Laser-induced Predissociative Fluorescence Research Articles

Crank-angle-resolved laser-induced fluorescence imaging of NO in a spark-ignition engine at 248 nm and correlations to flame front propagation and pressure release

Inside the combustion chamber of a spark-ignition engine, NO fluorescence is excited with a narrow-band tunable KrF excimer laser. The fluorescence light is detected by an intensified CCD camera that yields images of the NO distributions. Rotational-vibrational transitions of NO are excited by the A(2)Σ+ ? X(2)Π (0, 2) band system around 248 nm. Single laser shot planar NO distributions are obtained with good signal-to-noise ratio at all crank angles and allow us to locate areas of NO formation during combustion. The pressure within the combustion chamber is measured simultaneously with the NO distributions, which allows the evaluation of correlations between indicated work and NO formation. The crank-angle-resolved sequences of two-dimensional NO distributions and averaged pressure traces are presented for different engine-operating conditions. In addition, laser-induced predissociation fluorescence of OH excited by the same laser source is measured in order to visualize the corresponding flame front propagation and to compare the time of formation of NO relative to that of OH.

Laser-induced predissociative fluorescence: dynamics and polarization and the effect of lower-state rotational energy transfer on quantitative diagnostics

Laser-induced predissociative fluorescence is often used for diagnostics because its short-lived upper states are minimally disturbed by collisions. We discuss the effects of lower-state collisions with parameters relevant to our atmospheric H(2)-O(2) flame. A pulse of tunable KrF excimer-laser light induces the A ? X, Q(1)(11), 3 ? 0 transition in OH. We measure the intensity and the polarization of the resulting A ? X, Q(1)(11), 3 ? 2 fluorescence as a function of laser brightness. A simple model that uses no adjustable parameters produces a reasonable fit to the data. It predicts that, even at very modest laser energies, the fluorescence intensity is almost directly proportional to the rate constant for rotational energy transfer (RET) within the lower vibrational state. That rate constant can be a strong function of local conditions. Furthermore, under typical operating conditions the excimer will pump an amount of OH out of the lower state that is many times as large as that originally present. This occurs because RET within the X-state continuously replenishes the lower state during the laser pulse. Even when this occurs, the signal may still vary linearly with laser intensity, and the polarization may be nearly that expected for weak pumping. At the higher laser intensities, a significant fraction of the measured OH arises from two-photon photodissociation of the water from the flame reaction.

Visualization of Supersonic Flow Using O2-LIPF.

As a method for analysis of supersonic flow, visualization of the flow field structure using LIF of tracers seeded in a carrier gas has been employed. In this study, the capability of imaging of supersonic flow using O2-LIPF (laser-induced predissociative fluorescence) is examined. Since oxygen is one of the main constituents of air, O2-LIPF may enable visualization of air flow in a high-speed wind tunnel without tracers. However, because the visualization of the flow field below room temperature using O2-LIPF is very difficult, it has been applied only to high enthalpy and combustion flow, not to high-speed flow at low temperature. In this study, the supersonic oxygen flow is visualized by O2-LIPF using an ArF excimer laser and a high-sensitivity CCD camera. If the CCD camera is set at a long exposure time (1200 sec) and the scattered laser beam is suppressed sufficiently, the supersonic oxygen flow below room temperature can be imaged successfully using O2-LIPF.

Visualization of Supersonic Flow Using O2-LIPF. 2nd Report. Improvement of a Detection System and Analysis of Detection Signal.

This paper describes the O2 LIPF (laser-induced predissociative fluorescence) method for visualization of high-speed flows. Under the condition of weak laser excitation, the linear dependence of O2-LIPF both on number density of oxygen molecules and on laser intensity is presented theoretically and verified experimentally using a static condition. The vibrational and rotational temperature distributions along a centerline of a supersonic free jet are calculated theoretically, leading to the theoretical fluorescence signal distribution. To improve the S/N ratio of a detection system including an image intensifier and a high-sensitivity CCD camera, three spectral filters are selected to eliminate the scattered laser beam, and three images visualized through them are compared. A low-speed oxygen flow is also successfully visualized using O2-LIPF.

