Planar laser-fluorescence imaging of combustion gases

Application of quantitative two-line OH planar laser-induced fluorescence for temporally resolved planar thermometry in reacting flows

A temporally resolved approach for measurement of two-dimensional temperature fields in reacting flows is experimentally investigated. The method, based on planar laser-induced fluorescence of the hydroxyl (OH) radical, is applicable in many combustion environments, including variable density flow fields. As a means of examining the accuracy of the technique, temperature images, from 1300 to 3000 K and 0.4 to 3 atm, have been acquired in shock-heated H(2)-O(2)-Ar flows with a two-laser, two-image ratio scheme. A complete measurement system for producing accurate, effectively instantaneous temperature images is described; the system includes single-shot monitors for laser-sheet energy distributions and spectral profiles. Temperature images obtained with the OH A(2)Σ(+) ? X(2)II (1, 0) P(1)(7)-Q(2)(11) transition pair exhibit a systematic error of only 7% over the entire range of conditions, with the error most likely dominated by shot-to-shot fluctuations in the lasers'spectral profiles. The largest error source in the instantaneous temperature images is photon shot noise. A group of OH transition pairs that provide good temperature sensitivity and strong signals for reduced shot-noise error over a range of flow-field conditions is also presented.

Temporally resolved, two-line fluorescence imaging of NO temperature in a transverse jet in a supersonic cross flow

Temporally resolved temperature imaging of a fuel jet in a hot, nonoxidizing supersonic cross flow is described. The temperature measurements are obtained with two lasers and two intensified cameras, with a two-line ratio of planar laser-induced fluorescence from nitric oxide, seeded either in the jet or in both the jet and the cross flow. Diagnostic issues related to the application of the two-line technique in high-speed combustion flows are addressed and include temperature sensitivity, transition selection, measurement resolution, fluorescence lifetime, temporal resolution, and intensifier and camera dynamicrange limitations. Single-shot and frame-averaged side-view temperature images of the flow field are presented, and the measurement uncertainties, which are dominated by photon statistical noise and pulse-to-pulse laser fluctuations, are discussed.

Temperature measurements in gases by use of planar laser-induced fluorescence imaging of NO

Two techniques based on planar laser-induced fluorescence of NO are applied to the measurement of two-dimensional temperature fields in gaseous flows. In the single-line technique, the NO fluorescence signal, which is in general a function of temperature, pressure, and mole fraction, can be reduced to a function of temperature alone. In this limit, a single measurement of fluorescence can be directly related to temperature. In contrast, in the two-line thermometry technique the ratio of fluorescence signals resulting from excitation of two different rovibronic states is related to the fractional populations in the initial states, which are solely a function of temperature. The one-line method is applied to the study of a laminar heated jet, and the two-line technique is used to measure temperature in a supersonic underexpanded jet. In addition, energy transfer in NO laser-induced fluorescence is analyzed with multilevel rate equation models. Finally, an accurate model is developed for prediction of the temperature dependence of the NO fluorescence signal.

Two-dimensional mapping of temperature in a lame by enerate four-wave mixing in OH

Phase conjugate images are produced by resonant degenerate four-wave mixing from rotationally excited OH molecules in a premixed, laminar methane/air flame. By comparing signal intensities in the images produced using transitions from different rotational levels, the temperature map of a planar section of the flame is derived.