FAYETTEVILLE, Ark. - Fire shaped the history of humans and today it fuels power plants and car engines, but researchers have yet to understand the inner workings of combustion. Physicists at the University of Arkansas have created an optical probe that combustion scientists can use to measure many different aspects of combustion’s components simultaneously.

Rajendra Gupta, professor of physics, will present his findings at the International Conference on Photoacoustic and Photothermal Phenomena at the University of Toronto in Canada.

"If you could understand combustion, you could presumably create engines that are more efficient and less polluting," Gupta said.

Even when methane, the simplest of carbon fuels, burns, it produces small quantities of molecular species—radicals that play an important role in combustion chemistry—before the reactants end up as carbon dioxide and water. These highly reactive species range in concentration from parts per thousand to parts per million.

Researchers have developed theoretical models that describe these species, and have measured them experimentally. However, the currently used measurement techniques have limitations: They produce relative measurements of molecular concentrations, not absolute concentrations. And they only can measure one parameter at a time.

Gupta’s technique will allow researchers to do multi-parameter measurements, including flow velocity, temperature and absolute concentrations for multiple molecular species.

Measuring the components and temperatures in combustion has proved challenging, because physical probes would change the properties of a given flame.

Gupta and his postdoctoral assistant Yunjing Li have used photothermal deflection to create changes in combustion temperature with a tunable laser and then detect the change with another laser beam. They have used this technique to measure the concentration of a species within a methane flame, its temperature and its flow velocity.

The researchers shoot a tunable laser beam into the flame. The beam is tuned to the frequency of the molecule the researchers wish to measure, so it only excites those molecules when it hits the flame. The researchers use another laser beam to detect the excited molecules. Gupta and Li concentrated first on OH, the hydroxyl radical, produced during combustion.

"If I detect a signal, I know I’ve detected hydroxyl," Gupta said.

However, detection often proves difficult; the signal can get lost in "noise" that emanates from the detector and the lasers themselves. To combat this noise, the researchers take measurements over a 50-second interval—500 pulses at 10 pulses per second—and integrate the signal; random noise drops out over time, but the molecular signal remains.

Fifty seconds is a long time in combustion.

"In a flame, things are changing very fast," Gupta said. He and his team are continuing attempts to increase the signal-to-noise ratio and therefore decrease the time needed for detection.

"Ideally, we would like to have a measurement in a single pulse," he said.

This work is supported by the Army Research Office.

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