Jeffrey A. Sutton

Rayleigh scattering cross sections of combustion species at 266, 355, and 532 nm for thermometry applications

Rayleigh scattering cross sections are measured for nine combustion species (Ar, N2, O2, CO2, CO, H2, H2O, CH4, and C3H8) at wavelengths of 266, 355, and 532 nm and at temperatures ranging from 295 to 1525 K. Experimental results show that, as laser wavelengths become shorter, polarization effects become important and the depolarization ratio of the combustion species must be accounted for in the calculation of the Rayleigh scattering cross section. Temperature effects on the scattering cross section are also measured. Only a small temperature dependence is measured for cross sections at 355 nm, resulting in a 2-8% increase in cross section at temperatures of 1500 K. This temperature dependence increases slightly for measurements at 266 nm, resulting in a 5-11% increase in cross sections at temperatures of 1450 K.

Optimization of CH fluorescence diagnostics in flames: range of applicability and improvements with hydrogen addition.

This study quantifies the range of premixed flame conditions for which CH fluorescece diagnostics are applicable, and it shows that the CH fluorescence signal can be increased if some of the hydrocarbon fuel is replaced with hydrogen. The CH fluorescence signal is found to be adequate for fuel-air equivalence ratios (phi) as small as 0.85 for both methane-air and propane-air flames. The CH signal increases until a maximum at phi = 1.25 and phi = 1.35 for methane-air and propane-air flames, respectively, and then decreases for richer conditions. A strategy to increase the CH fluorescence signal and decrease interference from soot precursors is proposed by addition of the proper amount of hydrogen to the hydrocarbon fuel. Hydrogen addition reduces the background signal from soot precursors by as much as afactor of 10 and increases the CH fluorescence signal by as much as 80%. The normalized CH fluorescence measurements are compared with computations that utilize GRI-MECH 3.0 chemistry. Sources experimental uncertainties are discussed.

Scalar dissipation rate measurements in flames: A method to improve spatial resolution by using nitric oxide PLIF

Scalar dissipation rate ( χ ) previously has been identified as one of the most important parameters inturbulent non-premixed flames. A new method to image χ is demonstrated which is a variation of the method of Strner, Bilger, Frank, and Long; nitric oxide (NO) is added to the jet fluid to mark the fuel, replacing previous fuel markers such as acetone. Planar laser-induced fluorescence of NO is combined with temperature images from Rayleigh scattering to measure mixture fraction and scalar dissipation rate with greatly improved signal-to-noise ratio (SNR) and spatial resolution. Calibration measurements of mixture fraction ( f ) and χ in a laminar flame and qualitative images of a turbulent flame are presented. The method has three advantages. (1) Differential diffusion problems (that have limited the use of acetone) are eliminated, since the diffusivity of NO is equivalent to that of the methane/nitrogen fuel mixture. (2) NO is an excellent marker of the fuel (unlike acetone) since it properly disappears from centerline to the flame boundary due to rapid reactions with H and CH radicals. A small correction (typically 5% of the centerline value) must be applied to account for slight leakage of NO through the CH layer, but this correction can be made entirely from the experimental data. (3) NO can be added in large amounts to achieve ecceptional SNR values exceeding 150 when the spatial resolution equals a typical Taylor scale of 400 μ m. Radial profiles of mixture fraction and scalar dissipation rate in the calibration flame compare favorably with previous measurements.

A Burner Platform for Examining the Effects of Non-Equilibrium Plasmas on Oxidation and Combustion Chemistry

In this communication, we describe the development of a new plasma/flame facility and burner platform that appears promising for directly investigating the effects of high-voltage, nanosecond-duration, repetitively pulsed plasma discharges on moderate- and high-temperature reaction chemistry. Such a configuration is ideal for identifying key processes and key species that can alter fuel oxidation, hydrocarbon intermediate formation, and radical formation/heat-release under combusting conditions. Initial results using emission spectroscopy demonstrate that the excited-state species such as OH*, CH*, and are significantly enhanced in the presence of repetitive nanosecond pulse discharges. This new plasma-flame facility also lends itself to kinetic modeling due to its simple quasi-one-dimensional geometry and uniformity of the nanosecond pulse discharge.

Towards High-Repetition Rate Rayleigh and Raman Scattering Imaging in Turbulent Jets and Flames

In this paper we will describe recent advances made in our laboratory in the development of high-repetition-rate Rayleigh and Raman scattering imaging capabilities. High-repetition-rate 1D Raman and 2D Rayleigh scattering imaging capabilities are being developed to image the time-varying mixture fraction and temperature fields in turbulent non-reacting and reacting flows. Initial results using a custom pulse-burst laser system at Ohio State University have demonstrated the ability to capture ten sequential 2D Rayleigh scattering images at a repetition rate of 10 kHz in both turbulent non-reacting jets and non-premixed jet-flames with pulse energies approaching 200 mJ at 532 nm. This paper will also describe the development and pending application of a new higher-energy, long-duration, next-generation burst-mode laser system and the use of higher resolution cameras for high-speed Raman/Rayleigh imaging.

