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Microgravity Experiments Seek to Test the Equations for Predicting Flame Behavior. In Other Words: How Does a Fire Burn During an Adventure?

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Mechanical Engineering 123(06), 62-68 (Jun 01, 2001) (7 pages) doi:10.1115/1.2001-JUN-3

This article discusses how experiments on microgravity plan to test the equations for flame behavior prediction. Houston is home to the Johnson Space Center, NASA’s headquarters for manned space flight programs. Ellington, a military base, serves as the space center’s home airport. NASA keeps a plane there called the Guppy, a freighter designed to ferry massive structures like those that will eventually wind up enlarging the International Space Station. The Guppy has a profile like Flipper swollen several stories high. The first time the team from Birmingham submitted its proposal, the idea was turned down, largely because of safety questions. The proposal went into rewrite and came back with a number of specific answers to questions of safety. The components for the experiment and the team’s trip to Houston were paid for largely by the Southern Co., a southeastern energy company where Hayes worked as a research intern when he was a student. He had talked to his boss, Steve Wilson, about money to support the microgravity experiment.

When March rolls across Texas, the bluebonnets bloom along the roadside clear to Mexico. In Houston, the Gulf Coast weather can be fickle, sometimes cool and balmy, sometimes warm and humid. You can stand in the dark and watch the methane flares burn over gas fields on the horizon. By day, you can watch a fighter shaped like a rubber dart take off from Ellington Field. Its turbines echo so loudly off the hangars they can make you believe there's another one coming in behind you.

And when the banks of cumulus blossom and lower overhead, sometimes you believe this part of the world is made of nothing but fast food and sky.

Of course, you wouldn't be entirely right. For instance, Houston is home to the Johnson Space Center, NASA's headquarters for manned space flight programs. Ellington, a military base, serves as the space center's home airport.

NASA keeps a plane there called the Guppy, a freighter designed to ferry massive structures like those that will eventually wind up enlarging the International Space Station. The Guppy has a profile like Flipper swollen several stories high. The gleam of its skin in the sun is hard to look at.

A civilian observer asks if it can really fly. Ellington is also home to NASA's KC-13S, the Weightless Wonder. And, by a circuitous route, that's what has brought the civilian, essentially a tourist, to stand just outside the flight line at Ellington in the first place.

This March, after months of preparation, one turndown, and almost a half-year of delay, four young men representing the University of Alabama at Birmingham finally had a ticket to ride the plane that delivers a sample of zero g.

Look, Ma, no gravity. Rob Hayes, when he was a student intern, approached his boss at the Southern Co. about sponsoring the UAB team's experiment.

Grahic Jump LocationLook, Ma, no gravity. Rob Hayes, when he was a student intern, approached his boss at the Southern Co. about sponsoring the UAB team's experiment.

They had put together an experiment to test a principle in microgravity. They were about to take their project to Ellington Field, mount it on NASA's KC-135, and fly with it.

The team invited the civilian observer to tag along, in the role of journalist.

They were taking part in the Reduced Gravity Student Flight Opportunities Program sponsored by NASA and overseen by the University of Texas at Austin. The UAB team was one of a dozen undergraduate groups from U.S. colleges and universities selected to fly during the week of March 12 this year.

The KC-135 aircraft that NASA calls the Weightless Wonder simulates the weightlessness of orbit at somewhere around 35,000 feet over the Gulf of Mexico.

Grahic Jump LocationThe KC-135 aircraft that NASA calls the Weightless Wonder simulates the weightlessness of orbit at somewhere around 35,000 feet over the Gulf of Mexico.

The team members were David Murphy, Joe Davis, John Carlson, and Rob Hayes. Davis is entering his junior year. Hayes and Carlson graduated from the university in December, but were back to finish what they had started.

Murphy, a sophomore, was the team leader. He had to go to all the meetings, even the early-morning briefings that his teammates could skip.

The program usually offers two flight tours a year for undergraduates. Donn Sickorez, university affairs officer at the Johnson Space Center, said that as many as 80 proposals may be submitted and up to 48 accepted for each tour.

