The Aluminum Association reports that from 1980 through 1995 the aluminum industry experienced several hundred explosions during casting operations. Such explosions can be accompanied by injuries and extensive property damage. Three devastating explosions occurred in 1986 alone.
The ORNL method was developed after researchers (mainly based on Aluminum Association sponsored test results) studied organic coatings used by the aluminum industry to suppress explosions. One widely-used coating has been recently discontinued largely because of toxicity concerns. ORNL's research has led to the discovery that injecting air or other "noncondensible" gases at the right places and rates should prevent steam explosions during contact of molten aluminum with water.
In the aluminum industry, an aluminum ingot is typically formed by pouring molten aluminum into a steel mold that is lowered into a steel-lined pit of water. The water cools the mold, eventually solidifying the aluminum. When enough molten aluminum accidentally pours over submerged surfaces, an energetic explosion can occur.
The ORNL studies are being conducted by Rusi P. Taleyarkhan, Vladimir Georgevich, and Seokho H. Kim, all of the Engineering Technology Division.
"We first studied the potential problem of steam explosions in water-cooled research reactors whose fuel elements are made of uranium-aluminum mixtures sandwiched between aluminum plates," says Taleyarkhan. "When molten aluminum first comes into contact with water, a protective steam film forms. Then some sort of 'trigger' causes the steam film to become unstable and collapse. As a result, the molten aluminum mass can break into literally millions of hot particles, causing water they come in contact with to flash to high-pressure steam."
To study why some coatings applied to mold and pit linings suppress metal-water explosions, the ORNL researchers developed a unique experimental apparatus called the Steam Explosion Triggering Studies (SETS) facility. In this facility, molten aluminum never comes in contact with water in a tank so there is no danger of a steam explosion. However, the facility can accurately simulate heat transfer from molten aluminum moving over submerged surfaces.
ORNL's studies of uncoated, rusted steel, concrete, or other similar surface samples show they are quite wettable-water spreads and coats the surface. Thus, for example, rusted steel traps a mixture of water and steam, making it available for explosive boiling heat transfer, which can send shock waves to trigger a steam explosion. Appropriately coating the surface eliminates the forces for triggering an explosion, ORNL researchers found. The coatings decompose and char, generating large amounts of gases that tend to drive entrapped water away. "Also," says Taleyarkhan, "these gases then migrate to the aluminum surface and absorb external shocks, like airbags."
Says Taleyarkhan: "We believe that air injected into the protective steam film would repel water that would otherwise contribute to a steam explosion and would cushion against external shocks that could make the film collapse."
Field demonstrations of the new gas injection technique are planned to show its usefulness to the aluminum industry. They will be carried out later this decade by ORNL and the Aluminum Association Inc. under a cooperative research and development agreement.
The research is sponsored by DOE's Office of Energy Research, Laboratory Technology Research Program, and by DOE's Office of Energy Efficiency, Office of Industrial Technologies.
ORNL, one of the Department of Energy's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research Corporation.