Unraveling the mysteries of the science of burning became the primary research focus of the Fourth United States Microgravity Payload Friday night as Columbia’s crew “fired-up” the middeck glovebox to begin the mission’s one and only combustion experiment.
Friday night -- midway through the crew’s work day -- Mission Specialist Dr. Kalpana Chawla ignited the first flames of an experiment that could provide insight into why jet engines occasionally flame-out. Jet engines, used in aircraft, natural gas power plants and some modern ships, use laminar or "thin sheet" gas flow. Fuel is injected into an airflow that is already moving. If conditions are not just right, the flame can move out of the combustion chamber and extinguish, causing the engine to flame out.
Chawla conducted several runs of the combustion experiment using a fuel mixture of 50 percent methane and 50 percent nitrogen, with differing fuel flow and air velocities. Each run produced a one centimeter blue flame, but the length of time that the flame remained stable and in the proper position varied with changes in fuel flows and air velocities. On the ground, the research team closely monitored its experiment on video sent down by the crew.
“First we were trying to observe how long the flame could remain stable and attached to the valve before it would pull away. After detaching from the valve, we wanted to observe how long it would take for the flame to extinguish,” said experiment co-investigator Dennis Stocker of NASA’s Lewis Research Center in Cleveland. “We hypothesized that flames would remain stable at a higher forced air flow in the microgravity environment, and that’s exactly what we’ve seen.”
Researchers hope to use information gained from this experiment to validate or correct computer models which simulate how various air flow velocities can influence the stability of flames. Findings may lead to the design of more efficient power plant combustors and safer jet engines for military and civilian aircraft.
After completing the combustion experiment runs, Chawla prepared the glovebox facility for a materials experiment intended to make possible more durable composite materials and even provide insight into why potholes form in roads.
As liquid metals solidify, a plane or thin slice of atoms form a line known as the solidification interface. As this line moves, particles are either pushed ahead of, or engulfed by, the growing solid mass. The science team, headed up by Dr. Doru Stefanescu of the University of Alabama in Tuscaloosa, wants to know why some particles are engulfed and others pushed. If particles are evenly distributed throughout the resulting composite material, the metal will be strong throughout its length and breadth. However, when particles or impurities are pushed into large non-uniform groups, weakened areas are created in the composite material.
To begin the experiment, Chawla placed a test cell, composed of two glass slides held together by a Teflon gasket, into the glovebox facility to be warmed. The cell contained microscopic glass beads ranging in size from 1 to 25 microns in diameter inside biphenyl, an organic acid.
Once the cell was warmed and the liquid inside melted, Chawla then mounted the cell between a heater and a cold block that traveled across the cell. As the cold block traveled, the liquid inside the cell froze. Through a microscope, she tracked the freezing process with a video camera for two hours. The process was recorded on video and sent down to the science team.
“So far, we’ve learned that our theory about the size of the particle groupings doesn’t drive engulfment rather, convection flow or sedimentation caused by gravity greatly affects the engulfment of impurities,” said Stefanescu.
He added that theoretical models for the engulfment of large groupings of particles need to be revamped to better reflect the effect gravity has on particles, and the effect particles have on composite strength and stability.
Overnight and into the morning the Confined Helium Experiment researchers continued to take ultra-precise measurements in cooled, liquid, “superfluid” helium — a material which conducts heat 1,000 times more efficiently than any other material.
The science team hopes to better understand how electrons might flow through ever-thinning channels etched into silicon computer chips. As more channels are being etched into chips, increasing their capacity for high-speed electronic operations, they are becoming thinner. Thinner chips, combined with the ongoing effort to make them even smaller, have forced chip designers to confront the possibility that channels may get so thin that electrons will not be able to flow through them freely. Knowledge from this study may help designers create the next generation of more powerful and smaller computer chips.
Looking ahead to the next 24 hours, the science focus will shift back to materials science. The MEPHISTO team will send electrical pulses through its bismuth and tin sample. Each pulse will last for one second, freezing the atoms at the interface -- the point where liquid meets solid. This will provide a series of curves that shows the evolution of the interface shape of the specimen. An irregular shape may indicate that the sample was distorted by spacecraft maneuvering.
In another materials experiment, the mercury-cadmium-telluride crystal growing in the Advanced Automated Directional Solidification Furnace will complete its 72 hour growth period Saturday afternoon.