Four dozen teams were accepted to fly in February and March this year. Many of them, including the Birrningham contingent, were originally supposed to go up sometime between August and November of 2000. According to Debbie Mullins, who works at the University of Texas as project administrator for the reduced gravity program, the schedule changed when NASA grounded the plane for routine maintenance. What had originally been planned as the summer 2000 campaign was merged into the spring flight weeks this year.

It costs NASA about $100,000 for each campaign of four flight weeks.

Like the typical undergraduate team, UAB would have two representatives flying on each of two consecutive mornings during the week.

Their experiment, "The Structure and Dynamics of Laminar Slot Diffusion Flames in Reduced Gravity," aimed to record the behavior of a flame from a slot burner port operating without the buoyancy effects of convection, which would not occur during microgravity. The data would be compared with theoretical models.

Equations for predicting slot flames differ from those governing flames from circular sources, such as candles and Bunsen burners. Flames from circular ports have been studied in the microgravity of space. According to the proposal, the team turned up no similar data on slot port flames.

The team's faculty advisor, John Baker, a mechanical engineering professor whose area of concentration includes combustion, will study the data gathered in flight.

The first time the team from Birmingham submitted its proposal, the idea was turned down, largely because of safety questions. Four students wanted to light propane on a NASA aircraft and leave it burning for an hour or so.

The proposal went into rewrite and came back with a number of specific answers to questions of safety.

The burner port was mounted in a cage of wire mesh, intended to keep fingers and floating debris out, and to protect the tiny flame as much as possible from errant breezes. The flame burned so low that it was almost invisible to the naked eye, but it was clear to a charge-coupled device and an infrared thermocouple, both of which were closely trained on it.

The thermocouple, from Omega Engineering in Stamford, Conn., kept tabs on the combustion. If the temperature fell, the fire was out. A signal would cut off the fuel.

The fuel line contained a normally closed solenoid valve, distributed by McMaster-Carr Supply Co. of Chicago. If power were interrupted for any reason, fuel would be stopped. Opening the door to the flame cage could break the circuit. So could kicking the plug out of the socket.

The available fuel for each flight consisted of five canisters with propane gas under a maximum pressure of 100 psi. Keeping the fuel in a gaseous state limited the volume of propane that would be aboard the aircraft at anyone time.

Dividing the volume by five further reduced potential hazards of handling propane during the flight. According to Carlson, if the entire contents of the fuel cylinders were to leak and remain trapped inside the experiment frame, the mixture would be too lean to ignite.

The team calculated the internal volume of the experiment frame at 354,354 cm3 A full canister would contain 75 cubic centimeters of propane gas at 100 psi , or about 689 kilopascals. If all the gas leaked out of all five cylinders , the propane volume would expand to more than 2,500 cm3 assuming a cabin pressure of 14.7 psia, approximately 101 kPa.

David Murphy, a UAB sophomore, levitated during the early stages of his flight. The straps on the floor permit flyers to stay in place during microgravity.

Grahic Jump LocationDavid Murphy, a UAB sophomore, levitated during the early stages of his flight. The straps on the floor permit flyers to stay in place during microgravity.

The volumetric fuel-air ratio inside the frame would be about 0.72 percent and inside the aircraft it would be less than 0.021 percent. These concentrations are well below the reported lower explosive limit for propane.

There were two manual shutdown switches. The data-gathering system included a CCD camera, from Cohu Inc. of San Diego, to record the image of the flame. A serial control titler from Horita Co. of Mission Viejo, Calif., would print the data readings on each frame of the video so there would be real-time readings, taken several times a second, of conditions that included temperature, acceleration, and pressure.

A trio of accelerometers from Endevco Corp. of San Juan Capistrano, Calif., measured acceleration, including gravitation, along the x, y, and z axes.

In addition to the Sony Hi-8 video recorder, a Dell lap top computer collected data. The laptop screen displayed graphs of the information in real time.

A late addition to the project was an LCD video monitor bought after the team arrived in Houston. The chief purpose of the monitor was to have visual confirmation during the experiment that the data was being collected and recorded on the videotape. It was the only practical way for members o"f the research team to see the flame.

