Fiery research: Computer models for propellant fires
Walt Gill of Sandia’s Fire & Aerosol Sciences Dept. 1532 calls it a pancake - a disk more than a foot in diameter covered with what looks like the debris you’d scrape off a particularly messy barbecue grill. It’s actually a crunchy, baked-on mixture of aluminum, aluminum oxide, carbon, and other chemicals that coats everything after a rocket propellant fire.
The gritty material is part of a study Sandia is doing under a three-year contract signed with the Jet Propulsion Laboratory (JPL), California Institute of Technology, with funding from NASA. The propellant fire modeling project began in February and expands beyond the risk analyses the Labs already does for DOE. A presidential directive requires DOE to assess the risk to the public of launching NASA space missions that carry radioactive material for power sources. It contracts with Sandia for those assessments.
A propellant fire is one of the major risks in launching space missions with radioactive material, says Ron Lipinski (6223), team leader for the risk analysis. Consider, for example, if fiery fragments from a launch accident landed on top of nuclear batteries and burned at a temperature high enough to melt one of the containment boundaries.
Ron’s team needs information to assess overall risks, and this year JPL teamed with Sandia’s fire sciences group to provide technical information on propellant fires for what’s called the databook, a report on potential accidents that could occur during launch. JPL is managing the activity.
Advanced modeling is key
Walt says the key to Sandia’s contribution is advanced computer modeling focusing on characteristics of propellant fires.
“It would be a combination of experimental data and model results that would work together to give them the information they need,” he says.
The databook becomes the foundation for assessing risk. “Sandia is able to use our broad range of expertise, from experiments with various environments to modeling to our risk analysis process and safety analysis,” Ron says.
The work builds on tools the Labs created in nuclear weapons programs, including data from propellant fires it has studied since the 1970s, Walt says. “What Sandia has contributed that’s original into this whole mix is the idea of putting test data together with multi-physics high-fidelity models and looking at them as one piece,” he says.
Coupling experimental data with high-fidelity modeling in the databook hasn’t been tried before, so researchers in Advanced Nuclear Fuel Cycle Technologies Dept. 6223 are building an interface between
Sandia’s physics-based computational model of a fire environment and DOE’s radiological response model.
The interface is one of three parts to the project. The team also will update the computer model and add features to predict how things will behave in a propellant fire, then develop experimental data to validate the model. Tests will be done in the contract’s final year.
“That’s what sets Sandia apart,” Walt says. “We model, predict, experiment, compare. The Sandia way is we make the model first. … It’s like the scientific method; you come up with a hypothesis and test it.”
At a very basic level, Anay Luketa (1532) likens modeling and simulation to a high school algebra function with a value in and a value out. “From an engineering perspective, modeling requires the development of equations that approximate physical processes we’re interested in. Once developed, they are put into computations where the inputs are in the form of geometry, material characteristics, boundary conditions, and initial conditions, and the outputs are such quantities as temperature and velocity.”
The complexity of this particular problem is increased by the makeup of rocket propellant, which has an oxidizer, a rubber binder, and aluminum powder for fuel. Under conditions representative of a launch accident, aluminum powder burns slowly, melts on the surface of the propellant, and is lofted up into the flame, where it burns like droplets and leaves gritty deposits on everything, Walt explains.
The risk assessment is interested in the chemical makeup of the deposit and the temperature in its layers, says Walt, hefting a large clear plastic bag of ashy-looking chunks. “This stuff comes from the aluminum that is burning in the flame. It hits the surface, goes out. It becomes really thick.”
Sandia is improving its overall model by focusing on models for the droplets and the coating’s chemical makeup.
Walt unrolls a poster showing a microscopic analysis of a 2 cm slice of the material. He points out different parts of what looks like a cross-section of a rock: “The white is aluminum. Gray is aluminum oxide and lighter gray is hydrochloric acid and more aluminum. The dark spots are empty space.”
Burl Donaldson, a New Mexico State University professor who worked at the Labs for a dozen years, studied what aluminum does in an atmospheric burn when he was at Sandia in the mid-70s.
