February 22, 2008
New X-Ray Technique Peers Through High-Speed Dense Liquids
Standard microscopy and visible light imaging techniques cannot peer into the dark and murky centers of dense-liquid jets, which has hindered scientists in their quest for a full understanding of liquid breakup in devices such as automobile fuel injectors.
Scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory have developed a technique to peer through high-speed dense liquids using high-energy X-rays from Argonne's Advanced Photon Source (APS).
"The imaging contrast is crisp and we can do it orders of magnitude faster than ever before," Argonne X-ray Science Division physicist Kamel Fezzaa said.
Fuel injector efficiency and clean combustion is dependent on the best mixture of the fuel and air. To improve injector design, it is critical to understand how fuel is atomized as it is injected. However, standard laser characterization techniques have been unsuccessful due to the high density of the fuel jet near the injector opening. Scientists have been forced to study the fuel far away from the nozzle and extrapolate its dispersal pattern. The resulting models of breakup are highly speculative, oversimplified and often not validated by experiments.
"Research in this area has been a predicament for some time, and there has been a great need for accurate experimental measurement," Fezzaa said. "Now we can capture the internal structure of the jet and map its velocity with clarity and confidence, which wasn't possible before."
Fezzaa and his colleagues, along with collaborators from Visteon Corp. developed a new ultrafast synchrotron X-ray full-field phase contrast imaging technique and used it to reveal instantaneous velocity and internal structure of these optically dense sprays. This work is highlighted in the Advance Online Publication of the journal Nature Physics.
A key to the experiment was taking advantage of the special properties of the X-ray beam generated at the APS. Unlike hospital x-rays, the synchrotron x-rays are a trillion times brighter and come in very short pulses with durations as little as 0.1 nanoseconds.
"The main challenge that our team had to overcome was to be able to isolate single x-ray pulses and use them to do experiments, and at the same time protect the experimental setup from being destroyed by the overwhelming power of the full x-ray beam," Fezzaa said.
Their new technique has the ability to examine the internal structure of materials at high speed, and is sensitive to boundaries. Multiphase flows, such as high-speed jets or bubbles in a stream of water, are ideal systems to study with this technique. Other applications include the dynamics of material failure under explosive or ballistic impact, which is of major importance to transportation safety and national security, and material diffusion under intense heat.
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Ulysses, the mission to study the Sun's poles and the influence of our star on surrounding space is coming to an end. After more than 17 years in space "“ almost four times its expected lifetime "“ the mission is finally succumbing to its harsh environment and is likely to finish sometime in the next month or two.
Ulysses is a joint mission between ESA and NASA. It was launched in 1990 from a space shuttle and was the first mission to study the environment of space above and below the poles of the Sun. The reams of data Ulysses has returned have forever changed the way scientists view the Sun and its effect on the space surrounding it.
Ulysses is in a six-year orbit around the Sun. Its long path through space carries it out to Jupiter's orbit and back again. The further it ventures from the Sun, the colder the spacecraft becomes. If it drops to 2ÃºC, the spacecraft's hydrazine fuel will freeze.
This has not been a problem in the past because Ulysses carries heaters to maintain a workable on-board temperature. The spacecraft is powered by the decay of a radioactive isotope and over the 17-plus years, the power it has been supplying has been steadily dropping. Now, the spacecraft no longer has enough power to run all of its communications, heating and scientific equipment simultaneously.
"ËWe expect certain parts of the spacecraft to reach 2ÃºC pretty soon," says Richard Marsden, ESA's Ulysses Project Scientist and Mission Manager. This will block the fuel pipes, making the spacecraft impossible to maneuver.
In an attempt to solve this problem, the ESA-NASA project team approved a plan to temporarily shut off the main spacecraft transmitter. This would release 60 watts of power that could be channeled to the science instruments and the heater. When data was to be transmitted back to Earth, the team planned to turn the transmitter back on. Unfortunately, during the first test of this method in January, the power supply to the radio transmitter failed to turn back on.
"The decision to switch the transmitter off was not taken lightly. It was the only way to continue the science mission," says Marsden, who is a 30-year veteran of the project, having worked on it for 12 years before the spacecraft was launched.
After many attempts, the Ulysses project team now considers it highly unlikely that the X-band transmitter will be recovered. They believe the fault can be traced to the power supply, meaning that the extra energy they hoped to gain cannot be routed to the heater and science instruments after all.
So, the spacecraft has lost its ability to send large quantities of scientific data back to Earth and is facing the gradual freezing of its fuel lines. This spells the end of this highly successful mission. "Ulysses is a terrific old workhorse. It has produced great science and lasted much longer than we ever thought it would," says Marsden. "This was going to happen in the next year or two, it has just taken place a little sooner than we hoped."
The team plan to continue operating the spacecraft in its reduced capacity for as long as they can over the next few weeks. "We will squeeze the very last drops of science out of it," says Marsden.
Photo Caption: The liquid breakup of a high-density stream from a fuel injector can easily be seen using an X-ray technique developed at Argonne National Laboratory. The technique could lead to better and cleaner fuel injectors.
On the Net:
Argonne's Advanced Photon Source
Argonne X-ray Science Division
UChicago Argonne, LLC
U.S. Department of Energy's Office of Science