Rotational Spectra Simulation in O2-LIPF.

The rotational spectra observed in O2-LIPF(laser-induced predissociative fluorescence) experiments using an ArF* excimer laser are theoretically investigated. First, the simulation code of the Schumann-Runge bands is developed and then the general characteristics of the bands are discussed from the viewpoint of the suitable wavelength for O2-LIPF. Although the tuning range of the ArF* excimer laser is not the best wavelength regime for the O2-LIPF, it is shown that the range of the excimer laser is adequate to determine rotational temperatures and that the bands (4, 0), (7, 1), (10, 2), (11, 2)and(14, 3) are observed to be intense there. Finally, the actual spectrum, considering the effects of the laser linewidth and the predissociation linewidth, is discussed and it is predicated that the (14, 3) band is most affected by those linewidth effects among the dominant bands above.

Polarization separated spatially resolved single laser shot multispecies analysis in the combustion chamber of a realistic SI engine with a tunable KrF excimer laser

For precise spontaneous Raman scattering measurements of species densities in hydrocarbon fueled combustion, it is vital to separate the Raman scattered light from interfering (broadband) fluorescence light. This paper describes the application of a detection scheme for separated and simultaneous acquisition of both polarization components of the laser-induced emissions (i.e., perpendicular and parallel to the polarization of the incident laser light). Using highly polarized incident laser light, the Raman scattered light maintains almost this polarization, whereas broadband fluorescence and most laser-induced predissociation fluorescence (LIPF) emissions are strongly depolarized. Therefore, it is possible to extract the Raman signal by subtracting the signals separately obtained for the two polarization directions (i.e., vertical polarized light consists of fluorescence and parallel polarized light consists of fluorescence and Raman scattering). This detection scheme is used to acquire high-precision single laser shot multispecies, polarization separated, and crank angle-resolved spectra in the combustion chamber of a firing spark-ignition (SI) engine. In addition, a combinative approach for cycle-resolved mixture composition and exhaust gas analysis is lemonstrated. In this approach, the air-fuel ratio before ignition and the NO LIF intensity in the burned gas are measured within the same engine cycle using the different polarization properties of Raman scattered and fluorescence light. The measurements are carried out in a standard production, four-cylinder SI engine that is modified only slightly to allow optical access. The engine data have not been changed by these modifications (e.g., the compression ratio is maintained at 10).

Experimental investigation of a SCRAMJET model combustor with injection through a swept ramp using laser-induced fluorescence with tunable excimer lasers

Tunable excimer lasers are used to obtain two-dimensional images to study the nonreacting and reacting flow field of a hydrogen-fueled SCRAMJET model combustor with underexpanded Ma =1.6 injection in the base of a swept ramp. The rectangular combustion chamber consists of an exchangeable injection module and a 3 o divergent expansion duct, both with optical access from all sides at different axial locations. The inlet conditions of the vitiated Ma =1.9 airflow are T =850 K static temperature at a pressure of 0.96 bar. Therefore, self-ignition of the injected hydrogen mixed with air is possible but was not reached throughout the whole flow field. As an alternate solution, shock-induced ignition by an oblique shock generated by a small wedge fixed at the upper wall of the combustor has been investigated. The spatial structure of the flow field is characterized by relative distributions of different species. Measurements of natural fluorescence of OH and laser-induced predissociative fluorescence (LIPF) of OH in axial and cross-sectional planes are presented. The averaged fluorescence distributions turned out to be highly reproducible and illustrate clearly the gasdynamical features of the flow field, as well as the highly three-dimensional mixing process by vorticity generated by the swept ramp in flow direction. Furthermore, ignition and mixing were improved significantly by the oblique shock wave generated by the additional wedge.

