Jonathan H. Frank

Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames

Measurements of temperature, the major species (N2, O2, CH4, CO2, H2O, CO, and H2), OH, and NO are obtained in steady laminar opposed-flow partially premixed flames of methane and air, using the non-intrusive techniques of Raman scattering and laser-induced fluorescence. Flames having fuel-side equivalence ratios of φ = 3.17, 2.17, and 1.8 are stabilized on a porous cylindrical burner (Tsuji burner) in a low-velocity flow of air. Results are compared with calculations using a version of the Sandia laminar flame code that is formulated for the Tsuji geometry and includes an optically thin treatment of radiation. Because velocity profiles are not measured, the strain rate in each calculation is adjusted to match the measured profile of the mixture fraction. Measured profiles of temperature and species mass fractions are then compared with results of calculations using the GRI-Mech 2.11 and 3.0 chemical mechanisms, as well as a detailed mechanism from Miller. All three mechanisms give agreement with experimental results for the major species that is generally within experimental uncertainty. With regard to NO formation, the relative performance of the three mechanisms depends on the fuel-side equivalence ratio. GRI-Mech 2.11 gives reasonably good agreement with measured NO levels in lean and near-stoichiometric conditions, but it under predicts NO levels in fuel-rich conditions. GRI-Mech 3.0 significantly over predicts the peak NO levels the φ = 3.17 and 2.17 flames, but it yields relatively good agreement with measurements in the φ = 1.8 flame. The Miller mechanism gives good agreement with measured NO levels in the φ = 3.17 flame, but it progressively over predicts peak NO levels in the leaner flames. Comparisons of adiabatic and radiative calculations show that radiation can have a significant effect on the width and structure of partially premixed flames, as well as on the levels of NO produced.

A white light confocal microscope for spectrally resolved multidimensional imaging.

Spectrofluorometric imaging microscopy is demonstrated in a confocal microscope using a supercontinuum laser as an excitation source and a custom‐built prism spectrometer for detection. This microscope system provides confocal imaging with spectrally resolved fluorescence excitation and detection from 450 to 700 nm. The supercontinuum laser provides a broad spectrum light source and is coupled with an acousto‐optic tunable filter to provide continuously tunable fluorescence excitation with a 1‐nm bandwidth. Eight different excitation wavelengths can be simultaneously selected. The prism spectrometer provides spectrally resolved detection with sensitivity comparable to a standard confocal system. This new microscope system enables optimal access to a multitude of fluorophores and provides fluorescence excitation and emission spectra for each location in a 3D confocal image. The speed of the spectral scans is suitable for spectrofluorometric imaging of live cells. Effects of chromatic aberration are modest and do not significantly limit the spatial resolution of the confocal measurements.

Conserved scalar measurements in turbulent diffusion flames by a Raman and Rayleigh ribbon imaging method

Abstract A new method to obtain images of conserved scalars in turbulent flames is presented and implemented with simultaneous Rayleigh and fuel Raman measurements in a methane/air jet diffusion flame by the use of a single dye laser and two intensified CCD cameras. The laser beam is focused to a line and retroreflected with a slight offset to form a thin ribbon, sufficient to measure gradients in two dimensions. A robust, iterative data reduction technique is used to derive statistics of temperature, fuel mass fraction, mixture fraction ( f ), and scalar dissipation (χ). Results for a flame of Reynolds number 20,600 show that the lower moments, pdfs, and scatter plots of the computed quantities do not differ markedly from published results of point measurements in similar flames, strengthening confidence in this new approach. The computed components of χ show behavior similar to that in nonreacting flows; there is some anisotropy, with the ratio of the radial to the axial component in the shear region around 2.0. The azimuthal component, measured by off-axis laser beam alignment, is roughly equal to the radial component. The correlation between f and χ is small on the flame axis, but the correlation coefficient R fχ rises to around 0.4 near the edge, which is largely consistent with other recent results for cold jets and jet flames.

