Jun Kojima

Spatially resolved measurement of OH*, CH*, and C2* chemiluminescence in the reaction zone of laminar methane/air premixed flames

Spatially and spectrally resolved chemiluminescence were measured in the reaction zone of a laminar premixed methane/air flame at equivalence ratios of 0.9–1.5 in atmospheric pressure using a new Cassegrain mirror system and a spectrometer with an intensified charge-coupled device. The measurement volume of the Cassegrain optics was estimated to be 100 μm in diameter and 800 μm long. Local flame spectra of the reaction zone showed remarkably high intensity of OH * , CH * , and C 2 * emission bands. The distributions of OH * , CH * and C 2 * emission intensities near the flame front were roughly revealed. The emission zone thickness of C 2 * was found to be the thinnest among these radicals. The OH * emission zone had a sharp distribution within a reaction zone, but its thickness was a little wider than that of either CH * or C 2 * . Highly spectrally resolved local OH * , CH * , and C 2 * chemiluminescent spectra were obtained at the local flame front. It was found that the dependence of the intensity of each rotational and vibrational head of the chemiluminescence on the equivalence ratio was almost the same as that of spectrally integrated band emissions of OH * , CH * and C 2 * . It was found that the strong correlations between the peak intensity ratios of OH * /CH * , C 2 * , and C 2 * /OH * in the reaction zone to the equivalence ratio could be used to investigate the local flame stoichiometry. OH * /CH * can be a good marker to determine the local flame stoichiometry in the reaction zone of methane/air premixed flames for a wide range of equivalence ratios.

Laser pulse-stretching with multiple optical ring cavities

We describe a simple and passive nanosecond-long laser-pulse stretcher using multiple optical ring cavities. We present a model of the pulse-stretching process for an arbitrary number of optical ring cavities. This new model explicitly includes the effects of cavity delay time, beam-splitter reflectivity, total number of optical cavities, and describes the effects of spatial profile sensitivity. Using the model, we optimize the design of a pulse stretcher for use in a spontaneous Raman-scattering excitation system that avoids laser-induced plasma spark problems. From the optimized design, we then experimentally demonstrate and verify the model with a three-cavity pulse-stretcher system that converts a 1000-mJ, 8.4-ns-long input laser pulse into an approximately 75-ns-long (FWHM) output laser pulse with a peak power reduction of 0.10x and an 83% efficiency.

Measurement of the local flamefront structure of turbulent premixed flames by local chemiluminescence

Local chemiluminescence measurements of OH * , CH * , and C 2 * were made at the flamefront of premixed turbulent propane flames to study details of the reaction zone and flamefront structure. The relationship between excited OH, CH, and C 2 concentrations and the intensity of chemiluminescence emissions was investigated by PREMIX (GRI-Mech), and a strong correlation was proven. Cassegrain optics were developed to detect the intensity of these chemiluminescent emissions simultaneously at a single point to obtain flame spectroscopic data for a local point. The relationship between the scale of turbulence and the thickness of the flamefront was investigated using simultaneous measurement of the three chemiluminescences and laser Doppler velocimetry. Particle velocimetry was also used to demonstrate combustion flow characteristics of interest. It was found that local chemiluminescence can provide very useful spectroscopic data on the flamefront region. Time-series chemiluminescence can describe the timescale of the reaction and the flows at the flame front. Furthermore, the structure of the flamefront can be characterized by the duration of the chemiluminescence and by the patterns of its occurrence, which were found to correspond to the thickness of the flamefront and the Damkohler number. A new technique for measuring the local flamefront structure using local chemiluminescence measurements is proposed, and its performance is proven.

Local chemiluminescence spectra measurements in a high-pressure laminar methane/air premixed flame

The spatially resolved chemiluminescence in the reaction zone of a laminar premised methane/air flamewas measured. The pressure dependence of the flame front structure and chemiluminescence spectra were examined up to 1.5 MPa. Local point measurements of chemiluminescence were obtained using the Cassegrain optics system that we have developed, which has a high spatial resolution of d=0.1 mm and L =0.8 mm. Two types of measurements were performed: a local point chemiluminescence measurement with an ICCD camera with monochromator and selected chemiluminescence intensity measurements to obtain its profile and the flame front thickness at the flame front. The chemiluminescence at different pressures was examined to identify the main OH * , CH * , and C 2 * spectra. Background spectra related to CO-O * were observed at pressures greater than 0.5 MPa. Detailed OH * and CH * spectra were measured at different pressures, and the results indicate that each vibrational and rotational peak is in the same location, regardless of the pressure. The chemiluminescence intensity ratio of OH * /CH * at different pressures was examined. Less dependence on pressure was observed, although the individual chemiluminescence intensities changed with increasing pressure. The CO-O * broadband spectrum was significant at 0.5 MPa: its intensity remained at the same level at pressures greater than 0.5 MPa. This background spectrum was not observed at 0.1 MPa. In order to examine these chemiluminescence intensity profiles with the reaction zone and their dependence on pressure, the peak locations of OH * , CH * , and C 2 * were measured. The OH * peak was located inside the CH * and C 2 * curves for all measured pressures.