Comparing Rocstar and Fuego
“We have the same problem 35 years later,” says Burl, a part-time researcher on the team. “I don’t think anyone represents that we will put this issue to rest, but we hope to learn more about it.”
Anay says it’s one thing to study a single aluminum droplet - there are numerous technical papers on that - but it’s another to study it in the complex physics of a propellant fire.
“It’s very hard in a real propellant environment to capture the true dynamic behavior experimentally,” she says.
She’s been studying a code called Rocstar, developed by the University of Illinois under a DOE-funded Advanced Simulation & Computing program. It’s designed to run massively parallel and has been tested through a rigorous verification and validation process similar to what Sandia applies to its codes. She’ll spend much of the summer seeing if Rocstar will meet the project’s needs. Sandia also has its own fire modeling code - called Fuego, Spanish for fire.
Walt says the team wants to run both programs and compare them.
The team also can leverage ongoing work on weapons systems involving melting and burning aluminum to study how materials respond, he says. Anay says much of the outside data on aluminum doesn’t deal with high temperatures or fire environments.
Bill Erikson (1516) says the model has to capture convection, or the flow of hot gases over a surface; thermal radiation transfer; and the thermal loading associated with the accumulating grit with its varying particle sizes and distribution.
Burning aluminum particles are extremely hot, reaching about 4,940 degrees Fahrenheit or more, Bill says. As they radiate out, countless tiny, very hot particles slam into surfaces, leaving the gritty chemical buildup.
Sandia researchers have designed models for the heat transfer occurring with the deposition, and they’ve added changeable properties to account for growing deposit layers, Bill says. Still, he says, the models don’t yet capture such things as chemical reactions where molten aluminum hits other surfaces.
Eventually, the team’s work will be added to large codes. “It’s one thing to understand; it’s another thing to make them predict something,” Walt says. “This is the result of a lot of work by a lot of people to give us these big machines and these big models.”
The team also will do integral validation - validating a model by putting all the pieces together, such as through an accident scenario.
“You do the calculation, predict what it’s going to do, and then you do the experiment and see if that’s what happened. You put everything in there, not just one part,” Walt says. “So these experiments can get large and complex. It might take six months to set up and a minute to do it.”-- Sue Major Holmes
Sandians honored by President Obama for early career accomplishments
by Bill Murphy
Sandia researchers Stan Atcitty (6121) and Dan Sinars (1648) have been named by President Barack Obama as recipients of the Presidential Early Career Award for Scientists and Engineers (PECASE). This is the highest honor bestowed by the US government on outstanding scientists and engineers who are early in their independent research careers.
Stan and Dan are among 96 researchers - including 13 from DOE laboratories - from 11 federal agencies named PECASE recipients this year. A variety of DOE’s program offices are funding the work by the department’s recipients.
In making this year’s announcement, President Obama said, “Discoveries in science and technology not only strengthen our economy, they inspire us as a people. The impressive accomplishments of today’s awardees so early in their careers promise even greater advances in the years ahead.”
Energy Secretary Steven Chu offered his congratulations to DOE’s recipients, encouraging them “to continue on their paths to becoming the next generation of innovators, who will help America stay competitive in a rapidly advancing world. Their cutting-edge research is helping to meet our energy challenges, strengthen our national security, and enhance our economic competitiveness.”
In addition to a citation and a plaque, each PECASE winner is continuing to receive department funding for up to five years to advance his or her research.
Stan was nominated for the PECASE award by DOE’s Office of Electricity Delivery and Energy Reliability “for advances in power electronics for the electric grid, including the development of a high-temperature silicon carbide power module and an ultra-high-voltage silicon carbide thyristor, for research on grid integration of energy storage, and for mentorship in the Native American community.”
Stan’s work on the high-temperature silicon carbide power module and on the high-voltage silicon carbide thyristor were honored with prestigious R&D 100 awards in 2009 and 2011 respectively. The semiconductor device allows next-generation “smart grid” power electronics system to be built up to 10 times smaller and lighter than current silicon-based technologies.
