Combustion Measurement of Counter Flow Diffusion Flame under High Pressure Using Coherant Anti-stokes Raman Scattering (CARS) Thermometry

H2-air combustion is widely employed in power generation systems and high speed propulsion systems which are high-pressure environments. Therefore, it is imperative to perform diagnostics in order to understand behaviors of H2-air flames under high pressure to improve the design of systems such as those mentioned previously. In order to perform measurements at high pressure, a facility has been constructed for stabilizing steady-state, laminar counterflow diffusion flames (CFDFs). Both, qualitative and quantitative measurements will be performed in this high-pressure facility. Numerical simulations of high pressure, H2-air flame structure are also being performed using COSILAB with finite chemistry reaction mechanisms. These simulations will be used to validate experimental results, specifically, nozzle centerline temperature profiles and flow strain rates at flame extinction. Temperature profiles in H2-air CFDFs at 1 atm have been obtained using Vibrational coherent anti-Stokes Raman scattering (VCARS) technique and those results have been validated compared to numerical results. Strain rates at flame extinction were noted at atmospheric conditions and in the high pressure facility (2 atm to 10 atm for 17% H2) and the results were compared against numerical results. Numerically, the temperature profiles of H2-air flames CFDFs exhibited narrowing with increasing pressures. Also, at higher pressures higher peak temperatures were computed for 17% to 50 % H2 at 10 SLPM. The strain rate at which flames went to extinction also increased with pressure (between 1 to 10 atm.). Numerical simulations and experimental results will further refine thermodynamic and hydrodynamic models used for simulating more realistic H2­-air combustion environments

Development of Combined Dual-Pump Vibrational and Pure-Rotational Coherent Anti-Stokes Raman Scattering (DPVCARS and PRCARS) System

Coherent anti-Stokes Raman scattering (CARS) [1,2] is a spatially-resolved, time-resolved spectroscopic technique for quantitative measurements in reacting flows [3 – 6]. This work demonstrates a combination of N2/O2/CO2 dual-pump vibrational coherent anti-Stokes Raman scattering (DPVCARS) system and two-beam pure-rotational coherent anti-Stokes Raman scattering (PRCARS) system. It is based on the previous development of combined VCARS and PRCARS system which was used to obtain temperature measurements in non-premixed H2-air flames. The new combined system will be used to measure the temperature profiles and major species concentrations such as N2/O2/CO2 in laminar counter-flow non-premixed (CH4/Air) and partially-premixed (CH4/H2/Air) flames. The new system is being characterized in H2/Air diffusion flames stabilized over a Hencken burner. CO2 will be added to the oxidizer stream for the system to assess the precision of the system while performing concentration measurements. The new combined system has shown good precision temperature using PRCARS (better than 3%) and N2/O2 mole-fraction ratio (better than 5%) using DPVCARS

Development of New Optical Sensors for Measurements of Mercury Concentrations, Speciation, and Chemistry

A diode-laser-based ultraviolet absorption sensor for elemental mercury was developed and applied for measurements in a high-temperature flow reactor and in the exhaust stream of a coal-fired combustor. The final version of the sensor incorporates a 375-nm single-mode laser and a 784-nm distributed feedback (DFB) laser. The output of these lasers is sum-frequency mixed in a non-linear beta-barium borate crystal to generate a 254-nm beam. By tuning the frequency of the DFB laser, the ultraviolet beam frequency is tuned across the transition frequency of mercury at 253.7-nm. The tuning range is large enough that an off-resonant baseline was clearly visible on both sides of the Hg transition. Because of this large tuning range, the effects of broadband absorption can be effectively eliminated during data analysis. Broadband absorption is a major concern for lamp-based mercury sensor, and the sample to be monitored must be chemically treated prior to analysis to remove species such as SO{sub 2} that absorb near 253.7 nm. No pretreatment is required when our diode-laser-based sensor is used for elemental mercury measurements. Our laser sensor measurements were compared to measurements from a commercial mercury analyzer (CMA), and were generally in good agreement except that the insitu diode-laser-based sensor measurements tended to give somewhat higher concentrations than the CMA measurements, presumably due to absorption of elemental mercury in the sampling lines needed for the CMA measurements. At Texas A&M University, the homogeneous oxidation of elemental mercury was studied using a high-temperature flow reactor

