Lynn A. Melton

Soot diagnostics based on laser heating.

Through numerical calculations we have investigated the possibility of developing soot diagnostics based on laser heating of the soot particles. Two strategies, one using the laser-modulated incandescence of the particles, and the other using direct detection of the evaporated C2 molecules, were examined. Both strategies can yield size distribution and volume fraction information provided the laser wavelength is near the graphite absorption band at 260 nm; otherwise, only volume fractions can be obtained.

Vapor/Liquid Visualization for Fuel Sprays

Abstract In a demonstration experiment, the vapor and liquid disiributions in a hollow-cone spray have been separately photographed using laser-excited fluoresent dopants which form organic exciplexes. In color photographs, the Fluorescence from the liquid appears blue-green and the fluorescence from the vapor appears purple. With high-speed black and while film, “stopped action” pictures of the liquid and/or vapor portions of the spray were obtained. This technique may lead to the development of real-lime visualization diagnostics For evaporating fuel sprays.

Fluorescence methods for determination of temperature in fuel sprays

Fluorescent additives which allow the determination of droplet temperatures in hydrocarbon fuel sprays have been developed. These systems, which exploit the chemistry of organic exciplexes; should make possible the determination of droplet temperatures within 1 degree C. Two specific systems are described: 3,10-dicyanophenanthrene/diethylaniline over the 20-135 degrees C range, and naphthalene/tetramethyl-p-phenylene diamine over the 140-265 degrees C range. These additives should allow 2-D real-time nonintrusive visualization of the droplet temperatures in an evolving fuel spray.

Recent advances in photoacoustic calorimetry: Theoretical basis and improvements in experimental design

Recent developments in time‐resolved photoacoustic calorimetry (PAC) are discussed. An equation is derived relating the amplitude of the maximum photoacoustic signal to parameters of a photoacoustic cell, to physical properties of the solvents, and to the energy deposited as heat. The equation is validated by a series of scaling experiments: it correctly correlates the dependence of the PAC signal on cell thickness, on energy deposition, and on solvent properties. The dominant sources of background signal have been determined and the background signal reduced substantially relative to previous work. It is now possible to measure energies and lifetimes of transients in solutions with optical densities as low as ∼5×10−4. Data are presented for the energy of 2‐cyclopentenone triplet, a transient for which interpretable PAC measurements were previously not possible. Its triplet energy is 73.1±1.1 kcal/mole. With reduced background and a faster digital storage oscilloscope, a quite short transient lifetime, 7....

Direct Calibration Procedures for Exciplex-Based Vapor/ Liquid Visualization of Fuel Sprays

Abstract Direct calibration procedures for both the liquid and vapor phases have been developed for use with exciplex-based vapor/liquid visualization systems, in particular for the system composed of 1 % (w/w) N,N,N′N′-tetramethyl-p-phenylenediamine (TMPD)/10% (w/w) naphthalene/89% (w/w) n-decane. For the vapor phase, a sealed temperature controlled cuvet provides a series of in situ reference samples. For the liquid phase, imaged droplets from the edge of a non-evaporating spray are used to provide a range of in situ reference samples. The calibration factors, determined as the measured fluorescence intensity/amount of vapor (or liquid)/unit laser intensity, can then be used to provide absolute calibration for the fluorescent images that are collected from an evaporating fuel spray. Calibrated images, which show the average and transient behavior of evaporating fuel sprays, are presented for the vapor and liquid phases.

Fluorescence Lifetime Imaging: An Approach for Fuel Equivalence Ratio Imaging

This paper describes dopants, apparatus, and procedures which can be used for planar laser-induced fluorescence (PLIF) imaging of the fuel/oxygen equivalence ratio, the most important mixture parameter in combustion studies. The lifetime imaging techniques described here provide data beyond the data available from conventional intensity imaging, and should allow the equivalence ratio to be determined in real time in a two-dimensional slice of a fuel spray. Lifetime imaging methods require the sequential gating of two 2-D detectors during the fluorescence decay. Proof-of-concept experiments indicate that lifetimes ranging from 5 to 50 ns can be imaged with good accuracy. The requirements for fluorescent dopants and considerations for use under flame conditions are also discussed.

Vapor/liquid visualization in fuel sprays

In a series of demonstration experiments, we have shown that exciplex visualization systems can be used to photograph either the liquid or the vapor pattern in a hollow cone fuel spray. Thus, vapor density information can be obtained without the analysis of droplets. Furthermore, through image analysis of digitized negatives, we have obtained a quantitative presentations as false color maps and contour density plots. The exciplex visualization systems exploit the reversible equilibrium which may exist when an excited organic molecule (M * ) reacts with a suitable ground state organic molecule to form an emitting exciplex (E * ). The emission from M * can be made characteristic of the vapor and the emission from E * can be made characteristic of the liquid. Because the emission from E * is shifted with respect to that from M * , filters can suppress either the M * or E * emissions, and hence with laser sheet excitation, two dimensional vapor or liquid sections of the fuel spray can be photographed.

Photoacoustic calorimetry: a new cell design and improved analysis algorithms

A new cell design has been developed for use in photoacoustic calorimetry. In contrast to previous designs, this cell has a hundred‐fold better sensitivity (signal/energy absorbed), allows measurements to be made on optically thick solutions, allows greater laser pulse energy to be used (with consequent increase in signal levels), allows greater time resolution of transient decays in the time regime below 100 ns, and reduces the effects of reflections. The determination of photophysical parameters by use of photoacoustic calorimetry requires iterative reconvolution, and improved algorithms for these processes are presented.

Exciplex-Based Vapor/Liquid Visualization Systems Appropriate for Automotive Gasolines

This paper reports the development of exciplex-based vapor/liquid visualization systems based on exciplexes formed from tertiary amines and fluorine-substituted benzene and/or toluene. These systems are expected to be virtually coevaporative with solvents (fuels) boiling in the temperature range 70 to 110°C and thus are expected to track the vaporization of automotive gasoline effectively. A system consisting of 10% triethylamine/0.5% fluorobenzene/89.5% hexane should be coevaporative with a normal boiling point of 69°C. A system consisting of 10% n-propyldiethylamine/0.5% 4-fluorotoluene/89.5% isooctane should be coevaporative with a normal boiling point of approximately 100°C. Although the coevaporation of these systems is excellent, the exciplexes revert to varying extents to excited monomer at temperatures near 100°C. Thus there is considerable cross talk from the liquid into the vapor spectral region. The tertiary amines generally require excitation at wavelengths below 250 nm; the fluorobenzene or 4-fluorotoluene can be excited at 266 nm. Monomer emission peaks at 290 nm; exciplex emission peaks at 350 nm.

Two-Dimensional Gas-Phase Temperature Measurements Using Fluorescence Lifetime Imaging

Two-dimensional (2D) temperature images of turbulent hot nitrogen flows have been obtained by use of a novel technique involving fluorescence lifetime imaging. Nitrogen flows were doped with naphthalene vapor. The fluorescence lifetime of naphthalene decreases from 180 ns at 25 °C to 35 ns at 450 °C, and this temperature dependence can be used to calculate the temperature from the fluorescence lifetime in oxygen-free environments. With the current apparatus and with naphthalene as the fluorescent dopant, the error in the absolute temperature was estimated to be ±15 °C in the range of 25 to 450 °C. This application demonstrates that fluorescence lifetime imaging, which does not depend on the absolute concentration on the fluorescent dopant, may be useful in a variety of turbulence, temperature, and other combustion-related studies.