Measurements of Nitric Oxide in Flames Using Electronic- Resonance-Enhanced (ERE) Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy Ning Chai 1, Waruna D. Kulatilaka 1, Sameer V. Naik 1, Normand M. Laurendeau 1, and Robert P.Lucht 1, Joel P. Kuehner 2, Sukesh Roy 3, James R. Gord 4 1 School of Mechanical Engineering, Purdue University, West Lafayette, IN Department of Physics and Engineering, Washington & Lee University, Lexington, VA Innovative Scientific Solutions, Inc., 2766 Indian Ripple Road, Dayton, OH Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, OH Background Nitric oxide (NO) is an important air contaminant. Measurements of NO in high pressure environment is important but very difficult to implement. Current available techniques include LIF, CARS, etc. LIF presents excellent sensitivity at low pressures but subject to line broadening and interferences from O 2 and CO 2, among other factors. CARS with electronic- resonance-enhancement can increase the signal by orders of magnitude potentially; thus, provides a new approach to detect NO in high-pressure flames. ERE-CARS has some demonstrated merits in terms of measurements of NO. These include the pressure scaling behavior (signal increases with pressure up to 2 bar and remains nearly constant thereafter) and the electronic-quenching insensitive characteristics (signal is insensitive to collisional effects from major species CO 2 and O 2 ). Objectives The objectives of this study is to investigate the applicability of ERE-CARS for measurements of NO in: a.H 2 /air flame on Hencken burner without seeding NO b.C 2 H 2 /air sooting flames under fuel-lean and fuel-rich conditions c.H 2 /air counter flow flame and to extend current work to single-shot measurements in flames Experimental System Experimental system Acknowledgments U.S. Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Air Force Office of Scientific Research (Dr. Julian Tishkoff, Program Manager) Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base Conclusions 1. ERE-CARS measurements of NO performed in three different flame environments. H 2 /air (Φ = 1.15) flame stabilized on a Hencken burner. NO concentrations of ppm C 2 H 2 /air (Φ = ) flames on a Hencken burner. NO measurements in highly sooting flames H 2 /air non-premixed flame within counter-flow burner. Measured NO profiles are in very good agreement with calculated profiles - a surprising result given the drastic variation in collisional rates throughout the flame. 2. Preliminary results of single-laser-shot measurements of NO are encouraging. The desired dispersion ~ 0.10 cm - 1 /pixel. Solution: grating groove density 1200 grvs/mm – 3600 grvs/mm. Using a 0.1 cm -1 /pixel resolution, we can potentially measure temperature with N 2 and/or H 2 Pump beam ≈ 532 nm Stokes beam ≈ 591 nm Probe beam ≈ 236 nm ERE-CARS signal ≈ 226 nm Energy Level Diagram H 2 /air flame on Hencken burner Nascent NO was measured Detection limit ~ 50 ppm Acetylene/air flames on Hencken burner ERE-CARS measurement of NO is less sensitive to soot interference as compared to LIF measurments  The theoretical spectra are computed by using a revised Sandia CARSFT code which incorporates an electronic-resonance-enhancement factor. H 2 /air flame within counter-flow burner ERE-CARS measurements of NO using Q-branch lines with rotational quantum number of 9.5, 13.5, 17.5 are compared with theoretical calculations. The experimental data are not corrected for collisional quenching effects even with steep temperature and concentrations gradients across the flame. Single-laser-shot broadband Stokes CARS Preliminary results [P = 1 atm, T = 300 K, Q 1 (4.5)] FWHM of Stokes Laser Beam ~ 3 cm -1 Dispersion of the system ~ 0.4 cm -1 /pixel.