Multiline hydroxyl tagging velocimetry measurements in reacting and nonreacting experimental flows L. A. Ribarov, J. A. Wehrmeyer, S. Hu, R. W. Pitz Abstract A compact micro-lens optical system is devel- oped that produces a 7·7 multi-line optical grid for Hydroxyl Tagging Velocimetry (HTV) and generates at least 49 resolvable velocity vectors. Single-photon photo- dissociation of ground-state H 2 O by a 193-nm ArF ex- cimer laser ‘‘writes’’ a 7·7 beam molecular grid with very long gridlines of superequilibrium OH and H photoprod- ucts in either room air flowfields or in H 2 -air flames due to the presence of H 2 O vapor. The displaced OH tag line positions are revealed through fluorescence by A 2 S + (v¢=0) ‹ X 2 P i (v¢¢=0) OH excitation using a 308-nm pulsed frequency-doubled dye laser. Time-of-flight analy- sis software determines the instantaneous velocity field in either an air nozzle or in a hydrogen/air flame. The OH tag lifetime is measured and compared to theoretical predic- tions using detailed chemistry. The lifetime of the OH tag is significantly enhanced by the presence of O atoms from 193-nm photodissociation of O 2 . 1 Introduction Accurate and reliable velocimetry measurements are essential in our understanding of the fluid-mechanical and thermo-chemical properties of experimental flowfields. Nonintrusive velocimetry methods have been developed that do not perturb the interrogated flowfield and provide a high degree of accuracy. Most often, velocity is measured with particle-based methods such as laser Doppler veloci- metry, Doppler planar velocimetry, or particle imaging velocimetry (Samimy and Wernet 2000). In these methods, the gas is seeded with tracer particles whose velocity is measured. However, the tracer particles with their large inertia relative to the gas flow molecules, especially in supersonic flow fields, are often susceptible to flow field irregularities, e.g., shocks, temperature and velocity gra- dients, etc., and may not follow accurately the interrogated flow streamlines (Maurice 1992). In flames, thermophoretic forces cause particle and gas velocities to deviate in the high temperature gradients found in burning boundary layers and stagnation planes (Talbot et al. 1980; Gomez and Rosner 1993; Sung et al. 1994). Lorenz-Mie light scattering from the seed particles also interferes with simultaneous optical diagnostics near the excitation wavelength. Finally, an excessive buildup of seed particles on test section access windows presents another serious challenge as clearly illustrated in recent work (Santoro et al. 2001). Laser-based nonintrusive velocity measurements have been developed without the addition of particles. These molecular velocity methods sometimes measure the Doppler shift of molecularly scattered light (Measures 1968). These are particularly useful in high-speed com- pressible flows where the Doppler shift is large. In the resonantly excited Doppler-shift methods, the fluorescent emission is measured with respect to the laser excitation wavelength to monitor the Doppler frequency shift. For example, Doppler shifts of sodium (Zimmermann and Miles 1980), iodine (McDaniel et al. 1983), copper (Mari- nelli et al. 1991), nitric oxide (Paul et al. 1989) and hydroxyl (Allen et al. 1994; Klavuhn et al. 1994) have been measured to determine gas flow velocity. The Doppler shift of Rayleigh-scattered laser light has been measured with Fabry-Perot interferometers and molecular filters to yield a velocity field (Seasholtz et al. 1992; Forkey et al. 1996; Miles and Lempert 1997). Doppler-shifted molecular velocimetry methods tend to be inaccurate at low speeds where the small Doppler shift is difficult to measure. Molecular tagging methods can be applied to a wide range of experimental flowfield velocities. In molecular flow tagging, the specific molecular marker is written into the interrogated gas and velocity is determined by com- puting the displaced marker’s position over a known time period. Several molecular tagging methods add a foreign gas ‘‘seed’’ to the gas flow prior to interrogation by the laser light source. In some seeded molecular tagging methods, a single laser produces a glowing grid in a gas that can be recorded by a camera. Examples are phos- phorescence of biacetyl in nitrogen flows (Hiller et al. Experiments in Fluids 37 (2004) 65–74 DOI 10.1007/s00348-004-0785-3 65 Received: 11 June 2003 / Accepted: 6 January 2004 Published online: 3 March 2004 Ó Springer-Verlag 2004 L. A. Ribarov, J. A. Wehrmeyer, S. Hu, R. W. Pitz (&) Department of Mechanical Engineering, Vanderbilt University, Station B, Box 1592, Nashville, TN 37235, USA E-mail: robert.w.pitz@vanderbilt.edu Present address: L. A. Ribarov Aero-Thermodynamics Group, United Technologies Research Center, 411 Silver Lane, MS 129–29, East Hartford, CT 06108, USA The authors gratefully acknowledge the support of NASA-Glenn (grant NAG3–1984, Dr. R. Seasholtz, technical monitor), BMDO- ARO (DURIP award DAAG55–98–1-0197, Dr. D. Mann, technical monitor), and AFOSR (DURIP award F49620–99–1-0120, Dr. J. Tishkoff, technical monitor). The authors thank Arnold Engi- neering Development Center (AEDC), Tennessee, for use of their ArF excimer laser and for their support under SVERDRUP/AEDC Group Contract No. T01–55. The technical discussions with Dr. B. Wieneke (LaVision, GmbH) regarding some of the initial image processing are gratefully acknowledged.