High-Speed Camera Produces Radical Images
Movies of molecular reactions in a turbulent flame are providing a better understanding of the interactions between reactions and turbulence, and could help engineers improve the efficiency of combustion engines. Scientists at Lund University's Institute of Technology used a high-speed planar laser-induced fluorescence system from DRS Hadland Ltd. that can capture up to 100 million images a second.
Researchers at the institute's Combustion Physics Div. filmed the formation of OH radical molecules in turbulent flames. The presence of these radicals helps to define the boundaries of the flame front, which is an important measure of combustion efficiency. The group used DRS's Imacon 486C ultrahigh-speed imaging system with a pyramid beamsplitter to provide eight sequential high-resolution images from a single output. The group also integrated a high-gain amplifier into the optical path of the camera to improve its sensitivity in the UV and to enhance its gain so that it can capture single-photon events.
"The maximum repetition rate we used was 10,000 pictures a second," said Clemens Kaminski, who led the research. "Currently we are limited to eight images in a series because of limitations in hardware and memory resources." Although the images themselves occupy a total of just 2 MB, a typical experimental run may produce gigabytes of information.
High-speed fluorescence imaging is improving knowledge of how OH radicals form in turbulent flames and could eventually help engineers improve the efficiency of combustion engines. Courtesy of DRS Hadland Ltd.
Eight individual intensified charge-coupled devices would be prohibitively expensive and extremely difficult to align, added Kaminski. Also, the nature of the imaging application requires high sensitivity and the ability to capture short exposures to compensate for the intensely luminous background of the flames. "There's no other reasonable way of capturing such small and fleeting events," he said.
The work is sponsored by European auto manufacturers keen to reduce environmental pollution by making more efficient engines. "Now we can look at the influence turbulence has on the chemistry of combustion," said Kaminski. The next step will be to see if the same imaging systems can be used to capture turbulent reactive flows in three dimensions.
The researchers induced fluorescence in the OH molecules with four double-pulse Nd:YAGs from Thomson-CSF Laser of Evry, France. Each laser used an oscillator and single amplifier stage, producing eight pulses that could be synchronized to arbitrary external events.
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