Two-Color Polarization Spectroscopy Measurement of Nitric Oxide

Nitric Oxide (NO) is a greenhouse gas that contributes to smog and acid rain. Commercial combustion engines and turbines are significant sources of NO emission. Two-color Polarization Spectroscopy (PS) will be used to measure the collision-induced resonances of NO in gas mixtures. The effect of collision partners, such as He and Ar, on the line-shape of NO molecule will be studied. This experiment requires the use of two dye laser systems to generate 226nm beam from frequency mixing of 355nm and 622nm. This enhanced the scan range and improved the ease of operation. One dye laser will be used to generate circularly polarized pump beam, which will be tuned to excite the transitions across the X-A (0,0) band of NO. Another dye laser will be used to generate linearly polarized 226nm probe beam, which will be used to probe the transitions. A photomultiplier tube will be used to collect the polarization signal. Nitrogen will be mixed with the NO gas mixtures to measure the sensitivity of this technique. In the current stage of the project, the pump beam is aligned and its wavelength is controlled by a LabVIEW programmed motor. Laser induced fluorescence data was collected to calibrate the scanning frequency with NO’s excitation frequency. The excitation spectrum of NO from this specific pump-probe transition pair will help us understand the fundamental collision dynamics of NO and create a more quantitative technique of NO concentration measurement

Preliminary Testing of Plasma-Induced Combustion

Plasma-induced combustion (PIC) has been shown to improve the reliability, efficiency, and delay time of ignition in flight systems like augmentors and scramjets. These high-velocity systems are mostly used in military applications, and improvement may help commercial viability. To understand this chemical process, the concentration of radicals, particularly H radicals, must be tracked through the flame using laser diagnostics. This requires a steady source of plasma-assisted combustion to be secured and well-understood. A plasma torch flowing partially premixed air and methane was installed and successfully operated, and preliminary testing was carried out. Primarily it was observed that PIC created stable flames at equivalence ratios as low as 0.3, though under the same conditions the flame would not light without PIC even at an equivalence ratio of roughly 0.7. In addition, photos of the flame demonstrate the presence of CH radicals. A jump in current was observed at certain electrode voltages, at which the current would spike and heat transfer would increase. Turning off the plasma extinguished the flame immediately. These observations and others point to the potential of plasma to assist combustion. In addition, future laser experiments will benefit from the recorded procedure and documentation of the plasma torch installation and operation

Coherent antistokes Raman scattering diagnostics of plasma in synthesis of graphene-based materials

Scalable production of carbon nanostructures to exploit their extraordinary properties and potential technological applications requires an improved understanding of the chemical environment responsible for their synthesis. In this study, the temperature and concentration of molecular hydrogen is measured using coherent antistokes Raman scattering (CARS) spectroscopy in the plasma of a Microwave Plasma Chemical Vapor Deposition reactor under parametrically controlled conditions. The reactor pressure is varied from 10 to 30 Torr and the plasma generator power from 300 to 700 W, simulating the conditions required for the synthesis of carbon nanotubes and graphene. Temperature measurements are conducted within the plasma sheath and 5–10 mm away from the sheath to elucidate the spatial distribution of temperature within and around the plasma region. The results of the CARS experiments indicate only a weak correlation between the rotational temperature of hydrogen and the distance away from the plasma sheath at 10 Torr. The temperature of hydrogen varies approximately from 700 K to 1000 K. However, a strong correlation at 30 Torr is apparent, in which the temperature increases from 1100 K to 1900 K at both 500 and 700W. The concentration measurements of molecular hydrogen imply that the degree of dissociation in the plasma is very low and that the rotational temperature of molecular hydrogen and the translational temperature of the heavy species in the plasma are equal to within experimental error. The spectroscopic techniques applied in this research may prove to be suitable in-situ monitoring methods for the scalable manufacturing of carbon nanomaterials