The components for the experiment and the team's trip to Houston were paid for largely by the Southern Co., a southeastern energy company where Hayes worked as a research intern when he was a student. He had talked to his boss, Steve Wilson, about money to support the microgravity experiment.

Wilson, manager of the power technologies department in the company's research and environmental affairs organization, said Southern regularly funds a variety of research projects. As he saw it, the subject of the experiment was clearly in Southern's line of work. The students' work intended to validate combustion models. By removing gravitation from the process, their data would test the equations for predicting flame behavior.

Wilson's department came up with money from a fund devoted to fundamental or startup research developments.

Mark Berry, a senior research engine er at Southern Co., stepped in as a technical advisor. Berry, who holds a master's degree from UAB, had been one of Baker's grad students.

Berry became a hands-on mentor, spending time with the students to help them organize their activities, from ordering parts to final assembly. He showed up during evenings when the team was building the experiment, mostly to provide moral support.

"They could put it together," he said.

Hayes is now a full-time mechanical engineer for the Southern Co., and David Murphy, the UAB team leader, has Hayes's old j ob. Davis, too, has a job on the side, with Miller & Weaver Inc., consulting engineers in Birmingham.

Other support for the team came from 80/20 Inc. of Columbia City, Ind. The company supplied the experiment frame, including base plate, struts, Lexan panels, and safety mesh at a steep discount. The precut components came as a kit to Birmingham, where the team members assembled it.

According to Murphy, Radical Systems of Huntsville, Ala., did a lot of work on the data acquisition system for which the company didn't charge.

In order to participate, Carlson needed the green light from his employer, ESI Inc. of Tennessee in Kennesaw, Ga.

"They were very supportive of me when I told them about the flight ," Carlson said. "They had no problem giving me nine days off after I had just started working here."

When the teams arrived in Houston, there were several days of preparation. The first step was to assemble their hardware in Hangar 990 at Ellington Field on the Thursday before flight week.

Test directors inspected the equipment and pointed out changes, usually safety precautions, that needed to be made.

For instance, an unprotected circuit board is more vulnerable in microgravity than the uninitiated might suspect. Early in the UAB team's first flight, Carlson made a quick one-hand catch of a metal washer floating with potential havoc through the cabin.

Frames need padding on all the edges because it is impossible to predict where the crown of anyone's head is going to be at anyone time.

After NASA's people checked the equipment, they posted the flight schedule. UAB would be among the first six teams to go-Murphy and Carlson on the 13th. Hayes and Davis the next day. The six remaining teams would fly on the 15th and 16th.

After NASA's people checked the equipment, they posted the flight schedule. UAB would be among the first six teams to go-Murphy and Carlson on the 13th, Hayes and Davis the next day. The six remaining teams would fly on the 15th and 16th.

On Friday, the flyers and alternates met the tourist tram shortly before 7 a.m. to begin a day of physiological training.

First, they rode past the display of flight frames and the longhorn cattle pen at the Johnson Space Center to Building 30. As they entered the building, they were directed to the auditorium.

Directly across the lobby, behind heavy security doors, lay the fabled Houston, as in "Houston, the Eagle has landed," or "Houston, we have a problem."

John Carlson, floating over the experiment, which wasn't part of the experiment, adapted to micro, macro, lunar, and Martian gravitation with ease.

Grahic Jump LocationJohn Carlson, floating over the experiment, which wasn't part of the experiment, adapted to micro, macro, lunar, and Martian gravitation with ease.

Cameras came out and flashed. Everyone knew what lay behind that security wall. Hadn't we gotten a glimpse for ourselves, courtesy of the NASA cable channel, back in our rooms?

The smoldering computer banks of Mission Control were tracking the progress of the Space Shuttle Discovery toward its rendezvous with the International Space Station, and in the auditorium across the way, the tyros were going to get a condensed course in the physiology of high altitude, low pressure, spatial disorientation, and motion sickness.

No one really expected a window to pop out of the KC-135, but taken together, the lessons may have carried a subtext, perhaps to remind the novices that the adventure they were embarking on had a serious side.