Plif imaging measurements of a co-axial rocket injector spray at elevated pressure

A small-scale rocket combustion chamber has been developed for fundamental investigations on liquid oxygen/gaseous hydrogen (LOX/GH2) combustion under high-pressure conditions. The combustion chamber is designed for pressures up to 2M Pa equipped with a unielement shear co-axual injector. The intention of the experimental work is to apply optical diagnostic techniques to show their feasibility in two-phase high-pressure flows and to characterize the phenomenology of the LOX/GH2 spray flamelike liquid oxygen injection, atomization, evaporation, turbulent mixing, and combustion. Planar laser-induced predissociation fluorescence (PLIPF) and spontaneous emission of OH have been applied at three different chamber pressures in the near field of a shear co-axial injector in the optically accessible rocket chamber. Single shot and averaged images have been measured to map the flame zone, delivering qualitative information of the reactive flow field. Additionally, dispersed fluorescence spectra of OH in the wavelength range of the A2(v′=3)←X2H (v″=2) fluorescence band have been measured to evaluate interferential effects arising from O2 that occur with increasing chamber pressures.

Characteriation of spherical hydrocarbon fuel flames: Laser diagnosis of the chemical structure through the oh radical

For the first time, spherical diffusion flames of n -decane, n -heptane, and methanol established around fuel-drenched porous spheres were investigated by applying the two-dimensional laser-induced predissociation fluorescence method to measure the OH radical under microgravity conditions. Focus of this work was laid on the development of a UV-laser diagnostic system attached to the Bremen drop tower for the characterization of combustion under buoyant-free conditions and on its applicability to the diffusion zone of hydrocarbon droplet flames. The experiments carried out on the previously mentioned fuels showed capabilities and limitations of this method applied on liquid fuels. While it is relatively straightforward to investigate the chemical structure of methanol droplet flames because of the absence of Mie scattering soot particles and of nonresonant fluorescing species, it is difficult for larger hydrocarbons such as n -hepatane and n -decane. It is considered that with the development of a luminous zone, strong broadband absorption by fuel vapor and intermediates larger than methane in connection with high laser pulserepetion rates may cause interactions of the input energy with the chemical kinetics of the combustion process.

Multi Species Detection in a Liquid Fuelled Model Combustor using Tunable Excimer Lasers

Abstract Tunable excimer lasers are used to obtain 2-D images to analyse spatial distributions of different species in a liquid fuelled flame of a single burner model combustor with optical access from all sides at ambient pressure and a maximum air inlet temperature of 600 K. The flame is stabilised by a turbulent structure generated by air mixing jets in combination with the axial swirls of an air spoke injector The spatial structure of the flame is characterized by relative distributions of different species. Measurements of natural fluorescence of OH (without laser) and laser-induced fluorescence (LIF) of NO and vaporized n-heptane fuel as well as laser-induced predissociative fluorescence (LIPF) of OH are presented. Averaged temperature field measurements are performed based on sequential excitation of two rovibronic transitions in the Schumann-Runge band system of O2. Liquid fuel droplets are visualized by Mie-scattering. Relevant spectroscopy is done to find the laser and fluorescence frequencies needed to measure isolated species and to study the natural flame emissions. The averaged fluorescence distributions turned out to be highly reproducible and illustrate clearly the fuel evaporation and consumption process as well as the area of NOx formation within the flame. The obtained flame spectra indicate that the formation of prompt NOx plays an important role in the precombustion zone where the mixture is still rich and the flame-temperature is low

Raman measurement of mixing and finite-rate chemistry in a supersonic hydrogen-air diffusion flame

Uhraviolet (UV) spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) from a KrF excimer laser are combined to simultaneously measure temperature, major species concentrations (H 2, O 2, N 2, H 2O), and OH radical concentration in a supersonic lifted co-flowing hydrogen-air diffusion flame. The axisymmetric flame is formed when a sonic jet of hydrogen mixes with a Mach 2 annular jet of vitiated air. Mean and rms profiles of temperature, species concentrations, and mixture fraction are obtained throughout the supersonic flame. Simultaneous measurements of the chemical species and temperature are compared with frozen chemistry and equilibrium chemistry limits to assess the local state of the mixing and chemistry. Upstream of the lifted flame base, a very small amount of reaction occurs from mixing with hot vitiated air. Downstream of the lifted flame base, strong turbulent mixing leads to subequilibrium values of temperature and OH concentration. Due to the interaction of velocity and temperature in supersonic compressible flames, the fluctuations of temperature and species concentrations are found to be higher than subsonic flames. Farther downstream, slow three-body recombination reactions result in superequilibrium OH concentrations that depress temperatures below their equilibrium values.