Quantitative fuel vapor/air mixing imaging in droplet/gas regions of an evaporating spray flow using filtered Rayleigh scattering.

This Letter demonstrates the application of filtered Rayleigh scattering (FRS) for quantitative two-dimensional fuel vapor/air mixing measurements in an evaporating hydrocarbon fuel spray flow. Using the FRS approach, gas-phase measurements are made in the presence of liquid-phase droplets without interference. Effective suppression of the liquid-phase droplet scattering using FRS is enabled by the high spectral purity of the current Nd:YAG laser system. Simultaneous Mie-scattering imaging is used to visualize the droplet field and illustrate the droplet loading under which the FRS imaging is applied in the current spray flows. The initial quantification of the FRS imaging is based on calibration measurements from a flow cell of known fuel vapor/air mixtures, while future work targets the utilization of a Rayleigh-Brillouin spectral model for quantification of the FRS signals.

Towards the Development of High-Speed 1D Raman Scattering in Turbulent Non-premixed Flames

In this paper we will describe recent advances made in our laboratory in the development of high-speed (10-kHz acquisition rate) 1D Raman scattering imaging. The ultimate goal is a capability of quantitatively measuring all major combustion species and deducing time-varying mixture fraction profiles in turbulent combustion environments. In this paper we will review our initial results depicting high-speed Raman scattering imaging of O2, N2, CH4, and H2 in a turbulent non-reacting CH4/H2 jet issuing into air (Appl Phys B, 101(1), 2010, p. 1-5). This work represented the first temporally-sequential 1D image sequences of major species measuring via high-speed Raman scattering. This paper will also describe our most recent work towards the application in combustion environments by examining system performance through single-shot measurements (at 10 Hz) in wellcharacterized laminar flames.

Simultaneous High-Resolution kHz-Rate 2-D Conserved Scalar and 3-Component Velocity Field Measurements in Gas-Phase Turbulent Jets

Turbulent scalar mixing processes are of significant interest to a broad range of fields within the fluid and thermal sciences. An improved understanding of the underlying physics of how two independent fluid streams mix under turbulent flow conditions is important for a multitude of engineering systems and natural processes ranging from the mixing of a fuel and oxidizer in combustion chambers to the dispersal of pollutants within the atmosphere. The dispersion and mixing of scalars is dependent on the local three-dimensional velocity field, which for turbulent flows, fluctuates in both space and time. In this manner, it is important to have both scalar and velocity measurements with high spatial and temporal resolution across a large dynamic range of scales. Laser-based imaging diagnostics have made it possible to investigate the underlying structure of turbulent flows with high spatial resolution; however, the majority of commonlyused scalar measurement techniques have been limited to low acquisition rates in gas-phase flows. Typically, gasphase measurements utilize techniques which rely upon high laser pulse energies (i.e., 100’s mJ/pulse) due to the relatively “weak” signals collected from the scalar of interest. Such levels of output pulse energy are not currently possible with commercially-available high-speed laser systems. In the present work we demonstrate simultaneous acetone planar laser-induced fluorescence (PLIF) and stereoscopic particle imaging velocimetry (sPIV) at 10 kHz. The acetone acts as a tracer to mark the conserved scalar field and the sPIV measurements yield all three components of the turbulent velocity field. The PLIF measurements are facilitated by a High-Energy Pulse-Burst Laser System (HEPBLS), which can generate 266-nm pulse trains for burst durations exceeding 20 ms at 10 kHz with >140 mJ/pulse. The fourth-harmonic output of the HEPBLS is approximately an order of magnitude higher than any previously-reported kHz-rate 266-nm output. The high pulse energies available from the HEPBLS allows our preliminary measurements to be performed with low tracer seed levels (4% acetone in the main jet and 0.75% acetone in the co-flow) which significantly reduces absorption effects and facilitates quantitative measurements. A sample image set of the scalar field, where the velocity field is left off for visual clarity, is shown in Fig. 1 demonstrating the detail in both time and space that is possible with the current measurements. The sPIV measurements are made with an EdgeWave IS80-2-LD double-pulsed PIV laser capable of 40 Watts of average power per laser head. While PIV measurements are possible using the HEPBLS, an independent laser system is employed for the PIV measurement to allow for an optimization of the temporal spacing between the PIV double pulses and an accurate temporal placement of the PIV laser pulses in reference to the scalar measurement. Both of these considerations are important when attempting to accurately correlate an inferred velocity field to an instantaneous scalar measurement. The two laser systems are coupled with two Vision Research VR710 (sPIV) and one VR711 (PLIF) cameras for the data collection. The PLIF imaging system includes a 240-mm focal length acromat lens coupled with an 85 focal length f#1.4 Nikkor camera lens to maximize the signal while maintaining the desired resolution of 55 um x 55 um per pixel. The sPIV cameras are each mounted with a scheimpflug adapter and a 200-mm focal length Nikkor Macro lens leading to a resolution of 28 um x 28 um per pixel. For the given repetition rate of 10 kHz the fields of view of the PLIF and PIV cameras are 27.5 mm x 60.5 mm and 11 mm x 25 mm, respectively. The current work is centered on developing a new measurement capability for examining the time-dependent coupling between the turbulent velocity field and conserved scalar mixing in non-reacting gas-phase axisymmetric jets over a broad range of Reynolds number. Our preliminary results show significant promise for the methods described above. Simultaneous 3-component velocity and 2-D scalar imaging will provide the necessary data (both visualization and spatio-temporal statistics) to study the complex interaction between the velocity and scalar fields. Beyond an increased understanding of the underlying physics, new spatiallyand temporally-resolved data will provide new, detailed information for assessing and validating turbulence models.

