Laser Probe Characterizes Combustion

Brent D. Johnson

Combustion is a complex phenomenon. When even a simple hydrocarbon such as CH₄ burns in air, it produces a number of minority species, such as hydrogen and oxygen, that play important -- sometimes crucial -- roles in combustion chemistry. A better understanding of the process at this fundamental level could lead to the development of more efficient, nonpolluting engines and other combustion devices as well as fire retardants.

A photothermal deflection spectroscopy technique was used to conduct experiments on hydroxyl radicals produced in a hydrogen/air flame when excited by frequency-doubled dye and pulsed lasers. Courtesy of the University of Arkansas, University Relations.

However, to verify theoretical models of combustion, one needs to measure the local temperatures and the concentrations of various species produced. Physical probes are not appropriate because they can change the very concentration or temperature that one is trying to assess. For this reason, there is an enormous interest in the development of laser probes, which, generally, are nonperturbing.

In a project supported by the US Army Research Office, Rajendra Gupta and Yunjing Li of the University of Arkansas have developed a laser technique based on photothermal deflection spectroscopy that can measure absolute concentrations of minority species.

Their experiments have been conducted on hydroxyl (OH) radicals produced in a hydrogen/air flame. They use light emitted by a Lambda Physik frequency-doubled dye laser, pumped by the second harmonic of a pulsed Nd:YAG laser from Spectra-Physics to excite the radical. They chose the dye laser because it offers a source of radiation that could be tuned to a specific transition of the molecules.

Excited OH molecules decay primarily by collision, and the energy is deposited into the thermal modes of the flame gases, altering the refractive index of the medium in the laser-irradiated region. The researchers detect any change in the refractive index by observing the deflection of a 633-nm beam from a JDS Uniphase HeNe laser that acts as a probe beam. A measurement of this deflection yields the absolute concentration of OH.

Absolute values can be measured because energy is measured in the thermal modes, where between 99.8 and 99.9 percent of the energy goes, rather than in the radiative modes, Gupta explained. Because only 0.1 to 0.2 percent of the energy goes into fluorescence, the absolute concentration value derived from measuring fluorescence would be uncertain by about 100 percent, whereas the photothermal deflection value would have a negligible uncertainty.

The temporal evolution of the photo-thermal deflection signal yields the local flame temperature via a measurement of the thermal diffusivity of the medium, and a measurement of the time of flight of the change in refractive index from the dye laser beam to the HeNe beam yields the flow velocity of the medium.

Although many methods yield relative values of concentration, Gupta's technique measures three important combustion parameters simultaneously: concentration of absolute minority species, temperature and flow velocity.