Laser Induced Fluorescence for Temperature Measurement in Reacting Flows
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LASER INDUCED FLUORESCENCE FOR TEMPERATURE MEASUREMENT IN REACTING FLOWS R.G. JOKLIK National Institute of Standards and Technology, Gaithersburg, MD 20899
ABSTRACT OH vibrational Thermally Assisted Fluorescence (THAF) temperature measurements have been demonstrated in both premixed and diffusion flames. The accuracy of the measurements is generally better than 100 K over a wide range of flame conditions for which the collisional quenching rate varies considerably. Application of this technique for temperature measurement in Chemical Vapor Deposition (CVD) flows, for which the quenching rate is relatively constant, should exhibit greater accuracy. THAF measurements in these flows are limited by signal to noise considerations, and should be possible down to pressures of 103-104 Pa or less.
INTRODUCTION The application of laser based optical diagnostic techniques to reacting flows is attractive due to the non-intrusive nature of these techniques and to the excellent spatial and temporal resolution with which measurements can be made. Laser Induced Fluorescence (LIF), with its comparatively large signals, is particularly suitable for making measurements on the many reactive intermediates that are found in such flows. In addition to concentration, LIF measurements can also yield information on internal state distributions, and hence temperature. Several different LIF approaches have been developed for measuring temperature, including excitation scans for measuring ground state temperatures [1-4], saturated and linear two-line fluorescence [5,6], and thermally-assisted fluorescence (THAF) [7-11]. In this work the application of THAF to temperature measurements in flames is described, and its possible application for temperature measurements in reacting flows used for materials synthesis, such as those typical of CVD systems, is discussed. The THAF technique uses the collisional redistribution of population among internal degrees of freedom following laser excitation to measure temperature. This is accomplished by spectrally resolving the fluorescence from the collisionally populated states. The THAF temperature measurements in flames are based on vibrational transfer in A2 E' OH following (0-0) band laser excitation from the X2r1 ground state. Initial studies employing this approach were carried out by Crosley and coworkers [10, 11] in a premixed methane-air flame for a limited number of conditions and demonstrated that temperature measurements could be made successfully, although calibration was required for each flame condition due to the variability of the collisional quenching rate. This early work has been extended through a detailed study of the effects of quenching in a wide variety of methane, ethylene, and acetylene flames [ 12], and a calibration procedure has been developed that accounts for these effects without requiring knowledge of the local flame conditions. Examples of THAF temperature measurements made in several premixed and diffusion flames are discussed below. Mat. Res. Soc. Symp. Proc. Vol. 250.
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