Computing a flame's turbulent flickers.

When a spark plug ignites a mixture of fuel and air in a car's engine, the resulting flame flashes through the combustion chamber. It spreads in an extremely complicated manner, creating a violent, luminous, miniature storm of gaseous whirlpools and swift currents.

This turbulent flame, with its transitory blends of active and quiescent regions, also generates pollutants -- undesirable by-products of combustion. Researchers have long sought to understand this process well enough to determine how best to minimize pollutant formation and increase combustion efficiency.

Using supercomputers and advanced methods of computation, scientists and engineers have developed a number of useful models of turbulent flow, whether over airplane surfaces, inside oil-carrying pipelines, or along ocean currents. But turbulent flames also involve heatgenerating chemical reactions, which strongly influence such flows. Now, researchers are starting to include this chemical effect to create improved models of turbulent combustion.

"Our goal is to try to understand the fundamental interactions between fluid mechanics and chemistry in turbulent combustion," says Jacqueline H. Chen of the Combustion Research Facility at the Sandia National Laboratories in Livermore, Calif. "We're basically using the computer as a microscope to zoom in on the small scales at which mixing occurs to see what those interactions are."

Chen and colleagues Shankar Mahalingam of the University of Colorado at Boulder and Luc Vervisch of the National Institute of Applied Sciences (INSA-Rouen) in Mont-Saint-Agnan, France, start with the Navier-Stokes equations, which describe the motions of a fluid. Incorporating a rudimentary, one-step representation of a heat-generating chemical reaction, they use an approach initially developed at the Stanford/NASA-Ames Center for Turbulence Research to solve the resulting equations in full detail.

Simulation data indicate that turbulence stretches and distorts the flame, and it creates pockets where the flame is unable to sustain itself. At the same time, the flame itself tends to dampen turbulence in its vicinity.

"You get a wrinkled flame surface with holes punched out of it," Chen says. "There would be some spots where the flame has extinguished itself."

Overall, the interaction between chemistry and turbulence can enhance mixing. This effect, in turn, can lead to local reaction rates quite different from those measured in a steady flame.

Chen and her collaborators are now planning to incorporate more realistic chemistry in their model, focusing on the reaction between methane and air and the production of nitrogen oxides. "We want to increase the complexity of the chemistry," Chen says.