The danger of foolhardiness was a recurring theme. As part of its orientation on Thursday afternoon, the group was told: "This is a very safe work environment, but it is a very dangerous playground."

In the afternoon, training moved to the Neutral Buoyancy Laboratory, a large building that in one corner contains a water tank a little bigger than the average shark aquarium. Astronauts float in the water to rehearse the assembly of components in space.

The flight groups headed for the high-altitude chamber. Each participant received an oxygen mask and a flight cap like Snoopy's.

After preliminary instruction, the group passed under the shadow of the buoyancy tank and proceeded to a corner that held a box with a narrow door, and several seats, and consoles with switches and hoses.

Reactions differ to a low-pressure chamber. Some people can walk in, take a seat, and wait for instructions. For others, cramped quarters wake latent claustrophobia. Even a small uptick in anxiety can make a journalist, for instance, forget his assigned seat number. He will have to wait for everyone else to sit before he learns he is No. 6, just like Patrick McGoohan in The Prisoner.

Joe Oavis. a junior in the engineering program at UAB. bounced back from a quick bout with nausea near the start of his flight and was able to enjoy a ride that he had waited a year or more to take.

Grahic Jump LocationJoe Oavis. a junior in the engineering program at UAB. bounced back from a quick bout with nausea near the start of his flight and was able to enjoy a ride that he had waited a year or more to take.

The chamber was sealed and the people inside breathed a miserly stream of pure oxygen through their masks for half an hour.

Mike Fox, who was outside the box calling the shots, asked if everyone was adjusting. No. 6's thumb did not pop up.

"What's wrong, No. 6?" asked Fox, the physiological training officer at the Johnson Space Center.

"I can't breathe through my nose," No. 6 said.

" Then you 'll have to b e a mouth breather," Fox replied.

No. 6 said that was fine with him. He convinced no one, including himself.

The half-hour on oxygen reduced the nitrogen content of the blood. The precaution prepared everyone for the rapid reduction of chamber pressure to simulate an altitude of 25,000 feet, or a little more than 7.5 kilometers.

According to Bud Collier, one of the morning's instructors, a normal human being that far up without supplemental oxygen has three to five minutes of useful consciousness-or enough awareness and motor control to respond to trouble. After that, you've passed out, don't care, or can't tell the difference.

When the box pressure is down to spec, the participants in odd-numbered seats remove their oxygen masks while the evens watch. The object of the exercise is to learn the symptoms of hypoxia, especially one's own.

The odds did fine, most still looking almost alert when Fox called time after five minutes and they went to emergency oxygen.

Most of the evens did well, too. No. 6 was short of breath from anxiety before his mask came off, but even he lasted the best part of four minutes, before he started to darken in the face.

Javier Roque, an aerospace physiologist with the KelseySeybold Clinic of Houston , which is under contract to NASA, was one of three pros chaperoning the gang in the box. He helped No. 6 back into his mask. Although No. 6 believed he had the presence of mind to flip the "emergency" switch by himself, Roque was quicker.

After oxygen had flowed for a second or two, Roque pointed out a symptom of recovery: "See how pink he is already."

The next big event was the test readiness review on Monday morning. Test directors confirmed that safety precautions were in place and secure. They also listened to the teams explain the aims of their experiments and their planned procedures onboard the airplane.

Carlson, elected spokesman, explained how safety measures required both flight team members to relight the flame after a fuel canister is replaced. One must hold the solenoid open while the other strikes the light.

"What happens if one of you goes down?" asked one of the test directors, meaning "bowled over by motion sickness ."

Carlson answered, if that happened, that was it for the day. Everybody nodded, and UAB was cleared to board.

Monday afternoon was occupied with jockeying the equipment aboard and securing it. Anything not tied down, no matter its size, would float for the period of zero g, and then would fall. Hard.

Then came another essential part of the preparation: tips to avoid motion sickness . Another name for this plane is the Vomit Comet, so the topic was on everybody's mind.

For one thing, a flyer is supposed to eat a reasonable, low-fat breakfast before the flight. No bacon, no gravy, Kool-Aid at your own risk.

