A look inside the combustion process

  • 17-Mar-2009 08:34 EDT

The top image shows microsecond synchrotron-based X-radiography from Argonne National Laboratory. The bottom image shows a transient multiphase fluid dynamics simulation. Both reveal unique dynamic shock-wave structures emanating from the tip of a high-pressure diesel jet, showing quantitative agreement between experiment and simulation.

Improving the combustion process requires knowing how it works in its smallest details, in terms of micrometers and microseconds. Specialized equipment developed in the last 10 years help researchers understand combustion.

“Laser-diagnostics are uniquely capable of probing and analyzing the in-cylinder processes in an operating engine,” said John Dec of Sandia National Laboratory’s Combustion Research Facility. Laser beams formed into thin sheets slice through a reacting diesel fuel jet, the HCCI (homogeneous-charge compression ignition) combustion chamber, or zones of fuel/air mixing, and the laser-induced light emission is isolated and imaged. It maps distributions of soot, fuel concentration, NOx, temperature, and CO, while velocity fields are mapped with particle imaging or laser Doppler velocimetry.

“Our experimental investigations are often done with numerical modeling to obtain a better understanding of combustion and emissions-formation processes,” said Dec.

Argonne National Laboratory scientists use X-ray technology to study the detailed structure of fuel sprays. X-rays do not encounter the multiple scattering problems typical of methods that use visible light, according to Christopher Powell, Engine Research Scientist at Argonne. High-intensity beams of monochromatic X-rays generated by its Advanced Photon Source quantitatively examine sprays from injectors.

“X-rays don’t have the limitation that visible light does, which is like shining a light in fog,” said Powell. “We discovered the spray breaks up much faster in modern diesel engines than was previously thought. The density of the fuel drops off very quickly. There is actually a high-density region of fuel near the nozzle and then another high-density region at the leading tip of the spray. Our data provides measurements inside the sprays so that advanced models can be developed that capture the real fuel distribution.”

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