Snapshot Absorption Spectroscopy

Abstract

Experimental improvements have been made in the UV-visible absorption spectroscopy technique applied to propellant flame diagnostics. The two-dimensional feature of an intensified charge-coupled device (CCD) detector was used to simultaneously record multiple, spatially distinct absorption spectra over a region of 0.35 cm. Temporal resolution has been increased to 1 ms by pulsing a simmering xenon arc lamp. The resulting increase in the light intensity by 30 to 70 times over the nonpulsed output provides the necessary light flux to achieve single-pulse, multiple absorption spectra. Species with low concentrations can be measured with the inclusion of multiple-pass optics to increase the effective pathlength through the combustion region. Due to broadband UV-visible absorption observed in propellant flame spectra, typically only 20% of the incident light is transmitted. However, inclusion of a neutral-density filter during the measurement of the incident intensity I0 allows the dynamic range of the detector to be effectively increased by a factor of 5. With these improvements, temperature and OH and NO concentration maps, with 1 ms temporal resolution, have been determined with two different propellant flames.