During periods of microgravity, you can avoid confusion from your inner ear: Don't turn your head independently of your body. In heavy g, don't move at •all. And remember to hold your head up. Most fliers opted to take a motion-sickness medication, a mixture of scopolamine and Dexedrine.

The olive drab flight suits issued to the amateurs had zippered breast pockets. Everyone was issued a plastic bag and told to keep it there, loose and at the ready. It appeared that we were all wearing pocket squares.

A military version of a Boeing 707, the KC-135 was designed as a Stratotanker, to refuel aircraft in flight.

As the Weightless Wonder, this one flies parabolic loops over the Gulf of Mexico. After a steep climb that gives passengers a rush of acceleration equal to 1.8 times gravity, the plane begins to crest in its arc of travel, and the law of gravitation seems for a startling moment to be suspended. Everybody floats, and so does everything else on board that isn't battened down.

It was an earlier plane on the same run that made Tom Hanks weightless for Apollo XIII. The current aircraft is the Weightless Wonder V. It will make about 40,000 parabolas before being retired from this service.

One way to think of it is that your body is a rock somebody threw into the air. For a few seconds at the top of the climb, your speed, aiming to keep you going straight, and Earth's pull, trying to take you down, are in a sort of balance. Of course, you build up a debt of karma for that license, so downward acceleration comes back with a vengeance when you come out of it.

The period of weightlessness lasts for about 25 seconds. On the warning from test director John Yaniec, "Feet down, coming out," floaters quickly right themselves, as they prepare to sit up or lie flat to undergo 1.8 gs again.

The neophytes have to do that. The test directors and the flight doctor, Chuck La Pinta, do this four times a week. They stand to keep an eye on the uninitiated.

According to test director Dom Del Rosso, "Just because you're used to it doesn't mean the laws of nature can't come up and bite you."

Del Rosso mentioned a technique he uses, flexing the muscles of his legs and his stomach, to slow the drain of blood from his head. But, as he explained, "In 2 gs, if you're bending down and have to move fast, you can gray out."

Each parabola, during which the plane may climb and fall 8,000 feet or more, lasts about 65 seconds. The plane tops out somewhere near 35,000 feet. Those in the cabin do not experience the ride as up and down, but as a change in acceleration from almost 2 gs to zero and back again. Varying the ascent can alter the effect. Flight one simulated lunar and Martian gravitation for five passes each, in addition to 30 at zero.

The flame in zero g behaved in ways no one on the team fully expected.

Hot air rising from a flame creates convection, which draws oxygen-rich air from below to feed the combustion. The absence of gravitation leaves hot air no particular cause to rise, and without convection there's no steady flow of air coming in to serve the oxidation of the fuel.

How to Get There

NASA'S REDUCED GRAVITY Student Flight Opportunities Program gives students a chance to develop useful professional skills as they design and carry out working experiments. The program introduces the space agency to a variety of creative minds, and vice versa. It also generates a lot of publicity for NASA.

The undergraduate program is sponsored by NASA and is one of several educational projects overseen by the Texas Space Grant Consortium, operating out of the University of Texas in Austin.

The consortium, which includes more than 30 public, private, and academic organizations, has sister groups in the other states, all operating under the National Space Grant Program.

Information on the consortium's activities and programs, including when and how to compete for a place on the Weightless Wonder, are posted on the group's Web site, www.tsgc.utexas.edu.

Proposals come before a peer review committee of people from the state consortium and the Johnson Space Center. Debbie Mullins, the program's administrator at the University of Texas, said that since 1995, when the reduced gravity project began, student teams have submitted a total of 446 proposals and 295 have been accepted.

When this issue of Mechanical Engineering went to press, the consortium had not posted deadlines for the Spring 2002 program. The teams in this summer's campaign were announced late in April and are scheduled fly in July and August.

The Web site contains an archive of teams and proposals accepted for earlier campaigns. For instance, in March of last year, four physics students from Wellesley College took part in the program. Their experiment was titled "Group Investigating Rotating Liquids during Space conditions just wanna have Fluids Under No-gravity, or (G.I.R.L.S. just wanna have F.U.N.)." They studied the effects of microgravity on bubbles in fluid.

