Sep. 22, 1998 WEST LAFAYETTE, Ind. -- Purdue University researchers are blazing the trail in the use of lasers to detect and measure pollutants in burning fuel, and their efforts could lead to more fuel-efficient, cleaner-burning jet engines. Normand Laurendeau, the Reilly Professor of Combustion Engineering at Purdue, has spent the past 15 years developing novel ways to "look" into flames and determine the amount of pollutants, such as nitric oxide, that are produced during combustion.
Nitric oxide, a primary pollutant produced by aircraft engines, is a major concern because it can damage the Earth's protective ozone layer, Laurendeau says.
"In our latest work, we measured the amount of nitric oxide produced in spray flames, a process where liquid fuel sprays through a small hole to form droplets, which are then ignited," Laurendeau says. "This is the basic combustion process in jet aircraft engines. Before we did this, there was no evidence at all that these measurements could even be made inside fuel sprays, and we're the only lab in the country that has done this."
Laurendeau's doctoral student Clayton Cooper presented results from the spray-flame experiments at the International Symposium on Combustion in Boulder, Colo. The researchers also published an article on their research methods in the July issue of the journal Applied Optics.
Laurendeau is working with jet-engine manufacturers to determine the amount of pollutants that would be produced in more fuel-efficient, next-generation engines, which are designed to use a kind of fuel spray called lean direct injection. He and his students are the first to conduct such diagnostic experiments on this design.
Based on his research, Laurendeau says this type of engine design could significantly reduce nitric oxide production, although more tests are needed to determine exactly how much.
"Our work with lean direct injection spray flames has so far been limited to flames produced at atmospheric pressures," he says. "In a real jet engine, the pressures are much higher, and that's the next step for me and my students -- combining sprays with high pressure."
Cooper, from Batesville, Ark. , has been working in Laurendeau's lab, the Flame Diagnostics Laboratory, for three years. He is building the high-pressure facility to see if the laser-measurement techniques will work on spray flames in that environment.
"There are problems getting the spray flames to stabilize in a high-pressure chamber, and we also have to be concerned with interference from the fluorescence of other species of molecules in the flame," Cooper says. "There are going to be numerous challenges, but I think we can do it."
Lean direct injection engines are now under development, and Laurendeau says the Purdue research will help assure the designers and manufacturers that they can reduce the nitric oxide produced by the new engine.
"If we can get accurate data for designers, then they can use that data to develop models, and essentially design a gas turbine engine on a computer," Laurendeau continues. "This saves money for industry, and ultimately the consumer, because the cost of running tests during the design phase is absolutely enormous."
Laurendeau says automobile manufacturers also are interested in seeing if his techniques can be applied to examining pollutants produced by car engines, which run on a slightly different type of combustion process than gas turbine engines.
Laurendeau's techniques rely on laser-induced fluorescence. He shines a powerful laser on a flame, and the molecules of the pollutants in the flame absorb the energy in the laser light. The molecules then "relax," losing that excess energy in the form of light -- a process called fluorescence. Optical detectors then pick up that light, and a computer analyzes its wavelength and intensity to tell the researchers what type and quantities of molecules are present in the flame.
Laurendeau says this technique is very effective for learning about the combustion process, but he and his students had to overcome several challenges to make it work.
"The fluoresced light can be garbled by light from other sources," Laurendeau explains. "For example, the liquid fuel droplets reflect and scatter the laser light, which gives you a giant background signal that has to be separated out from the real signal you want to see. Also, there's a problem called 'quenching,' where the pollutant molecules lose their energy not by fluorescing light, but just by bumping into one another, which you also have to correct for."
Laurendeau and his students developed a method to compensate for the quenching called laser-saturated fluorescence.
"Basically, we take a measurement in two parts," he explains. "First, we shine the laser on the flame to obtain a 'signal' from the fluorescing nitric oxide. But that reading also contains a lot of background noise. We then change the wavelength of the laser so that we don't get a signal from the nitric oxide -- we're just measuring background. After we have all the data, we subtract the background noise out, and we're left with just nitric oxide measurements. The whole thing can be done in an afternoon."
Laurendeau also is experimenting with putting a special filter in front of the detector that could get rid of the background noise altogether.
When Laurendeau started his flame experiments in the early 1980s, little funding was available because no one yet knew whether it would work and there were no immediate practical applications. But with a small grant from the Department of Energy, and subsequent grants from the National Aeronautics and Space Administration, Laurendeau showed that he could measure pollutants in flames in partial vacuum, at atmospheric conditions, at high pressures, and, most recently, in spray flames.
"When we first started, industry didn't put much stock in the idea of using lasers to find the concentrations of pollutants in flames," Laurendeau says. "I wasn't sure when we started doing spray flames that we could get any quantitative results -- we surprised ourselves."
Laurendeau's research currently is funded by NASA, General Electric and other industrial sponsors.
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