Marshall B. Long

Laser emission from individual droplets at wavelengths corresponding to morphology-dependent resonances

Inelastic emission characteristics from individual ethanol droplets (60-microm diameter) containing Rhodamine 6G dye and pumped by a cw laser (514.5 nm) were investigated. Laser emission was confirmed by noting the spectral, temporal, and output-versus-input intensity behavior. The liquid-air boundary of the droplets provides the optical feedback at selected wavelengths corresponding to the morphology-dependent resonances of a spherical droplet.

EXPERIMENTAL AND COMPUTATIONAL STUDY OF CH, CH*, AND OH* IN AN AXISYMMETRIC LAMINAR DIFFUSION FLAME

In this study, we extend the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. A procedure is outlined by which the number densities of ground-state CH (X 2P), excited-state CH (A2D, denoted CH*), and excited-state OH (A2R, denoted OH*) are measured and modeled. CH* and OH* number densities are deconvoluted from line-of-sight flame-emission measurements. Ground-state CH is measured using linear LIF. The computations are done with GRI Mech 2.11 as well as an alternate hydrocarbon mechanism. In both cases, additional reactions for the production and consumption of CH* and OH* are added from recent kinetic studies. Collisional quenching and spontaneous emission are responsible for the de-excitation of the excited-state radicals. As with our previous investigations, GRI Mech 2.11 continues to produce very good agreement with the overall flame length observed in the experiments, while significantly under predicting the flame liftoff height. The alternate kinetic scheme is much more accurate in predicting lift-off height but overpredicts the overall flame length. Ground-state CH profiles predicted with GRI Mech 2.11 are in excellent agreement with the corresponding measurements, regarding both spatial distribution and absolute concentration (measured at 4 ppm) of the CH radical. Calculations of the excited-state species show reasonable agreement with the measurements as far as spatial distribution and overall characteristics are concerned. For OH*, the measured peak mole fraction, 1.3 2 10 18 , compared well with computed peaks, while the measured peak level for CH*, 2 2 10 19 , was severely underpredicted by both kinetic schemes, indicating that the formation and destruction kinetics associated with excited-state species in flames require further research.

Simultaneous two-dimensional mapping of species concentration and temperature in turbulent flames

A new technique has been developed that permits simultaneous two-dimensional mapping of two scalar components (e.g., species and temperature) in a turbulent reacting flow. The technique uses two optical multichannel analyzers, each of which detects light scattered by a different mechanism (e.g., Rayleigh and Raman) and thus provides different information. The technique has been applied to both premixed and nonpremixed flames, and results are reported for each. The simultaneous information obtained in these experiments should provide new data on the interaction of turbulence and combustion in these chemically reacting flows.

Instantaneous Ramanography of a turbulent diffusion flame

The first reported instantaneous two-dimensional map of the fuel-gas concentration in a turbulent diffusion flame using quantitative Ramanography is described. Details of the experimental configuration are presented, along with data obtained in both cold flows and turbulent reacting flows.

Evaporation and condensation rates of liquid droplets deduced from structure resonances in the fluorescence spectra

A new optical technique, based on morphology-dependent peaks in the fluorescence spectra, is-used to determine the evaporation and condensation rates of a linear stream of ethanol droplets. The droplets are monodispersed, in close proximity to one another, and impregnated with fluorescent dye molecules. On irradiation of the droplets with a single N(2) laser pulse, the evaporation or condensation rates can be deduced from the wavelength shift (to the blue or to the red, respectively) of the spectrally narrow (<0.1-nm) structure-resonance peaks in the fluorescence spectra.

Two-Dimensional Rayleigh Thermometry in a Turbulent Nonpremixed Methane-Hydrogen Flame

Abstract A time-resolved two-dimensional Rayleigh seatiering technique for mapping the temperature Held or a nonpremixed methane-hydrogen flame is presented. The laser Rayteigh scattered tight from an illuminated plane intersecting the flow field was digitized in an 8,000 data point array. The two-dimensional jnd quantitative character or the data makes it possible to apply calculations to the data, e.g., ensemble averaging and pattern recognition, to obtain more information on the flame structure. The results of these calculations are presented along with instantaneous measurements of the temperature field of the flame.

Instantaneous three-dimensional concentration measurements in turbulent jets and flames

An experiment is described in which the instantaneous three-dimensional gas-concentration distributions in turbulent jets and flames were recorded. A resonant scanning mirror was used to sweep a laser sheet through the volume of a flow field. During the 1.4-μsec duration of the laser pulse, an electronic framing camera imaged the scattered light from the gas in as many as 12 parallel planes within the flow. A two-dimensional charge-coupled-device array was used to digitize the images from the framing camera in real time. The measurement period was brief enough to freeze the motion of the gas.

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.

Computational and experimental study of soot formation in a coflow, laminar ethylene diffusion flame

A sooting, ethylene coflow diffusion flame has been studied both experimentally and computationally. The fuel is diluted with nitrogen and the flame is slightly fifted to minimize the effects of the burner. Both probe (thermocouple and gas-sampling techniques) and optical diagnostic methods (Rayleigh scattering and laser-induced incandescence) are used to measure the temperature, gas species, and soot volume fractions. A detailed soot growth model in which the equations for particle production are coupled to the flow and gaseous species conservation equations has been used to investigate soot formation in the flame. The two-dimensional system couples detailed transport and finite-rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. The formation of benzene is primarily controlled by the recombination of propargyl radicals, and benzene production rates are found to limit the rate of inception, as well as the net rate of soot growth. The model predicted soot volume fractions well along the wings of the flame but underpredicted soot volume fractions by a factor of four along the centerline. Oxidation of particulates is dominated by reactions with hydroxyl radicals that attain levels approximately ten times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted temperatures.

Investigation of the transition from lightly sooting towards heavily sooting co-flow ethylene diffusion flames

Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution of the fuel and predicted peak soot levels are in reasonable agreement with the data. Computations are also presented for modifications to the model that include: (i) use of a more comprehensive chemical kinetics model; (ii) a revised inception model; (iii) a maximum size limit to the primary particle size; and (iv) estimates of radiative optical thickness corrections to computed flame temperatures.