Dan was nominated by DOE’s Office of Science “for developing innovative techniques to study the properties of instabilities in magnetized-high-energy-density plasma, enabling quantifiable comparison between experiment and simulation needed for validating cutting-edge radiation-hydrodynamics codes, and for demonstrating substantial leadership qualities in high-energy-density-laboratory-plasma physics.”
In 2011, Dan was awarded a DOE Office of Science Early Career Research Program award of $2.5 million over a five-year period for measuring fundamental instabilities in magnetically driven Z-pinch explosions.
Dan’s team was the first to capture, in a series of 3-D images separated by nanoseconds, the undesirable but apparently unavoidable appearance of a damaging instability (called Magneto-Rayleigh-Taylor, or MRT) in Z-pinch magnetic fields otherwise known to create conditions that fuse atoms for possible electrical energy generation.
Sandia President and Labs Director Paul Hommert said, “I congratulate Stan Atcitty and Dan Sinars for this prestigious award. I’m sure everyone at Sandia shares my pride in their accomplishments. Those of us at Sandia who have watched their careers develop over the past few years are hardly surprised by this high-level recognition from President Obama, but we are very gratified. Their recognition, along with that of colleagues from throughout the DOE enterprise, is testimony to the quality of research in DOE’s national laboratories.”
The PECASE awards were established in 1996, and are administered by 11 federal agencies. Besides DOE’s Office of Science and NNSA, the other nominating agencies are the National Science Foundation, NASA, Department of Veterans Affairs, Department of Health and Human Services, Department of Defense, Department of Agriculture, Department of Education, Department of Commerce, and the Smithsonian Institution.
The awards will be presented at a White House ceremony on July 31.-- Bill Murphy
Authors reprise 2010 paper showing increased productivity from more efficient lighting
by Neal SingerIn an unusual follow-up to the publication of a scientific paper, two researchers have reprised in the journal Energy Policy their groundbreaking finding that improvements in lighting — from candles to gas lamps to electric bulbs — historically have led to increased light consumption rather than lower overall energy use by society.
The same unexpected result they predicted in 2010, might also apply to light-emitting diodes (LEDs), poised to take over from the Edison light bulb as the next, more efficient light source of choice.
Increased light equals increased productivity
But the main point of Jeff Tsao (1120) and Harry Saunders (The Breakthrough Institute in Oakland, Calif.) was that, as three centuries have shown, increased light availability leads to increased productivity. Workers are no longer forced to stop shortly after nightfall, as they had in primitive, candle illuminated huts, but instead could continue producing through the night in homes, offices, factories, and even at outdoor locations not serviced by power lines.
Thus, the original paper drew attention to the increased productivity made possible by better lighting, rather than societal energy-savings mistakenly cited as a feature of improved lighting technologies.
But misinterpretations of the original paper by the widely read Economist magazine and the New York Times led to the confusion that Tsao’s team had shown that lighting efficiency improvements were not improvements at all. This is because no reductions in overall energy usage or overall lighting costs would occur.
The researchers’ upcoming article, titled “Rebound Effects for Lighting,” opens, “Our 2010 article on solid-state lighting in the Journal of Physics with several colleagues (Tsao et al, 2010) has generated considerable interest (and confusion, unfortunately) in the popular press and in the blogosphere. This communication seeks to clarify some of this confusion for the particular benefit of energy economists and energy policy specialists.”
The original article was titled “Solid-state Lighting: an Energy-Economics Perspective.”
The new article appears under “Articles in Press” on the Energy Policy website.
“We were motivated to publish something, even if short, in Energy Policy, because that journal serves a community very different from that served by the Journal of Physics, where our original article was published,” Jeff says.
“We thought that many in the energy economics community were still unaware of the work, and of the benefit — even when there is no direct energy-use savings — of energy efficiency and other welfare-enhancing technologies.”
Other authors of the 2010 article included Mike Coltrin, Jerry Simmons and Randy Creighton (retired). Harry Saunders is also associated with Decision Processes Inc. in Danville, Calif.
The work was supported by Sandia’s Solid-State Lighting Science Energy Frontier Research Center, which is funded by DOE’s Office of Basic Energy Sciences.-- Neal Singer