Time-resolved Planar Laser-induced Fluorescence Research Articles

Time-Resolved PLIF Imaging of OH Radicals in the Afterglow of Nanosecond Pulsed Discharge in Combustible Mixtures

A time-resolved planar laser-induced fluorescence (PLIF) imaging technique has been used to investigate the spatial distribution of OH radical dynamics in the afterglow of a pulsed nanosecond discharge. Experiments were carried out using a lean methane/air mixture, at temperatures of 300 K and 500 K and at atmospheric pressure. The nanosecond pulsed discharge was formed in a pin-to-pin electrode system. Time-resolved PLIF imaging indicated uniform OH radical dynamics along the discharge channel. Comparison of OH radical emission dynamics with discharge emission dynamics from excited nitrogen revealed a close similarity in spatial distribution and allowed to clarify the mechanisms of atomic oxygen formation.

Residence time measurement of an isothermal combustor flow field

Residence times of combustors have commonly been used to help understand NOx emissions and flame blowout. Both the time mean velocity and turbulence fields are important to the residence time, but determining the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three-dimensional nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using a time-resolved planar laser-induced fluorescence (PLIF) technique, but a second camera for particle image velocimetry (PIV) was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures. Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after injection is stopped: this equivalent to the sum of the convection time and the half-life values. The technique was applied to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core, where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NOx production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales.

Spark ignition of turbulent methane/air mixtures revealed by time-resolved planar laser-induced fluorescence and direct numerical simulations

By use of high-speed planar laser-induced fluorescence (PLIF) imaging, the evolution of turbulent reactive flows was recorded and studied in real time in a filmlike manner. The technique was used to track the concentration field of the OH radical, which was produced during spark ignition of a turbulent methane/air mixture. The results were compared qualitatively to a two-dimensional direct numerical simulation of the same system using a detailed chemical mechanism and a detailed transport model.

KrF laser-induced photobleaching effects in O_2 planar laser-induced fluorescence signals: experiment and model

KrF excimer lasers are often employed as high-power excitation sources in planar laser-induced fluorescence (PLIF) imaging experiments to measure the distributions of O(2), OH, and H(2)O-all important species in combustion phenomena. However, due to the predissociative nature of these molecules, the high laser pumping rates typically required in such PLIF experiments may significantly deplete the ground-state population. The proper interpretation of the ground-state number density and/or the temperature from the fluorescence signals then requires the inclusion of photobleaching effects. We compare the results of a five-level rate-equation model incorporating photobleaching effects to the time-resolved PLIF signals from O(2) as obtained in the products of a fuel-lean CH(4) air flame. The results indicate that the fluorescence signals in a typical predissociated PLIF imaging experiment are subject to significant amounts of photobleaching. In an effort to provide a convenient way to account for photobleaching, a simple three-level model is developed. This model provides an analytic solution that describes satisfactorily the time-integrated fluorescence signal when compared with both the five-level model and the measurements. The results also indicate that at the low laser irradiances required to minimize the effects of photobleaching, the correspondingly low fluorescence signal levels make the acquisition of single-shot PLIF images a challenge to currently available camera systems.

Time-resolved PLIF imaging of Cu in a laser-ablated copper plasma plume

Planar laser-induced fluorescence (PLIF) has been used to determine the relative number density of ground state copper atoms in laser-ablated plasma plumes. An ablation laser power flux of /spl sim/1.5 GW/cm/sup 2/ is applied to a solid copper target in a background gas, producing a plasma plume suitable for studying homogeneous copper vapor condensation. Density is measured at postablation time delays ranging from 5 /spl mu/s to 10 ms with 1-100 torr of either argon or helium as the background gas. Planar laser-induced fluorescence images are used to spatially resolve the relative density within the plume, The decrease in density is due to the homogeneous condensation of copper vapor to form particulate.