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In this work we utilize two simultaneous laser diagnostic techniques to measure velocity fields and CH radical profiles. Particle Streak Velocimetry (PSV) is used to record axial velocity profiles, while Planar Laser Induced Fluorescence (PLIF) is utilized to measure relative concentration profiles of the CH radical. Previously, Carter et al. (1998) performed simultaneous CH PLIF and Particle Image Velocimetry (PIV) measurements in a turbulent, non-premixed flame. Han & Mungal (2003) performed simultaneous PIV and CH PLIF measurements in turbulent jet-flames in co-flow. Simul- taneous CH and OH PLIF has also been demonstrated (St ˙arner et al. 1992; Donbar et al. 2000; Ratneret al.2000).

2.4.1

Particle Streak Velocimetry (PSV)

Velocity measurements in this study consist of axial velocity profiles measured along the axis of the stagnation flow/flame. Velocity measurements are typically performed in premixed flames using the techniques of Laser Doppler Velocimetry (LDV: Wu & Law 1984; Zhuet al.1988), or, more recently, Particle Image Velocimetry (PIV: Dong et al.2002; Hirasawa et al.2002). In this study, velocity measurements were performed with the technique of Particle Streak Velocimetry (see Appendix B and Bergthorson et al. 2005a). PSV offers several advantages over other velocity measurement techniques in the study of premixed laminar flames. Particle loading required for accurate velocity measurements is an order of magnitude, or more, lower than that required for LDV or PIV. In a single PSV image frame, one or two particles traversing the vertical extent of the image are sufficient for profile measurement. In contrast, PIV measurements require a dispersion of particles throughout the domain in any one (short-time) exposure. In PIV, the higher required particle number densities and the high spectral intensity of the Nd:YAG laser pulses can cause interference in laser induced fluorescence measurements (Carter et al. 1998). With LDV, high particle number densities are required to obtain converged statistics in a reasonable time. Particle loading can also be an important factor in chemically reacting flows, as the heat capacity (Ancimeret al.1999) and surface-catalytic properties of particles can potentially alter flame/combustion behavior. The technique is fast; a single image frame can capture the entire velocity field, allowing PSV to be implemented in short run-time experiments. In axisymmetric-steady flow the axial velocity component along the centerline of the flow field can be reliably measured. Particle paths do not cross or overlap, and out-of- plane particle displacements are small and easily discernible when they occur (in-focus/out-of-focus

streaks). Further, the high sensitivity of the scattering cross section to particle size, in the size range employed, allows easy identification of agglomerates that may not track the high spatial-gradient regions in the flow. Streaks used for PSV processing were from in-plane, non-agglomerated particles. The PSV technique utilized here has been documented previously (Bergthorsonet al.2005a), and a complete description of the PSV experimental setup and analysis technique is given in Appendix B.

2.4.2

Planar Laser Induced Fluorescence (PLIF)

In order to assess the performance of the chemistry and transport models employed in this work, it was desirable to perform detailed measurements of a reactive intermediate and compare the results with numerical simulations. In the study of hydrocarbon flames, several laser diagnostic techniques have been applied. Major species profiles can be measured using Coherent Anti-Raman Scattering (CARS) techniques, while minor species concentrations in flames are typically measured using Laser Induced Fluorescence (LIF) techniques (Eckbreth 1996). Major species profiles have been shown to be insensitive to the imposed strain on the system (Lawet al.1994). The technique of Planar Laser Induced Fluorescence (PLIF) allows the two-dimensional concentration field of reactive intermediates to be measured (Hansonet al.1990).

Most PLIF applications to combustion experiments study OH. OH radicals are produced within the reaction zone, but due to the relatively slow destruction reactions, tend to persist in the flow where the temperature is high (Crosley 1989). Sample profiles for several intermediates, including OH, in a Φ = 1.0 methane-air flame are presented in Fig. D.6 (see Appendix D). OH fluorescence is relatively easy to measure, as it produces a high signal due to the high number densities of OH within the reaction and product zones. However, the UV fluorescence requires the use of high f/# UV optics, resulting in reduced collection efficiency, and S-20 photocathode materials with relatively low quantum efficiency. According to Crosley, intermediate species that rise and fall within the reaction zone are much more revealing. CH exists near the flame front and reveals where the combustion chemistry is taking place (Crosley 1989). Its narrow spatial profile is well-correlated with flame location and provides a sensitive test of strained-flame models. Also, CH has been suggested as being an important participant in prompt-NO production (Crosley 1989; Norton & Smyth 1991). Accurate modeling of CH production is thus essential for the prediction of these important pollutant emissions, especially for low-temperature flames where the prompt mechanism can dominate (Renfro

et al.2001). Succesful predictions of CH profiles can also help validate the chemistry and transport models utilized in detailed numerical simulations (Luqueet al.1996). Thus, for this work, species profiles of the CH radical are measured using Planar Laser Induced Fluorescence (PLIF).

PLIF measurements of the CH radical can be performed using excitation from the ground state (X) to the first (A), second (B), or third (C) excited electronic states. One of the most successful excitation-detection schemes relies on excitation to the B state near 390 nm, and detecting the

fluorescence from theA–X transition (e.g., Carteret al.1998; Sutton & Driscoll 2003). This scheme results in a large separation between excitation and detection wavelengths, allowing the use of a long-pass filter to block the scattered excitation light, while transmitting a high percentage of the fluorescence. High transmission is essential due to the low fluorescence signal resulting from the small CH concentrations in flames, typically a few parts-per-million. As well, Sutton & Driscoll (2003) have shown that by measuring the fluorescence signal on a CH resonance line, and subtracting the signal obtained off-resonance, relative CH concentrations can be measured as a function of fuel type and mixture fractions. While absolute CH concentration measurements have been performed, these studies have been limited in the range of parameters studied and are typically restricted to sub- atmospheric pressures (e.g., Luque & Crosley 1996; Luqueet al.1996). An exception is the study of Luqueet al.(2002), where absolute measurements of the CH radical were made in a burner flame at atmospheric pressure. Relative concentration profiles will be studied in this work, although studies such as Luqueet al.(2002) can be utilized to anchor the results at a single point, converting relative concentration profiles to absolute measurements. Appendix C and Bergthorsonet al.(2005a) provide more detailed information on the CH PLIF diagnostic used in this study.