Final Report: Investigation of Polarization Spectroscopy and Degenerate Four-Wave Mixing for Quantitative Concentration Measurements

Laser-induced polarization spectroscopy (LIPS), degenerate four-wave mixing (DFWM), and electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) are techniques that shows great promise for sensitive measurements of transient gas-phase species, and diagnostic applications of these techniques are being pursued actively at laboratories throughout the world. However, significant questions remain regarding strategies for quantitative concentration measurements using these techniques. The primary objective of this research program is to develop and test strategies for quantitative concentration measurements in flames and plasmas using these nonlinear optical techniques. Theoretically, we are investigating the physics of these processes by direct numerical integration (DNI) of the time-dependent density matrix equations that describe the wave-mixing interaction. Significantly fewer restrictive assumptions are required when the density matrix equations are solved using this DNI approach compared with the assumptions required to obtain analytical solutions. For example, for LIPS calculations, the Zeeman state structure and hyperfine structure of the resonance and effects such as Doppler broadening can be included. There is no restriction on the intensity of the pump and probe beams in these nonperturbative calculations, and both the pump and probe beam intensities can be high enough to saturate the resonance. As computer processing speeds have increased, we have incorporated more complicated physical models into our DNI codes. During the last project period we developed numerical methods for nonperturbative calculations of the two-photon absorption process. Experimentally, diagnostic techniques are developed and demonstrated in gas cells and/or well-characterized flames for ease of comparison with model results. The techniques of two-photon, two-color H-atom LIPS and three-laser ERE CARS for NO and C{sub 2}H{sub 2} were demonstrated during the project period, and nonperturbative numerical models of both of these techniques were developed. In addition, we developed new single-mode, injection-seeded optical parametric laser sources (OPLSs) that will be used to replace multi-mode commercial dye lasers in our experimental measurements. The use of single-mode laser radiation in our experiments will increase significantly the rigor with which theory and experiment are compared

LINE STRENGTHS FOR SINGLE-PHOTON TRANSITIONS BETWEEN DOUBLET LEVELS

The focus of this paper is the calculation of radiative transition rates between doublet electronic levels in diatomic molecules. A different formulation for listing Hönl-London factors is introduced that enables the straightforward inclusion of higher-order correction factors for effects such as centrifugal distortion and Lambda-doubling in the calculation of the radiative transition rates. The formulae for the Hönl-London factors are developed using Hund’s case (a) basis states and the correction factors for spin-orbit splitting, centrifugal distortion, spin-rotation interactions, and Lambda-doubling and included in the Hund’s case (a) coefficients for the state wavefunctions. Inclusion of Herman-Wallis effects in the calculation of the radiative transition rates is also illustrated for the hydroxyl radical and nitric oxide. The Herman-Wallis correction factors are incorporated in a straightforward fashions in the tables of line strength factors

Femtosecond coherent anti-Stokes Raman scattering measurement of gas temperatures from frequency-spread dephasing of the Raman coherence

Gas-phase temperatures and concentrations are measured from the magnitude and decay of the initial Raman coherence in femtosecond coherent anti-Stokes Raman scattering (CARS). A time-delayed probe beam is scattered from the Raman polarization induced by pump and Stokes beams to generate CARS signal; the dephasing rate of this initial coherence is determined by the temperature-sensitive frequency spread of the Raman transitions. Temperature is measured from the CARS signal decrease with increasing probe delay. Concentration is found from the ratio of the CARS and nonresonant background signals. Collision rates do not affect the determination of these quantities