We've all done it in chemistry
class when the teacher wasn't looking: cranked up the bunsen burner and
watched the flame climb higher until it disappeared.November 29, 1997
United States Microgravity Payload-4 - Flight
Day 10
We've all done it in chemistry
class when the teacher wasn't looking: cranked up the bunsen burner and
watched the flame climb higher until it disappeared.
That's the heart of a special combustion experiment - the Enclosed Laminar Flames (ELF) - aboard Space Shuttle Columbia. Knowledge gained from it may one day be applied to cleaner, more fuel efficient engines and even better gas fireplaces at home.
Fires in space attracted a lot of attention aboard Columbia earlier mission this year. ELF looks at a very different aspect than those experiments.
"We are trying to see how the flue environment works," explained Dr. Lea-Der Chen, the ELF principal investigator from the Combustion and Propulsion Laboratory at the University of Iowa in Iowa City.
ELF is one of three experiments in the fourth U.S. Microgravity Payload (USMP-4) that will be conducted in the Middeck Glovebox rather than in the payload bay. The first round of about 20 burns was conducted Friday evening and Saturday morning.
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"In general, we saw that our hypothesis of the flame lifting off at higher forced flows was verified," said Dennis Stocker, a co-investigator at NASA's Lewis Research Center. But some results are puzzling - not all the flames followed the expected trends - and the team is analyzing them those in preparation for Sunday evening's second round of tests.
Chen is doing it with a tiny bunsen burner little bigger than the needle on a syringe in an apparatus (right) that would fit in a briefcase. A thin tube, only 1.5 mm (less than 1/16th in.) across at the nozzle, admits a 50/50 blend of methane and nitrogen, while air is pulled it through a grate at one end of the chamber. A small wire coil, like an electrical cigarette lighter, flips into position to light the gas as it mixes with the air, then flips out of the way.
From
there, the thin, almost invisible blue flame emerges then fades. Air mixing
with the flame dilutes its heat so only warm carbon dioxide and water vapor
are pulled into the Middeck Glovebox vents and recycled into the cabin air.
It's a far cry from the flames inside a jet engine or even on your stove or natural gas fireplace. But Chen says that the fundamental behaviors he will see in the flame will let him and his co-investigators build better math models of what happens inside large engines, and then improve those engines.
"We're trying to map the stability limits," Stocker explained, "where it's stable, where it lifted, where it blows out."
That's where it resembles a student improperly using a bunsen burner. If the gas supply is turned high enough, the base of the flame will separate from the nozzle and rise. Eventually, the flame extinguishes even though a generous supply of gas is available. (For this reason, don't try it in class or at home. Unburned gas is an explosion hazard.)
The reason for this is rooted in the basics of fire we learned in elementary school: you need fuel plus oxygen plus heat to make a fire. Take away one and the fire goes out. With the burner turned ever higher, it seems that the fuel pours in faster than the burning molecules can carry their heat back to newly arrived gas. That's only one possible explanation.
"There are various theories," Stocker explained. "If, for whatever reason, you aren't getting enough of a mixture of fuel or air or heat at the base of the nozzle, that can influence the behavior of the flame and even put it out."
Indeed sound or even movement can do it, too, which happened as Stocker demonstrated the ELF training model. When a guest moved closer, the slight breeze disrupted the flame and it went out. Sound at the right frequency also can put the flame out or make the flame dance above the burner (although this is not part of the experiment plan). Remember, too, that petroleum fires often are literally blown out by explosives.
Unlike many materials experiments, Chen will have no samples to analyze after USMP-4: you can't freeze flames, except in pictures. And that's why he has sent his experiment into space.
Tests on Earth are can be difficult to interpret because buoyancy strongly affects the flame flow. The hot combustion gases rise as denser cold air falls in behind them. That's why candle flames rise they way they do. Unlike many of the combustion experiments conducted earlier this year, Chen wants some air flow, but he needs it without the turbulence caused by rapid convection. Even low-g flights on an airplane (like those used to train astronauts) are not smooth enough to reveal the details Chen is after.
"Our study is looking at combustion itself without the complexity that turbulence causes," Chen said. "It's laminar (smooth) only."
ELF's
fuel is injected at velocities up to 1 meter/second (3.3 ft/sec) as compared
to the 30 to 40 m/s of fuel being injected into an automobile engine.
Two color video cameras observe the flame from the front and top of the glovebox (right). With the lights out inside the glovebox, the flame will be quite visible. Still, to help show it better, the device includes a rake of 25 fibers of silicon carbide, a tough ceramic that glows yellow-white when heated, plus five thermocouples to measure temperatures. The rake can be swung into position to reveal cross-sections of the flame since it burns only where fuel and oxygen mix. The core, consisting just of fuel (with inert nitrogen mixed in) will be cool. Five digital displays show the temperatures and a clock gives Universal Time.
As the flow rate increases, the flame will lift off from the burner tip and move ever farther away (as shown in the pictures at top). As the flow rate decreases, the flame base travels back to the nozzle.
But it doesn't reattach when conditions are back to the start. The flow actually has to be slower before that happens (an effect called hysteresis).
Chen has enough gas on board, in small bottles, for ELF to burn for up to two hours. That will be distributed, of course, among a number of discrete experiment runs in six groups. These will investigate the flame's stability limits - when it lifts off or reattaches, and how far it will rise before flameout - and measure the temperature profiles as fuel or air flow rates are increased and decreased.
It will be a closely controlled version of what happens on a much larger and more turbulent form in engines on Earth.
"We are looking at idealized processes in ELF," Chen said. "We will expand this knowledge to more complex problems in engines and even in fireplaces. It's fair to say we think that the knowledge gained here will move ahead our capability to understand complex engines."
Stocker said that it may shorten the design time for new engines where engineers now must try a number of injector designs and flow conditions until they get what works best. It may also lead to improved engine efficiency and reduce pollution from unburned or improperly burned fuels.
Chen said it might even make for more efficient gas-powered fireplaces which he considers waste a lot of energy.
Check
out the daily
Mission Status Reports prepared by Marshall's Public Affairs Office.
Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory S.
Wilson