Combustion Under Pressure – A New Understanding Revealed

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.

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.

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.”