Aizaz H. Bhuiyan

Dual-pump coherent anti-Stokes Raman scattering system for temperature and species measurements in an optically accessible high-pressure gas turbine combustor facility

The development and implementation of a dual-pump coherent anti-Stokes Raman scattering (DP-CARS) system employing two optical sub-systems to measure temperature and major species concentrations at multiple locations in the flame zone of a high-pressure, liquid-fueled gas turbine combustor are discussed. An optically accessible gas turbine combustor facility (GTCF) was utilized to perform these experiments. A window assembly has been designed, fabricated, and assembled in the GTCF to allow optical access from three directions using a pair of thin and thick fused silica windows on each side. A lean direct injection (LDI) device consisting of an array of nine integrated air swirlers and fuel injectors was operated using Jet-A fuel at inlet air temperatures up to 725 K and combustor pressures up to 1.03 MPa. The DP-CARS system was used to measure temperature and CO2/N2 concentration ratio on single laser shots. An injection-seeded optical parametric oscillator (OPO) was used as a narrowband pump laser source in order to potentially reduce shot-to-shot fluctuations in the CARS data. Large prisms mounted on computer-controlled translation stages were used to direct the CARS beams either into the main leg optical system for measurements in the GTCF or to a reference leg optical system for measurements of the non-resonant spectrum and for alignment of the CARS system. The spatial maps of temperature and major species concentrations were obtained in high-pressure LDI flames by translating the CARS probe volume in the axial and vertical directions inside the combustor rig without loss of optical alignment.

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.

Dual-pump CARS Measurements in a Gas Turbine Combustor Facility using the NASA 9-Point Lean Direct Injector

Lean direct injection (LDI) has been among the seve ral combustion strategies being studied by NASA Glenn Research Center (GRC) to reduce oxides of nitrogen (NO x) emissions while maintaining high combustion effic iency [1]. The NASA 9-point top-hat LDI assembly is a multiplex fuel injector c ontaining nine fuel injection tips and multiple burning zones that replace one conventiona l fuel injector. The nine injectors are placed in a 3 × 3 square matrix arrangement. We have developed a gas turbine combustor facility (GTCF) at the High Pressure Laboratory in Purdue’s Zucrow Laboratory complex. The GTCF has been modified for optical access and dual-pump coherent anti-Stokes Raman scattering (DP-CARS) measurements have been performed at supersonic cruise conditions. A window assembly has been designed, fabricated, and assembled in the GTCF at Purdue University for advanced laser diagnostic stu dies. The window assembly allows optical access from three mutually perpendicular di rections using a pair of thin and thick fused silica windows on each side. The assembly is cooled using water while filmcooling air is provided for the inside of the thin windows. The thin windows are designed for thermal load while the thick windows a re designed for pressure loading. Combusting flows are studied using the central inje ctor of the aforementioned 9-point lean direct injection (LDI) device. The combustor has been operated using Jet-A fuel at inlet air temperatures up to 725 K and combustor pr essures up to 10.2 atm. DP-CARS temperature and major species concentration measurements have been performed in the GTCF at various operating conditions. An injection -seeded optical parametric oscillator (OPO) is used as a narrowband pump laser source so as to improve the accuracy and precision of the CARS measurements. Spatial maps of temperature and major species concentrations have been obtained in high-pressure LDI flames by translating the CARS probe volume in axial and radial directions inside the combustor rig without loss of optical alignment.

H2 Mole Fraction Measurements in a Microwave Plasma Using Coherent Anti-Stokes Raman Scattering Spectroscopy

In an effort to provide insights into the thermochemical composition of a microwave plasma chemical vapor deposition (MPCVD) reactor, the mole fraction of H2 is measured at various positions in the plasma sheath, at pressures of 10 and 30 Torr, and at plasma powers ranging from 300 to 700 W. A technique is developed by comparing the Q(1)01 transition of experimental and theoretical spectra aided by the Sandia CARSFT fitting routine. Results reveal that the mole fraction of H2 does not vary significantly from its theoretical mixture at the parametric conditions examined. Furthermore, the ν″=1→ν′=2 vibrational hot band was searched, but no transitions were found. An analytical explanation for the increase in the temperature of H2 with the introduction of N2 and CH4 is also presented. Finally, because the mole fraction of H2 does not appear to deviate from the theoretical composition, the rotational and translational modes of H2 are shown to be approximately in equilibrium, and therefore, the rotational temperatures may be used to estimate the translational temperatures of H2.

