Micro-fireballs Lighting the Way to Better Engine Designs
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Science on Flight Day 12 of MSL-1
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When you mention burning hydrogen, many people think of the nearly invisible rocket exhaust from the Space Shuttle's three main engines. The last thing you would expect is that a burning droplet of alcohol would produce enough water to put itself out. Or that a fireball the size of a pinhead would linger for eight minutes.
These are some of the results coming out of experiments with fires so small that sometimes they barely exist. From these results, scientists hope to develop a better understanding of the countless fires - in internal combustion engines - that we use to run modern society.
(Image above shows a droplet of fuel burning in the
Fiber-Supported Droplet Combustion experiment moments after ignition.)
The Fiber-Supported Droplet Combustion (FSDC-2), shown at right, has flown before on the Shuttle, on the U.S. Microgravity Laboratory 2 (USML-2) mission in 1994. Where a number of test runs were missed on that mission as the science team learned how best to use the facility, the MSL-1 crew has been able to give the science team twice as many tests as they requested, and even do some unscheduled double droplet experiments.
"These things are running like clockwork," said Dr. Forman Williams, the principal investigator from the University of California at San Diego. "The results are very reproducible and the crew is running them twice as fast as we expected." Reproducible is an important word to scientists. It means that if the same test is run twice, the results are virtually identical, and that other scientists can run your experiment and get the same results. Then you have a confirmed discovery.
FSDC-2 burns large droplets suspended on a silicon carbide thread to
hold them in place. The droplets are large enough - 3 to 6 mm (1/8 to 1/4
inch in diameter) that the thread has little effect as the fuel droplets
burn. The experiments are run in a small chamber installed inside the Middeck Glovebox described in an earlier story.
FSDC-2 had 52 test burns scheduled. By Saturday morning (still flight day 11) the crew had run 125 burns and was on the verge of exhausting the fuel supply. With the last drops remaining, they were going to try tests with two droplets at a time, something, which has never been done.
Already, the FSDC team has at least one surprise to analyze.
"One of the early surprises is the way that alcohol burns," Williams said. Alcohol is a family of chemicals, the two most common being ethanol - more familiar as grain alcohol - and methanol - the deadly wood alcohol.
"These two alcohols burn completely different," Williams said. The ethanol drops burned to extinction, or until no fuel was left. But the methanol droplets extinguished themselves, which has implications for anyone designing engines that burn alcohol. Alcohol is strongly hygroscopic, meaning it absorbs water ("pure" alcohol is impossible to produce).
Both alcohols produce water vapor as a byproduct, but it appears that methanol reabsorbed more of the vapor from its own exhaust. Then, when the water was heated it boiled away from the drop, taking enough heat energy with it to cool the flame and stop combustion. In an engine, that means you have to burn more fuel to do the same work.
Williams said that the results could not be fully duplicated on the ground simply by tinkering with all possible running conditions on an engine. With data from FSDC, scientists will be able to tell better how to fine tune alcohol engine design.
Other experiments on FSDC have included burning mixtures of heptane and hexadecane, two complex hydrocarbons. The interesting results here was that the droplet did not burn evenly, but the heptane - the more volatile fuel - burned off first, leaving behind the hexadecane. Often this led to the droplet, especially large ones, extinguishing itself as it was cooled. Again, this has implications for designing clean burning engines. Gasoline, for example, is not a single chemical but a blend of hydrocarbons.
Williams and his colleagues will have plenty of data to evaluate because the astronauts ran double the number of planned experiments.
"They love this experiment because you can get in there and get
your hands on it," he said. "You can be like a scientist in a
lab on the ground as you burn these little fires.
Little, spherical fires are what scientists are studying with the poetically named SOFBALL experiments. SOFBALL stands for Structures Of Flame Balls At Low Lewis numbers. The Lewis number is a measure, in a fuel mixture, of two types of diffusion, heat and mass. A low Lewis number means the heat diffuses outward slower than the molecules.
Unlike the other combustion experiments, which use liquid droplets exposed to air, SOFBALL uses fuel and oxygen already mixed. In this set of experiments, the fuel is hydrogen gas. A spark plug, connected to the same type of electrical circuit found in a photographer's strobe light, ignites the gas.
And there's the paradox. Common sense tells us that the flame should leap across the chamber, burning everything in an instant. Instead, it produces the tiny glowing dots - 1 to 10 mm (1/25 to 1/4 inch) across) that float for up to 500 seconds before they are extinguished by fans in the chamber.
"If you dilute the mixture, it gets weaker and weaker until it disappears," explained Angel Abbud-Madrid, a SOFBALL co-investigator from the University of Colorado. "In low-g you can extend the life of that flame because you don't have the effects of convection." The hydrogen-oxygen mixtures are mixed with sulfur hexafluoride, carbon dioxide, or normal air. The concentration of fuel is so low that the fire can barely exist. In some cases, the mixtures have not lit off, which in itself is useful information to help scientists set the lower limits of flammability.
The idea of flame balls was suggested by an eminent Russian scientist, the late Dr. Yakov Zeldovich, in 1944 (astronaut Janice Voss is wearing his watch in his honor on this mission). He predicted they would be so unstable that they could not be observed. But they are, as accidentally discovered by principal investigator Paul Ronney of the University of Southern California. Ronney was running combustion tests in a drop tower and found that as the fuel concentration was reduced, the fire in the container went first to a spherical shape, then broke down into smaller cells and finally into the pinhead-size flame balls.
The spark does not ignite the entire gas, but a shell.
"Instead of propagating outward, they contract as they burn," Abbud-Madrid said.
Not only are the flame balls small, they are invisible to the naked eye, just as the exhaust from the Shuttle's main engines is almost invisible. The images that you see on TV are from an infrared camera. The flame balls only produce about 5 watts of energy, compared to about 100 watts for a match.
A flame ball's energy is so low that it could brush against your skin and you would barely notice it. And therein lies part of the danger for space travelers. Hydrogen gas is aboard many spacecraft as part of the electrical power systems - in the Space Shuttle's fuel cells or in the batteries of other craft - and can be made by biological processes. Potentially, gas leaking into a cabin could be ignited by a static electrical source, form a flame ball, and then drift cross the cabin for a few minutes and ignite a truly sensitive material.
"One major discovery [on this mission] was to observe them in low-g," Abbud-Madrid said. The prediction was that they would burn for up to 200 seconds. In the two runs that were possible on the abbreviated STS-83 mission, scientists found that the flame balls lasted 500 seconds.
"We wish we could extend this 500 seconds and let the experiment go until it extinguishes on its own," Abid said.
In addition to a better understanding of fire safety aboard spacecraft, SOFBALL will help in designing hydrogen engines for use on Earth.
"You won't build an engine using flame balls," Abid said, "but you need to know the lower limits so you can achieve isotropic [even] burning and reduce pollution." Flame balls serve as a model to understand combustion better.
Already, Ronney's studies have led to a patent for improved combustion by preheating the air being drawn into an engine.
Preliminary results from MSL-1's combustion experiments will be among the topics covered at the 27th International Symposium on Combustion to be held at the University of Colorado, August 2-7.
For the next 5 days, you can follow along and learn about the science being performed on the mission through activities on this WWW site, as well as the "Liftoff" Mission Home Page, and the Shuttle Web Site. Check out our daily image and video highlights on the "Science In Action" page!!
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