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Low-Gravity Flights are Precursors to Space Station Fluids and Combustion Experiments
Burning Rate Emulator investigation showing an elliptical, ethanol-fuel-soaked wick flame as it appears during a suborbital flight progression. It then takes on a cone shape in low gravity. (NASA) Burning Rate Emulator investigation showing curved ethanol flame in low gravity. (NASA)
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A typical gas core for two-phase injection to help increase understanding of how to separate gases and liquids in microgravity as visualized by this image extracted from high speed video as part of the Two Phase Flow Separator Experiment (NASA) A typical gas core for two-phase injection to help increase understanding of how to separate gases and liquids in microgravity as visualized by this image extracted from high speed video as part of the Two Phase Flow Separator Experiment. (NASA)
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Wouldn't it be fun to do experiments while floating free inside an airplane? Scientists from NASA's Glenn Research Center in Cleveland get to do just that using reduced-gravity aircraft. These planes fly in roller-coaster arcs to create a few minutes of free fall, allowing for low-gravity research. These reduced-gravity aircraft experiments are precursors for International Space Station investigations.

So just why are they needed before experiments and equipment are sent to the space station? Researchers use this environment to test investigation and hardware concepts before developing final designs for long-term testing on the space station. These reduced-gravity aircraft experiments serve as trial runs to how equipment and tests may fare in space.

Although astronauts appear weightless aboard the space station, they are still under the influence of Earth's gravity. They actually are falling around the planet at the same rate they are orbiting. Both spacecraft and crew are going at the right speed horizontally to maintain their altitude above Earth and experience a continual state of free fall, commonly referred to as microgravity. It is the same with the airplane’s elliptical flights, but for a much shorter period of time—minutes instead of years.

On low-gravity flights from May 1 -- 4, 2012, scientists tested equipment for four station investigations. These studies were the Burning Rate Emulator, Two-Phase Flow Separator Experiment, Flow Boiling and Condensation Experiment and Packed Bed Reactor Experiment.

The Burning Rate Emulator is a gas fuel investigation attempting to emulate the burning of solids to improve our understanding of materials' flammability over a wide range of conditions. The approach relies on the fact that all burning solids are first converted into a gas. By understanding the rate of gasification and other physical properties of a given solid material, the experiments will emulate the burning process by carefully controlling a gas flame. If the theory is correct, it should be possible to imitate a wide range of burning solid materials by using only a small number of gaseous fuels, which are simpler to control and characterize.

The Burning Rate Emulator uses a hot wire to ignite an ethanol-soaked wick in low gravity.

The investigation's focus is to improve fire safety in reduced-gravity environments, such as those encountered by astronauts aboard the station. "An important result of the airplane tests showed that a large flame could be maintained in low gravity," said Paul Ferkul, Ph.D., Burning Rate Emulator principal investigator at Glenn. "It was questionable whether a large flame would be sustained without any air flow because on Earth there is always some movement of air." These observations from the airplane tests will help in the design of the experiment for the space station.

The goal of the Two-Phase Flow Separator investigation is to help increase understanding of how to separate gases and liquids in microgravity. Many systems on the space station contain both liquids and gases. However, most equipment can effectively handle either gases or liquids, but not both.

"For example, pumps used in life support systems that are designed to pump liquids will stall when gases enter the pump inlets," said Two-Phase Flow Separator Experiment Project Scientist John McQuillen, Ph.D., at Glenn. "Condensers are most efficient when handling only vapor or gases."

This high-speed video shows the typical gas core for two-phase injection in microgravity as part of the Two-Phase Flow Separator Experiment.

On Earth, liquid settles to the bottom of the container, due to gravity, and gas bubbles rise to the top. In space, gas bubbles remain suspended in the liquid and there is a need to separate the gas from the liquid. The low-gravity experiment determined that a separator would work in low gravity over a range of conditions, such as flow rates and changes in separator design. Eventually scientists will determine the separator response and stability aboard the space station so that this technology can be adapted to space environment applications.

The Flow Boiling and Condensation Experiment is another investigation that examines the flow of a mixture of liquids and the vapors they produce when in contact with hot space system equipment. Cooling hot surfaces in these systems occurs when cool liquids vaporize or boil when flowing past the hot surface. Scientists recognize the lack of understanding of the behavior of mixtures of liquids and their vapor flow in condensers and boilers in low gravity.

"The data necessary for the design of these system components for microgravity and partial gravity applications for behavior prediction is nonexistent," said Henry Nahra, Ph.D., Flow Boiling and Condensation Experiment lead engineer at Glenn.

This video from the Flow Boiling and Condensation Experiment investigation shows condensation film in microgravity.

In the low-gravity flow condensation experiments, saturated vapor -- a vapor whose temperature equals the temperature of boiling at the pressure existing on it -- and liquid and vapor mixtures were introduced in the flow condensation modules to better understand how heat conducts through the liquid film that forms on the condensation tubes. At high flow rates, the liquid is able to drag the vapor bubbles along with the liquid from the heated wall before they can form a vapor barrier on the tube. This allows the wall to be replenished with liquid and increases the performance of the condenser. The key is to find the lowest flow rate that gives the best performance that in turn will reduce the cost of pumping the liquid.

The purpose of the Packed Bed Reactor Experiment in low gravity is to determine how a mixture of gas and liquid flows through a packed bed in reduced gravity. A packed bed consists of a metal pipe or tube filled with small particles or objects, in this case glass beads.

"Packed beds are a critical operation used with many of the leading water reclamation technologies for advanced life support systems," said Packed Bed Reactor Experiment Principal Investigator Brian Motil, Ph.D., at Glenn. "They have important applications on the space station for purifying water; however, there is very little understanding of the effect of low gravity on their performance."

This video shows a pulse flow in microgravity as part of the Packed Bed Reactor Experiment investigation.

The low-gravity aircraft experiments so far have shown that a pulse flow occurs at lower flow rates than in normal gravity. Pulse flow is a condition where high interactions occur between the reactants. This means that in the microgravity environment, a reactor could be operated at a higher efficiency than on Earth with proper flow conditions.

Experiments tested under low-gravity conditions in an airplane will continue to provide scientists and engineers insight into designing new equipment and experiments for application on the space station.

by Linda Nero
NASA's Glenn Research Center