Assumed joint probability density function approach for supersonic turbulent combustion

In a recent experiment, Cheng et al. used UV spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence techniques for simultaneous measurements of temperature and concentrations of O2, H2, H2O, OH, and N2 (and the rms of their fluctuations) in supersonic turbulent reacting shear layers. Because present computational techniques are not suited for the prediction of all of the above measurements, a new approach has been developed and is being used to predict all relevant flow properties and the rms of their fluctuations (where appropriate). The approach explores the use of a multivariate Beta PDF for concentrations. In particular, a version developed by Girimaji to model scalar mixing in turbulent flows is employed. Predictions using this model were, in general, satisfactory in regions preceding ignition, but not in regions downstream of ignition. Part of the discrepancy is a result of our current inability to relate Favre and time averages. 10 refs.

Visualization of a Supersonic Flow Using O2-LIPF.

For an analysis of supersonic flow, visualization of the flow field structure using LIF of tracers has been employed. In this study, the capability of imaging of supersonic flow using O2-LIPF (laser-induced predissociative fluorescence) is examined. Since oxygen is one of the main components of air, O2-LIPF may make it possible to visualize the air flow in a high-speed wind tunnel without a tracer. However, because the visualization of the flow field below room temperature using O2-LIPF is very difficult, it has been applied only to high enthalpy and combustion flows, not to high-speed flow with low temperature In this study, the supersonic oxygen flow is visualized by O2-LIPF, using an ArF excimer laser and a high- sensitivity CCD camera. If the CCD camera is set at a long exposure time (1200 seconds) and the scattered laser beam is suppressed sufficiently, the supersonic oxygen flow below room temperature using O2-LIPF can be imaged successfully.

22 O2-LIPFを用いた希薄気体流の可視化

For an analysis of supersonic flow, visualization of the flow field structure using LIF of tracers has been employed. In this study, the capability of imaging of supersonic flow using O2-LIPF (Laser-Induced Predissociative Fluorescence) is examined. Since oxygen is one of main component of air, O2-LIPF may make it possible to visualize the air flow in the high speed wind tunnel without the tracer. However, because the visualization of the flow field below the room temperature using O2-LIPF is very difficult, ithas been applied only to the high entalpy and combustion flow, not to high speed flow with low temperature. In this study, the supersonic oxygen flow is visualized by O2-LIPF, using an ArF excimer laser and a high-sensitive CCD camera. If long exposure time (1200 seconds) of the CCD camera is set and the scattered laser beam is suppressed sufficiently, the supersonic oxygen flow below the room temperature using O2-LIPF can be imaged successfully.

Vibrational temperature measurements in a shock layer using laser induced predissociation fluorescence

The feasibility of applying laser induced predissociation fluorescence (LIPF) to the measurement of vibrational temperatures in a shock layer flow around a cylinder in a pulsed supersonic free stream is examined. Single shot spatially and spectrally resolved LIPF measurements are presented and a comparison with theoretical predictions is made.

Two-photon predissociative fluorescence of H2O by a KrF laser for concentration and temperature measurement in flames

Two-photon laser-induced predissociative fluorescence (LIPF) of H2O is examined as a potential measurement technique of H2O concentration and temperature in flames. Two-photons of 248 nm light from a narrowband KrF laser excite H2O to the highly predissociative \(\tilde C^1 B_1 \) state in a hydrogen-air flame. The subsequent bound-free emission \(\tilde C^1 B_1 \to \tilde {\rm A}^1 B_1 \) is observed from 400–500 nm in the flame at temperatures of 1000–2000 K and is found to be free of fluorescence interference from other flame species. This LIPF signal is not affected by collisional quenching due to the short lifetime of the \(\tilde C^1 B_1 \) predissociative state (∼2.5 ps). Broadband laser dispersion spectra, narrowband laser dispersion spectra, laser excitation spectra and probability density functions of the H2O fluorescence are obtained in the hydrogen flame. The H2O LIPF signal is found to be temperature sensitive and a two-line LIPF technique is needed for concentration and temperature measurement. The accuracy of a two-line LIPF technique for H2O concentration and temperature measurement is determined.

