Galen B. King

Pump/probe method for fast analysis of visible spectral signatures utilizing asynchronous optical sampling.

We report the results from a new pump/probe spectrometer for potential use in combustion diagnostics that employs asynchronous optical sampling. The instrument consists of two frequency-doubled mode-locked Nd:YAG lasers operating at slightly different repetition rates, synchronously pumping two dye lasers (rhodamine 6G) to generate the pump and probe beams. The spectral and temporal capabilities of the instrument are examined by obtaining a spectrum and an excited state decay of rhodamine B. The instrument response is shown to be proportional to pump power, probe power, and sample absorptance. Different frequency synthesizers and different modes of triggering are used to study their effect on signal stability.

Two-photon-excited fluorescence measurement of hydrogen atoms in flames

We report the first two-photon-excited hydrogen-atom fluorescence measurements in flames made to our knowledge. The n = 3 level of the H atom was excited by 205.1-nm radiation generated by Raman shifting a 224-nm beam produced by frequency mixing. Fluorescence was observed at 656.3 nm as a result of radiative decay from n = 3 to n = 2, the Balmer-alpha transition. A novel technique, photoionization-controlled loss spectroscopy, is proposed to eliminate the quenching dependence of the fluorescence signal.

Pump/Probe Spectroscopy by Asynchronous Optical Sampling

We report the first results from a new pump/probe technique called asynchronous optical sampling (ASOPS). The method employs a mode-locked, frequency-doubled Nd:YAG laser operating at a repetition rate of 81.5970000 MHz as the pump laser, and a synchronously pumped dye laser (R6G) operating at a repetition rate of 81.5870000 MHz as the probe laser system. The 10-kHz beat frequency produces a repetitive relative phase walk-out of the pump and probe pulses which replaces the optical delay line used in conventional instruments. Studies of rhodamine B in methanol demonstrate that the instrument response is proportional to pump power, probe power, and sample absorptance. The fluorescence lifetime of 4 × 10−5 M rhodamine B is determined to be 2.3 ns.

Influence of temperature and hydroxyl concentration on incipient soot formation in premixed flames

Abstract Critical equivalence ratios ( φ c ) have been measured as a function of temperature (1600–1880K) for premixed flames at atmospheric pressure. The five fuels studied are methane, ethane, propane, ethylene, and acetylene. The flames were stabilized on a flat flame burner and the temperatures were measured using sodium D-line reversal. A linear relationship is found between ln φ c and 1 T for each fuel. Based on a global kinetic model in which soot precursors are formed by fuel pyrolysis and oxidized by OH, a predictive correlation has been developed which shows the influence of temperature, OH concentration, and C H ratio on sooting tendency. This correlation describes all of the measured φ c versus temperature data, suggesting that the overall mechanism of soot formation is similar among aliphatic fuels.

A combustion facility for high‐pressure flame studies by spectroscopic methods

We have developed a high‐pressure combustion facility that includes the following: (1) a pressure vessel constructed from standard stainless‐steel piping components; (2) an internal x‐y burner translation system employing inexpensive stepper motors and translation mechanisms; (3) two flat‐flame burners; and (4) an electronic gas delivery system, which is interfaced to a microcomputer. The facility is designed for study of high‐pressure flames by spectroscopic methods and should also aid in the development of spectroscopic tools for high‐pressure combustion environments.

Scalar time-series measurements in turbulent CH4/H2/N2 nonpremixed flames: CH

Time-series measurements of CH concentrations [CH] are reported in a series of methane/hydrogen/nitrogen diffusion flames. Power spectral densities (PSDs) and autocorrelation functions are computed from the time series, permitting a detailed investigation of [CH] fluctuation time scales. The effects of fluorescence lifetime fluctuations are found to be negligible for quantitative determination of the PSDs. A similar study of hydroxyl concentrations in nonpremixed hydrogen/argon flames recently demonstrated that OH PSDs collapse to a single curve when normalized by the integral time scale, in agreement with mixture fraction statistics. However, for the present measurements, [CH] data on the fuel-side of the peak [CH] location exhibit low-frequency differences in their PSDs with respect to both the [OH] and air-side [CH] PSDs. These differences could be associated with the closer proximity of the CH radical to the shear layer and thus to enhanced fluctuations in the local mixture fraction. This possibility can be examined via measurements of other scalars.

Micromachining of Metals, Alloys, and Ceramics by Picosecond Laser Ablation

Microhole drilling and microstructure machining with a picosecond (ps) Nd: YVO 4 laser (pulse duration of 10 ps) in metals, alloys, and ceramics are reported. Blind and through microholes are drilled by percussion drilling as well as trepanning drilling, where the diameters of the holes are in the range of 20-1000 μm. Microfeatures are also machined and the flexibility of ps laser machining is demonstrated. The quality of drilled holes, e.g., recast layer, microcrack, and conicity, and that of the microstructures, are investigated by an optical microscope, a surface profilometer, or a scanning electron microscope. Ps laser ablation rate is studied by experiments and a simplified laser ablation model.

Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure C 2 H 6 /O 2 /N 2 flames

We demonstrate saturation of a transition of the OH molecule in high-pressure flames by obtaining saturation curves in C(2)H(6)/O(2)/N(2) laminar flames at 1, 6.1, 9.2, and 12.3 atm. In addition we present quantitative fluorescence measurements of OH number density at pressures to 12.3 atm. To assess the efficacy of the balanced cross-rate model for high-pressure flames, we compare laser-saturated fluorescence measurements, which were calibrated in an atmospheric-pressure flame, with absorption measurements at 3.1 and 6.1 atm. At 3.1 atm the absorption and fluorescence measurements compare well. At 6.1 atm, however, the concentrations given by laser-saturated fluorescence are ~25% lower than the absorption values, indicating some depletion of the laser-coupled levels beyond that at atmospheric pressure. By using a reasonable estimate for the finite sensitivity to quenching, we anticipate that fluorescence measurements that are calibrated at 1 atm can be applied to flames at ~10 atm with absolute errors within +/-50%.

Early-stage plasma dynamics with air ionization during ultrashort laser ablation of metal

In this study, the early-stage plasma evolution generated by an ultrashort laser pulse is investigated through pump-probe shadowgraph measurements and simulations. The measurements are performed to show the evolution of the plasma front, while the simulation model is used to further investigate the evolution process and mechanism. Specifically, the laser pulse propagation in air is simulated using the beam propagation method with the slowly varying envelope approximation. The lattice dynamics, the electron dynamics and the multi-scattering event, and the evolution of charged particles (free electrons and ions), are simulated using a molecular dynamics method, a Monte Carlo method, and a particle-in-cell method, respectively. With this simulation model, the refractive index and plasma evolutions are calculated and compared with measured results to validate the simulation model. Different plasma expansion processes, caused by the air ionization, are found with the focal point slightly above and below the target. Air ionization occurs in both cases, but their primary mechanisms are shown to be different.

Asynchronous optical sampling: a new combustion diagnostic for potential use in turbulent, high-pressure flames

Asynchronous optical sampling (ASOPS) is a pump-probe method that has strong potential for use in turbulent, high-pressure flames. We show that rapid measurement of species number density can be achieved by maintaining a constant beat frequency between the mode-locking frequencies of the pump and probe lasers. We also describe the instrumental timing parameters for ASOPS and consider the optimization of these parameters. Measurement of the nanosecond decay for electronically excited sodium in an atmospheric flame demonstrates the viability of the ASOPS technique in highly quenched flame environments.