Most experiments address questions in engineering or scientific terms. The San Francisco Art Institute, however, was selected to fly in 1998 with an experiment called "Investigating Creativity in a Microgravity Environment," in which the intent was to blend art and science.

It involved a series of activities that included action painting. During that part of the experiment, the artist stepped into a closed vinyl bag tethered to the floor and used various devices to project acrylic paint during the weightless portions of the flight. A canvas caught the paint when gravitation returned.

Before the flight, members of the team had no assurance that their tiny propane flame, almost invisible through the mesh, would stay lighted during the weightless phases of the flight.

With the optimism of experimenters, the team members agreed beforehand that, even if the flame could not stay lit without the effects of gravitation, that phenomenon in itself constituted data.

The early parabolas of the first day's flight proved that the flame would not fail in microgravity. It not only burned through zero g, but swelled out of the camera frame.

Carlson and Murphy at first believed that they had lost their light, except that the temperature readings reported by the titler told them otherwise. As gravitation returned with a vengeance, the flame resumed its former shape, viewed by the CCD in cross-section as a slim feather.

Asked later to suggest a possible explanation, Baker cautioned that his opinion was preliminary, but gave it a shot.

"In the reduced gravity environment, the impact of buoyancy was greatly reduced," he said. "Because the fuel flow rate is relatively small, the structure of the flow field is changed, and the flow from the slot burner is more like a line source.

"Because there is no buoyancy to remove the products of combustion, the oxygen has farther to travel (via molecular diffusion) and that is why the flame sheet is farther from the burner port."

He added, "I did not expect the flame to get as big as it did, but I did expect the flame to behave as I described."

Sharp fluctuations occurred in the shape of the flame during reduced gravity. These Baker attributed to g-jitter, caused by random changes in the acceleration and vibration of the plane.

Plans are already in the works to modify the experiment and resubmit it for another run. Next time, Baker said, the camera will be adjusted to keep the expanding flame within the video frame. Researchers may burn a different fuel, perhaps butane, he added.

The interior of the Weightless Wonder is another environment that evokes varied reactions.

Some of those on the flight cartwheeled and spun that is, once they got used to the strange ride. Others floated cautiously and spent much of the time on the sidelines hanging on.

Some became ill. About a half-dozen on the first flight were sent by La Pinta to the rear of the craft, where they buckled themselves into their seats.

Murphy was fine for the first part of the ride, but eventually the rapid swings from weighing twice as much as usual to weighing nothing at all caught up with him. He spent the last part of the trip lying on the floor, although he did not become nauseated. During microgravity, he held himself down with straps anchored to the deck.

On the second flight, Davis got sick early, purged, and then got over it. He had his characteristic grin back when he was photographed in mid-air holding a "Hi, Mom," sign.

That was a popular device. The Blazers had brought it aboard, and it showed up at various times at Rowan, North Carolina State, and Tennessee, at least.

The Space Center videotapes the flights, as programl1ung fodder for the NASA channel. For part of the flight there is a ground-to-air phone hookup, which makes the rounds from team to team.

Asked how he was enjoying the ride, Hayes reported that he felt great. He held up the "Hi, Mom" sign to the video camera. Maybe on purpose, he held it upside down: "Wow."

One observer believes nothing may be able to upset Carlson. He floated like an old hand at this. Bu t like everyone else not part of the flight crew, this was his first time in zero g.

There was Carlson, talking to the camera: ''I'm now over the experiment, which is not part of the experiment. I think my teanUl1ate is worried that I'm going to fall on top of him."

Then he shoved himself off, so he wouldn't land on Murphy, and got ready for "feet down, coming Out."

Jennifer Akers, a member of the team from Rowan University in New Jersey, manages this turn with help from reduced gravity and from test director Dom Del Rosso.

Grahic Jump LocationJennifer Akers, a member of the team from Rowan University in New Jersey, manages this turn with help from reduced gravity and from test director Dom Del Rosso.

Copyright © 2001 by ASME
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