Identification and imaging of OH (ν″ = 0) and O_2 (ν″ = 6 or 7) in an automobile spark-ignition engine using a tunable KrF excimer laser

Planar laser-induced predissociative fluorescence is applied to image state-specific densities of OH and hot O(2) inside an internal-combustion car engine. Improved instrumentation is described. It includes better imaging optics and a spectrometer that permits desired molecular quantum states to be selected and identified in real time. The OH (nu'' = 0) images are cleanly separated from the isooctane fuel and they display a thin superequilibrium region at the flame front. In contrast, vibrationally excited O(2) (nu'' = 6 or nu'' = 7) is uniformly distributed behind the front. Uneven and broken flame fronts are commonly observed.

Simultaneous temperature and multispecies measurement in a lifted hydrogen diffusion flame

Ultraviolet (UV) spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) techniques are combined and applied to a lifted hydrogen jet diffusion flame. Simultaneous, temporally and spatially resolved point measurements of temperature, major species concentrations (H 2, O 2, N 2, H 2O), and absolute hydroxyl radical concentration (OH) are obtained with a “single” excimer laser for the first time. For OH measurements, the use of LIPF makes quenching corrections unnecessary. Results demonstrate that fuel and oxidizer are in a rich, premixed, and unignited condition in the center core of the lifted flame base. In the lifted zone, combustion occurs in an intermittent annular turbulent flame brush and strong finite-rate chemistry effects result in nonequilibrium values of temperature, major species, and OH radicals. Downstream in the slow three-body recombination zone, the major species concentrations are in partial equilibrium, the OH concentrations are in superequilibrium, and the temperatures are in subequilibrium. Far downstream in the flame, equilibrium values of temperature, OH radical, and major species are found.

Direct observation of orbit rotation predissociation in the O2 Schumann–Runge system

Laser induced predissociation fluorescence spectroscopy was performed with a tunable Argon fluoride laser in the Schumann–Runge band of O2 in an open atmospheric flame. Simple rotationally resolved spectra of the 10←2 and 11←2 bands were obtained when the fluorescence was dispersed through a monochromator. Line widths of these transitions, which predominantly reflect state specific predissociation, were measured. It was found that the orbit rotation interaction between the B 3Σ−u state and the 3Πu continuum must be included in addition to the spin–orbit interaction between B 3Σ−u and 5Πu, 1Πu, 3Σ+u, and 3Πu in order to fit the data. The general coupling formulas for orbit–rotation interaction for the case of intermediate coupling between Hund’s case a and b were derived and orbit–rotation interaction constants for v=10 and 11 were estimated. This is the first experimental evidence of the orbit rotation interaction in the O2 Schumann–Runge system.

Fluorescence imaging inside an internal combustion engine using tunable excimer lasers

Tunable excimer lasers are used to obtain 2-D images of molecular (and some state-specific) density distributions inside a cylinder of a modified four-cylinder in-line engine that has optical access. Natural fluorescence (i.e., without a laser) is used for some OH pictures, normal laser-induced fluorescence (LIF) for those of NO and of the isooctane fuel, and laser-induced predissociative fluorescence (LIPF) for other OH pictures and for those of O(2). Relevant spectroscopy is done to find the laser and fluorescence frequencies needed to measure isolated species. LIPF works well at high pressures, is state specific, and is ideally suited to follow turbulent processes. No similar measurements in engines have been previously reported. Pictures are taken in succeeding engine cycles. Their sequence is either at a particular point of the engine's cycle to show cyclic fluctuations, or at succeeding portions of the cycle to illustrate the progress of the gasdynamics or of the combustion.