REACTION-RATE, MIXTURE-FRACTION, AND TEMPERATURE IMAGING IN TURBULENT METHANE/AIR JET FLAMES

Instantaneous two-dimensional measurements of reaction rate, mixture fraction, and temperature are demonstrated in turbulent partially premixed methane/air jet flames. The forward reaction rate of the reaction CO OH ⇒ CO2 H is measured by simultaneous OH laser-induced fluorescence (LIF) and two-photon CO LIF. The product of the two LIF signals is shown to be proportional to the reaction rate. Temperature and fuel concentration are measured using polarized and depolarized Rayleigh scattering. A three-scalar technique for determining mixture fraction is investigated using a combination of polarized Rayleigh scattering, fuel concentration, and CO LIF. Measurements of these three quantities are coupled with previous detailed multiscalar point measurements to obtain the most probable value of the mixture fraction at each point in the imaged plane. This technique offers improvements over two-scalar methods, which suffer from decreased sensitivity around the stoichiometric contour and biases in fuel-rich regions due to parent fuel loss. Simultaneous reaction-rate, mixture-fraction, and temperature imaging is demonstrated in laminar (Re 1100) and turbulent (Re 22,400) CH4/air (1/3 by volume) jet flames. The turbulent jet flame is the subject of multiple numerical modeling efforts. A primary objective for developing these imaging diagnostics is to provide measurements of fundamental quantities that are needed to accurately model interactions between turbulent flows and flames.

Scalar Dissipation Measurements in Turbulent Jet Diffusion Flames of Air Diluted Methane and Hydrogen

Abstract Simultaneous two-dimensional Rayleigh and fuel Raman images have been collected in air-diluted methane and hydrogen jet diffusion flames. Temperature, fuel mass fraction and mixture fraction images are derived by a two-scalar approach based on one-step chemistry and equal species diffusivities. This enables calculation of two components of the scalar dissipation rate x-The inherently weak Raman signal has been maximised by intra-cavity measurements, using a flashlamp-pumped dye laser. In addition, the Raman signal-to-noise ratio is drastically improved by a novel contour-aligned smoothing technique which exploits the high correlation between the Rayleigh and Raman signals. Quantitative measurements of scalar dissipation are presented, including probability density functions for components of x- Profiles of mean and rms mixture fraction show the usual features already documented in other published results for this type of flame. Probability density functions of ξ are close to Gaussian on the axis,...

Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames

Two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen is investigated in premixed hydrogen and methane flames with nanosecond and picosecond pulsed lasers at 226 nm. In the hydrogen flame, the interference from photolysis is negligible compared with the LIF signal from native atomic oxygen, and the major limitations on quantitative measurements are stimulated emission and photoionization. Excitation with a nanosecond laser is advantageous in the hydrogen flames, because it reduces the effects of stimulated emission and photoionization. In the methane flames, however, photolytic interference is the major complication for quantitative O-atom measurements. A comparison of methane and hydrogen flames indicates that vibrationally excited CO2 is the dominant precursor for laser-generated atomic oxygen. In the methane flames, picosecond excitation offers a significant advantage by dramatically reducing the photolytic interference. The prospects for improved O-atom imaging in hydrogen and hydrocarbon flames are presented.

POLARIZED/DEPOLARIZED RAYLEIGH SCATTERING FOR DETERMINING FUEL CONCENTRATIONS IN FLAMES

Rayleigh scattering has been shown to be a useful diagnostic technique for two-dimensional imaging studies of reacting and non-reacting flows. For example, by combining Rayleigh scattering with a simultaneous measurement of the fuel concentration (e.g., using Raman scattering), mixture fraction and temperature can be determined in flames. In this work, it is demonstrated that the fuel concentration can be obtained by measuring the polarized and depolarized components of the Rayleigh signal and taking their difference or a suitable linear combination. While the depolarized Rayleigh signal is smaller than the polarized signal by a factor of ≈100, this is still a factor of ≈10 larger than the Raman scattering. Application of the technique requires that one of the primary constituents of the fuel stream possess a depolarization ratio sufficiently different from that of the oxidizer. Methane is a convenient candidate as it has no measurable depolarization. Results are shown for methane flames diluted by argon as well as air.