Investigation of the Effects of Non-Equilibrium Plasma Discharges on Temperature and OH Concentrations in Low- Pressure Premixed Flames

In this paper, we investigate the effects of nanosecond, repetitively-pulsed, non-equilibrium plasma discharges on laminar, low-pressure, premixed burner-stabilized hydrogen/O2/N2 and hydrocarbon/O2/N2 flames using OH laser-induced fluorescence (LIF). Two different plasma sources, both of which generate uniform, low-temperature, volumetric, non-equilibrium plasma discharges, are used to study changes in temperature and radical species concentrations when non-equilibrium plasmas are directly coupled to conventional hydrogen/hydrocarbon oxidation and combustion chemistry. Qualitative imaging of flame chemiluminescence indicates that during plasma discharge, lean hydrocarbon flames “move” upstream in addition to showing a broadening of the flame chemiluminescence. For the same plasma discharge and flame conditions, quantitative results using spatially-resolved OH LIF and multi-line, OH LIF-thermometry show significant increases in ground-state OH concentrations in the preheating zones of the flame. More specifically, for a particular axial position downstream of the burner surface, the OH concentration increases, which also can be viewed as an effective “shift” of the OH profiles towards the burner surface. The increase in OH concentration is conceivably due to an enhancement of the lowertemperature kinetics including O atom, H atom and OH formation kinetics and temperature rise due to the presence of the low-temperature, nonequilibrium plasma.

Quantification and Accuracy of a CMOS-Based Raman Scattering Imaging System for High-Speed Measurements in Flames

In this paper we will describe recent work in our laboratory towards the quantification of a high-speed (> 10 kHz) combined 1D Raman-Rayleigh scattering imaging system utilizing CMOS-based cameras. While our previous work has demonstrated the ability to acquire high-speed Raman/Rayleigh scattering images using a pulse burst laser system (Gabet et al., 2010), further study of the acquisition system is necessary for quantitative results. For the majority of high-speed imaging experiments, CMOS cameras are used because conventional CCD cameras cannot operate at sufficiently high acquisition rates to capture the full range of temporal scales and fluctuations in turbulent flows. Unlike CCD cameras, which typically have uniform and linear pixel response, each pixel on CMOS cameras has a unique response which needs to be characterized individually (Patton et al., 2011; Weber et al., 2011). In addition, CMOS cameras are known to exhibit increased levels of noise, particularly when coupled with an image intensifier. Careful examination and calibration of CMOS-based acquisition systems is of particular importance to understand their limitations and accuracy for low-signal applications such as Raman scattering. This paper will focus on quantifying the precision and accuracy of Raman/Rayleigh scattering measurements of major species, temperature, and mixture fraction using our CMOS-based 1D Raman/Rayleigh system in a series of near-adiabatic H2/air flames and turbulent H2/N2 jet flames. A detailed analysis of the spectral response and signal-to-noise ratio (SNR) of major species (H2, N2, H2O, and O2) and temperature is presented. The ability to measure “single-shot” scalar values accurately in turbulent flames is assessed by comparing scalar results in the DLR H3 (50% N2/50% H2 Re=10,000) turbulent jet flame to previous work (Meier et al., 1996). The ultimate goal of our research is to measure the time-varying profiles of all major combustion species and deduce temporally resolved mixture fraction profiles in turbulent combustion environments.