Measurement and simulation of spontaneous Raman scattering in high-pressure fuel-rich H2–air flames

Rotational and vibrational spontaneous Raman scattering (SRS) spectra of H2, N2 and H2O have been measured in H2–air flames at pressures up to 30.4 bar as a first step towards establishing a comprehensive Raman spectral database for temperatures and species in high-pressure combustion. We have obtained an initial set of measurements that indicate the spectra are of sufficient quality in terms of spectral resolution, wavelength coverage and signal-to-noise ratio for use in the development of transferable standards for the cross-talk calibration matrix. The fully resolved Stokes and anti-Stokes shifted spectra were collected in the visible wavelength range (400–700 nm) using pulse-stretched 532 nm excitation and a spectrograph fitted with a non-intensified CCD detector and a high-speed shutter. Temperatures were determined via the intensity distribution of rotational H2 lines at stoichiometric and fuel-rich conditions. A discussion of the temperature measurement accuracy in terms of the number of laser shots, including a single-shot measurement, is presented. Theoretical Raman spectra of hydrogen were calculated using a semi-classical anharmonic-oscillator model with recent pressure broadening data and were compared with experimental data. The data and simulation showed good agreement at different equivalence ratios and pressures and indicate that high-J rotational lines of H2 may interfere with the N2 vibrational Q-branch lines, which could lead to errors in N2-Raman thermometry based on the line-fitting method. In addition, the relative intensities of the O- and S-branches to the Q-branch were determined theoretically and the result indicates that further studies of spectral interferences including contributions from O- and S-branches should be pursued. Finally, from a comparison of N2 Q-branch spectra in lean H2–air flames at nearly atmospheric (1.2 bar) and high pressure (30.4 bar), we found no significant line-narrowing or -broadening effects at a spectral resolution of 0.04 nm.

Multi-point time-series observation of optical emissions for flame-front motion analysis

A simple optical diagnostic technique to analyse flame-front movement is proposed. Time-series, multi-point optical emission measurements using an aberration-free Cassegrain probe coupled with multiple fibres were performed successfully in turbulent premixed flames. The spatial resolution of the Cassegrain optics was examined using a CCD camera in free space and also in turbulent flame conditions. Spatial intensity distributions of OH, CH and C2 chemiluminescence measured in a two-dimensional laminar flame were compared with the numerical calculation of one-dimensional profiles of these species, demonstrating the ability of the present optical technique to detect a flame front. The probability density of the instantaneous speed and orientation angle of flame-front displacement were determined locally through a geometric motion analysis of simultaneous, time-evolutional OH* signals at three points. Flow fields measured by PIV were compared with the flame-front velocities to examine differences between gas flow and flame-front dynamics. Errors due to flame curvature, thermal density change and inclined propagation were also estimated.

Single-shot rotational Raman thermometry for turbulent flames using a low-resolution bandwidth technique

An alternative optical thermometry technique that utilizes the low-resolution (order 101 cm−1) pure-rotational spontaneous Raman scattering of air is developed to aid single-shot multiscalar measurements in turbulent combustion studies. Temperature measurements are realized by correlating the measured envelope bandwidth of the pure-rotational manifold of the N2/O2 spectrum with a theoretical prediction of a species-weighted bandwidth. By coupling this thermometry technique with conventional vibrational Raman scattering for species determination, we demonstrate quantitative spatially resolved, single-shot measurements of the temperature and fuel/oxidizer concentrations in a high-pressure turbulent CH4–air flame. Our technique provides not only an effective means of validating other temperature measurement methods, but also serves as a secondary thermometry technique in cases where the anti-Stokes vibrational N2 Raman signals are too low for a conventional vibrational temperature analysis.

Subframe Burst Gating for Raman Spectroscopy in Combustion

We describe an architecture for spontaneous Raman scattering utilizing a frame-transfer CCD sensor operating in a subframe burst-gating mode to realize time-resolved combustion diagnostics. The technique permits all-electronic optical gating with microsecond shutter speeds (<5 micros) without compromising optical throughput or image fidelity. When used in conjunction with a pair of orthogonally polarized excitation lasers, the technique measures single-shot vibrational Raman scattering that is minimally contaminated by problematic optical background noise.

Development of Hyperspectral remote sensing capability for the early detection and monitoring of Harmful Algal Blooms (HABs) in the Great Lakes

Hyperspectral imagers have significant capability for detecting and classifying waterborne constituents. One particularly appropriate application of such instruments in the Great Lakes is to detect and monitor the development of potentially Harmful Algal Blooms (HABs). Two generations of small hyperspectral imagers have been built and tested for aircraft based monitoring of harmful algal blooms. In this paper a discussion of the two instruments as well as field studies conducted using these instruments will be presented. During the second field study, in situ reflectance data was obtained from the Research Vessel Lake Guardian in conjunction with reflectance data obtained with the hyperspectral imager from overflights of the same locations. A comparison of these two data sets shows that the airborne hyperspectral imager closely matches measurements obtained from instruments on the lake surface and thus positively supports its utilization for detecting and monitoring HABs.

Observation of Turbulent Mixing in Lean-Direct-Injection Combustion at Elevated Pressure

We report the first quantitative single-shot multiscalar data obtained from a realistic air-fed lean-direct-injection burner operating on gaseous methane (CH 4 ) fuel at elevated pressure (5 atm) using single-shot spontaneous Raman spectroscopy. From a statistical analysis of the multiscalar data, we present spatially mapped probability density functions of the concentration of CH 4 and O 2 , and the instantaneous temperature. The measured three-scalar correlations and probability density functions provide insights into the nature and extent of the mixing process and its impact on the subsequent combustion process. The data will also be useful for comparison with the various turbulence-chemistry interaction models such as large-eddy simulation. The swirl-stabilized flame investigated in this paper was characterized as operating in a partially premixed combustion regime that was dominated by turbulent mixing provided by the lean-direct-injection configuration. Although a majority of the single-shot data indicated complete or near-complete reactions including stoichiometric combustion, a considerable number of the data points exhibited incomplete combustion characterized by a substantial amount of residual fuel at intermediate temperatures or were simply unreacted with little or no preheating of the mixture.