COMBUSTION ANALYSIS
Free-fall fluorescence experiment sheds light on flames
Achim Strass
Researchers at the Institut für Physikalische Hochtechnologie (IPHT, Jena, Germany) and the Center of Applied Space Technology and Microgravity (ZARM, Bremen, Germany) are investigating combustion processes under zero- or microgravity conditions with two-dimensional laser-induced fluorescence (LIF), a standard technique for analysis of combustion processes. The laser-spectroscopy experiments are conducted at "Bremer Fallturm," a vacuum test facility that creates zero-gravity conditions by enabling a complete experimental setu¥to be dropped in a capsule down an evacuated tower from a height of 120 m. This means there are only 4.7 s for an experiment. The goal of these experiments is to make combustion more efficient and less environmentally polluting.
A microgravitational environment provides an opportunity to examine interactions between fuel droplets during ignition and combustion in greater detail than under standard conditions. Both buoyancy and convection are gravity-induced phenomena. Gravity influences the process because a more-dense phase experiences a greater gravitational pull than a lighter phase. Gases and fluids with regions of differing temperatures, such as flames, experience convection due to gravity. In combustion, fuel-droplet size is one of several fuel-flow-reactor conditions that scale inversely with gravity, that is, the droplet size increases in lower gravity.
Excimer lasers watch drop
Two 248-nm Lambda Physik (Göttingen, Germany) excimer lasers--combined in an oscillator/amplifier configuration--are used to observe chemical reactions in the dro¥capsule. The lasers are fixed at the to¥of the dro¥tower together with beam-shaping optics, and the beam is aimed down the measurement chamber. Laser pulse energies range u¥to 500 mJ at repetition rates u¥to 250 Hz. Cassegrain optics in the amplifier and a telescope with cylindrical lenses enable the beam cross section to be minimized to 25 mm2. At a distance of 115 m, the cross section spreads to less than 30 mm2.
To obtain meaningful data, the position of the laser/flame interaction zone must be kept stable to within 1 mm inside the capsule measurement chamber. Any guiding of the free-falling capsule must be avoided to kee¥microgravitational forces to less than 10-5 g, so submillimeter control of the laser beam over the dro¥range from 5 to 115 m is required.
The laser-induced fluorescence is imaged onto an intensified 256 ¥ 256-pixel CCD array--part of a complete image-processing system (including data-acquisition and storage hardware) that is integrated within the dro¥capsule. The CCD and associated frame grabber are synchronized to the laser pulses and produce a sequence of images as the dro¥capsule falls. In one of the first test experiments, OH-radical distribution around a burning methanol-containing, porous ceramic sphere was studied based on LIF images. The image sequence shows progression of the flame front, above and below the sphere, during the transition from normal gravity to microgravity in the drop. The flame front changes rapidly from vertically stretched to spherical (see figure on p. 34).
The measurements at Bremer Fallturm have shown that major experimental issues--such as stable laser-beam control--have been successfully addressed, enabling the researchers to make a range of critical observations of microgravity combustion using laser-induced fluorescence.
ACHIM STRASS is a contributing editor for Laser Report based in Munich, Germany.