Mixture fraction imaging in turbulent nonpremixed hydrocarbon flames

Three different experimental approaches for making instantaneous, two-dimensional (2D) measurementsof the mixture fraction in turbulent nonpremixed hydrocarbon flames are presented. The availability of spatial information on the mixture fraction provided by these techniques makes determination of the scalar dissipation feasible. Each of the techniques to be presented is based on simultaneously imaging fuel concentration and Rayleigh scattering. Measurement of the fuel concentration can be accomplished in a number of ways, including (1) Raman scattering from the fuel, (2) fluorescence from the fuel, or (3) fluorescence from a molecular tag added to the fuel. Preliminary results for experiments utilizing each of these approaches will be presented. In one experiment, joint Raman/Rayleigh measurements were performed in a turbulent methane flame.This approach worked quite well and was able to provide valuable mixture fraction statistics. However, the weak Raman signal required a tradeoff between signal/noise and spatial resolution. A second experiment involved imaging fuel fluorescence from an acetaldehyde flame. The acetaldehyde fluorescence/Rayleigh images obtained in this experiment were of high quality. On the fuel-rich side of the flame, however, the calculated mixture fraction shows a dip and plateau. These irregularities may be the result of fuel pyrolysis and could represent a limitation of this approach. A third method of mixture fraction measurement was to introduce acetone as a fuel tracer and image its fluorescence. Acetone fluorescence proved to be problematic when excited at a wavelength of 320 nm. Our measurements show indications of a temperature dependence of the fluorescence yield, making it difficult to interpret the signal.

Calibration of a wide-field frequency-domain fluorescence lifetime microscopy system using light emitting diodes as light sources

High brightness light emitting diodes are an inexpensive and versatile light source for wide‐field frequency‐domain fluorescence lifetime imaging microscopy. In this paper a full calibration of an LED based fluorescence lifetime imaging microscopy system is presented for the first time. A radio‐frequency generator was used for simultaneous modulation of light emitting diode (LED) intensity and the gain of an intensified charge coupled device (CCD) camera. A homodyne detection scheme was employed to measure the demodulation and phase shift of the emitted fluorescence, from which phase and modulation lifetimes were determined at each image pixel. The system was characterized both in terms of its sensitivity to measure short lifetimes (500 ps to 4 ns), and its capability to distinguish image features with small lifetime differences. Calibration measurements were performed in quenched solutions containing Rhodamine 6G dye and the results compared to several independent measurements performed with other measurement methodologies, including time correlated single photon counting, time gated detection, and acousto optical modulator (AOM) based modulation of excitation sources. Results are presented from measurements and simulations. The effects of limited signal‐to‐noise ratios, baseline drifts and calibration errors are discussed in detail. The implications of limited modulation bandwidth of high brightness, large area LED devices (∼40 MHz for devices used here) are presented. The results show that phase lifetime measurements are robust down to sub ns levels, whereas modulation lifetimes are prone to errors even at large signal‐to‐noise ratios. Strategies for optimizing measurement fidelity are discussed. Application of the fluorescence lifetime imaging microscopy system is illustrated with examples from studies of molecular mixing in microfluidic devices and targeted drug delivery research.

Mixture fraction imaging in a lifted methane jet flame

This work contributes to the study of the stabilization mechanism at the base of the classical lifted flame formed in a turbulent methane jet. Despite several recent efforts to elucidate the features of this mechanism, there is no consensus on the structural details of the flame base. In an effort to examine these mechanisms, the fine structural details at the flame base need recording. The authors present here high-resolution images of temperature and mixture fraction, using a two-scalar technique, simultaneously recording Rayleigh and fuel Raman images.