgravity.

Law is excited about the experiments his chamber design is already making possible. "Combustion has reached a very exciting stage," he claims. "It has evolved from an empirical science to an exact science. Now we can make a prediction, do a careful experiment, and test whether our theory is right." Without the ability to conduct experiments under difficult but realistic conditions, such as those of high pressure, he explains, "your information base is incomplete. No matter how beautiful your theory is, you need to have an experiment. That makes the data we are obtaining very valuable." Law is encouraged by the support he receives from NASA. "The NASA program is funding fundamental research with useful outputs," he says. "That's the best kind of research."

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