Laser 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 spatial distribution of the rotational temperature of hydrogen is investigated via coherent anti-Stokes Raman scattering 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, graphene and graphitic nanopetals. Temperature measurements are conducted within the plasma sheath and up to 6 mm away from the puck surface in order to elucidate the spatial distribution of temperature within and around the plasma region. The results indicate a linear increase in rotational temperature of H2 with respect to the distance normal from the puck surface. Temperatures also increase with pressure. At 10 Torr the temperature range is approximately 850–1150 K while at 30 Torr it is approximately 1200–1650 K for a plasma generator power of 500 W. In addition, the temperature increases with plasma generator power and the introduction of other substances such as CH4 and N2. These findings may aid in understanding the function of the chemical composition and reactions in the plasma environment of these reactors which, to date, remains obscure. The spectroscopic techniques applied in this work may prove to be suitable in-situ monitoring methods for the scalable manufacturing of carbon nanomaterials.Copyright

Two Color Polarization Spectroscopy for Measurements of Atomic Hydrogen Concentration using Single-Mode Laser Systems

A two-photon pump polarization spectroscopy probe (TPP-PSP) laser system has been designed for detection of atomic hydrogen (H-atom) in flames. Single frequency mode (SFM) laser output from injection-seeded optical parametric generators (OPG) coupled with pulsed dye amplifiers (PDA) have been used to produce the pump and probe laser beams for TPP-PSP experiment. In TPP-PSP, a 243-nm pump beam excites the 1S-2S two photon transition and the excited atoms in 2S level are probed by polarization spectroscopy between n=2 and n=3 manifolds using a circularly polarized 656-nm pump and a linearly polarized 656-nm probe laser beam. The SFM laser sources for the pump and probe beams allow accurate measurement of TPP-PSP line shapes. Atomic hydrogen was detected at concentrations as low as 11 ppm in atmospheric-pressure, near-adiabatic hydrogen-air flames. The measured atomic hydrogen concentration profiles are in good agreement with equilibrium calculations. The results obtained using SFM laser systems are very encouraging for quantitative measurements of atomic hydrogen in flames.

High-spectral-resolution Two-photon Pump Polarization Spectroscopy Probe Technique for H-atom Detection

We report the development of a two-photon pump polarization spectroscopy probe technique for detection of atomic Hydrogen using two single-mode laser sources. Significant effect of laser parameters on atomic transitions has been observed.

High-Spectral Resolution PLIF Imaging of Compressible Flows and Plasmas

Pulsed energy deposition using laser is an effectiv e method of local flow control. Therefore, careful investigation of the laser-induc ed plasma and the resulting flow field is required. Although some properties of the resulting plasma and flow field have been reported, there is still a need for detailed investigation of the flow field. We report the development of high-spectral-resolution planar laser induced fluor escence (PLIF) of nitric oxide (NO) for simultaneous measurement of pressure, temperature, and velocity in compressible flow field generated by laser-induced optical breakdown. This molecular-based technique eliminates the difficulties associated with particle lag related t o traditionally used particle-based techniques such as particle image velocimetry. An optical par ametric oscillator (OPO) coupled with pulsed dye amplifiers (PDA) was used as a radiation source to excite electronic transitions of NO in the A-X (0, 0) system. The output from injection-seede d OPO near 452 nm was amplified to ~13 mJ/pulse using two PDA stages, while maintaining the spectral linewidth of ~250 MHz fullwidth-half-maximum (FWHM). The OPO/PDA output was frequency-doubled and a laser sheet of the 226-nm UV light was produced to perform PLIF imaging in a plasma generated by focusing the second harmonic output (~532 nm) of another injection-seeded Nd:YAG laser. The spectrally narrow output from the OPO/PDA system gives the unique capability for simultaneous measurement of pressure, temperature, and velocity over a wide range of conditions in the investigated flow field.

Dual-Pump CARS Temperature and Major Species Concentration Measurements in Laminar Counterflow Flames and in a Gas Turbine Combustor Facility

[Abstract] A gas turbine combustor facility (GTCF) has been built and operated for measuring temperature and major species concentrations using dual-pump CARS in combusting flows at above atmospheric pressure. The facility includes a stainless steel window assembly that allows optical access from three sides with a pair of thin and thick windows on each side. It is water-cooled and provides air film-cooling passages; thin windows are designed for thermal load while thick windows are designed for pressure loading. High-speed imaging of combusting flows is performed using the center injector of a 9-point lean direct injection (LDI) device developed at NASA Glenn Research Center. The combustor is operated using Jet-A fuel at an inlet air temperature up to 670 K and combustor pressure up to 10 atm. In the future, dual-pump CARS temperature and major species concentration measurements will be performed in the GTCF. Spatial maps of both temperature and major species concentrations will be obtained by translating the CARS probe volume in axial and radial directions. These measurements will be used as validation data for CFD codes under development at NASA Glenn.

Dual-pump CARS Temperature and Major Species Concentration Measurements in a Gas Turbine Combustor Facility

A gas turbine combustor facility (GTCF) has been built and operated for measuring temperature and major species concentrations using dual-pump CARS in combusting flows at above atmospheric pressures. The facility includes a stainless steel window assembly that allows optical access from three sides with a pair of thin and thick windows on each side. It is water-cooled and provides air film-cooling passages; thin windows are designed for thermal load while thick windows are designed for pressure loading. High-speed imaging of combusting flows is performed using the center injector of a 9-point lean direct injection (LDI) device developed at NASA Glenn Research Center. The combustor has been operated using Jet A fuel at inlet air temperatures up to 725 K and combustor pressures up to 10 atm. Dual-pump CARS temperature and major species concentration measurements have been performed in the GTCF at inlet air temperatures up to 725 K and combustor pressures up to 7 atm. Spatial maps of temperature and major species concentrations have been obtained by translating the CARS probe volume in axial and radial directions inside the combustor rig. These measurements will be used for validation of CFD codes under development at NASA Glenn Research Center.