Explosion mitigation: polymeric fuel additive shows early promise
04 October 2015
Researchers at Caltech and JPL have discovered a polymeric fuel additive that can reduce the intensity of post-impact explosions following aviation accidents.
The process that distributes the spray of fuel for ignition in a jet engine (misting) also causes fuel to rapidly disperse and easily catch fire in the event of an impact.
The additive, created in the laboratory of Julia Kornfield at Caltech, is a type of polymer capped at each end by units that act like Velcro. The individual polymers spontaneously link into ultra-long chains called 'megasupramolecules'.
Megasupramolecules have an unprecedented combination of properties that allows them to control fuel misting, improve the flow of fuel through pipelines, and reduce soot formation. Moreover, they inhibit misting under crash conditions and permit misting during fuel injection in the engine.
Other polymers have shown these benefits, but have deficiencies that limit their usefulness. For example, ultra-long polymers tend to break irreversibly when passing through pumps, pipelines, and filters. As a result, they lose their useful properties.
This is not an issue with megasupramolecules, however. Although supramolecules also detach into smaller parts as they pass through a pump, the process is reversible. The Velcro-like units at the ends of the individual chains simply reconnect when they meet, effectively 'healing' the megasupramolecules.
When added to fuel, megasupramolecules dramatically affect the flow behaviour even when the polymer concentration is too low to influence other properties of the liquid. For example, the additive does not change the energy content, surface tension, or density of the fuel. In addition, the power and efficiency of engines that use fuel with the additive is unchanged — at least in the diesel engines that have been tested so far.
When an impact occurs, the supramolecules spring into action. These spend most of their time coiled up in a compact conformation. When there is a sudden elongation of the fluid, however, the polymer molecules stretch out and resist further elongation. This stretching allows them to inhibit the breakup of droplets under impact conditions — thus reducing the size of explosions — as well as to reduce turbulence in pipelines.
"The idea of megasupramolecules grew out of ultra-long polymers," says Ming-Hsin Wei, a research scientist and co–first author of a paper describing the work in the journal, Science. "In the late 1970s and early 1980s, polymer scientists were very enthusiastic about adding ultra-long polymers to fuel in order to make post-impact explosions of aircraft less intense."
The concept was tested in a full-scale crash test of an aircraft in 1984. It was briefly engulfed in a fireball, generating negative headlines and causing ultra-long polymers to quickly fall out of favour, Wei adds.
In 2002, JPL's Virendra Sarohia sought to revive research on mist control in hopes of preventing another attack like that of 9-11. The first breakthrough came in 2006 with the theoretical prediction of megasupramolecules by Ameri David, then a graduate student in Kornfield's lab.
David designed individual chains that are small enough to eliminate prior problems and that dynamically associate together into megasupramolecules, even at low concentrations. He suggested that these assemblies might provide the benefits of ultra-long polymers, with the new feature that they could pass through pumps and filters unharmed.
When Wei joined the project in 2007, he set out to create these theoretical molecules. Producing polymers of the desired length with sufficiently strong 'molecular Velcro' on both ends proved to be a challenge. With the help of a catalyst developed by Nobel laureate, Professor Robert Grubbs, Wei developed a method to precisely control the structure of the molecular Velcro and put it in the right place on the polymer chains.
"Looking to the future, if you want to use this additive in thousands of gallons of jet fuel, diesel, or oil, you need a process to mass-produce it," Wei says. "That is why my goal is to develop a reactor that will continuously produce the polymer — and I plan to achieve it less than a year from now."
A YouTube clip demonstrating the effects of the additive is available to view here.