T. Neger

Investigation of the early stages in laser-induced ignition by Schlieren photography and laser-induced fluorescence spectroscopy

Laser ignition has been discussed widely as a potentially superior ignition source for technical appliances such as internal combustion engines. Ignition strongly affects overall combustion, and its early stages in particular have strong implications on subsequent pollutant formation, flame quenching, and extinction. Our research here is devoted to the experimental investigation of the early stages of laser-induced ignition of CH4/air mixtures up to high pressures. Tests were performed in a 0.9-l combustion cell with initial pressures of up to 25 bar with stoichiometric to fuel-lean mixtures using a 5-ns 50-mJ 1064-nm Nd:YAG laser. Laserinduced fluorescence (LIF) was used to obtain two dimensionally resolved images of the OH radical distribution after the ignition event. These images were used to produce an animation of laser ignition and early flame kernel development. Schlieren photography was used to investigate the laserinduced shock wave, hot core gas, and developing flame ball. We extend existing knowledge to high-pressure regimes relevant for internal combustion engines.

Comparison of Different Methods of Abel Inversion Using Computer Simulated and Experimental Side-On Data

Abstract A recently devised new method for numerical Abel inversion is compared with four other commonly used methods. One of them, the convolution method, is employed in computer tomography for reconstructing asymmetrical objects. It is investigated whether this method can be adapted for the case of radial symmetry. As a first approach the comparison is performed by computer simulation. Special attention is given to the propagation of errors according to their origin. The result is a recipe for minimizing errors and for choosing the optimal method for reconstruction. The second step is a comparison of experimentally obtained radial profiles with functions resulting from Abel inversion of measured side-on data. Thus it is shown that the concept developed by computer simulation can be applied in practice

Optical Diagnostics of Laser-Induced and Spark Plug-Assisted Hcci Combustion

HCCI (Homogeneous Charge Compression Ignition), laser-assisted HCCI and spark plug-assisted HCCI combustion was studied experimentally in a modified single cylinder truck-size Scania D12 engine equipped with a quartz liner and quartz piston crown for optical access. The aim of this study was to find out how and to what extent the spark, generated to influence or even trigger the onset of ignition, influences the auto-ignition process or whether primarily normal compression-induced ignition remains prevailing. The beam of a Q-switched Nd:YAG laser (5 ns pulse duration, 25 mJ pulse energy) was focused into the centre of the cylinder to generate a plasma. For comparison, a conventional spark plug located centrally in the cylinder head was alternatively used to obtain sparks at a comparable location. No clear difference in the heat releases during combustion between the three different cases of ignition start could be seen for the fuel of 80/20 iso-octane/n-heptane used. However, with optical diagnostic methods, namely PLIF (Planar Laser-Induced Fluorescence), Schlieren photography and chemiluminescence imaging, differences in the combustion process could be evaluated.

Time‐dependent collisional‐radiative model for capillary discharge plasmas

A time‐dependent, zero‐dimensional collisional‐radiative model has been developed to study capillary discharge plasmas in view of possibilities for obtaining population inversion leading to laser action in the extreme ultraviolet and soft x‐ray spectral region, by amplifying the Balmer‐α (Hα) line of different hydrogenlike ions with nuclear charges from Z=3 to Z=6. The model is described in detail, and results for the case of a carbon plasma are presented. Limitations of the model are discussed and comparison is made between our calculations and recent experimental results.

Tomographic investigation of the particle density distribution of sodium atoms in a glow discharge using resonance heterodyne holographic interferometry

Abstract Local particle density distributions in a dc glow discharge causing phaseshifts less than one fringe are determined by resonance heterodyne holographic interferometry and optical tomography. For testing purposes the determination of the number densities of sodium atoms evaporated from an electrode consisting of a sodium intercalation compound (C64Na) was performed.

An experimental investigation of ablative capillary discharges as possible sources for amplified spontaneous emission in the XUV

A fast-rise-time capillary discharge was designed and investigated as a possible source of amplified spontaneous emission in the extreme ultraviolet spectral range. The capillaries were made of Li 2 CO 3 or SiO 2 with radii in the range 0.25-1 mm and lengths from 1-5 cm. Discharge currents of up to 15 kA and current rise-times down to 20 ns could be achieved. Spatially resolved spectra could be recorded with a time resolution of 5 ns. During the cooling phase an increase in the intensity of the H-like Li III 729 A line could be found, accompanied by a decrease in spatial divergence. By a length variation of 3-5 cm of SiO 2 capillaries with 0.5 mm radius an indication of the gain of the Li-like O VI 520 A line could be obtained.

Spatially resolved determination of atomic particle densities and line shapes within an arc plasma by tomographic resonance interferometry

The particle density of ground-state chromium atoms within one cross section of an arc plasma was measured spatially resolved, and the spatial distribution of the line shape of the chromium resonance line at 427.48 nm was partly determined. The measurements were performed with a newly developed setup that combines the methods of resonance interferometry and refractive tomography. The wavelength of a dye laser was scanned over the investigated transition, and the refractive index was measured spatially and spectrally resolved by use of tomography. For each spatial point the particle density and the local line shape were calculated from the measured spectral refractivity distribution by the method of resonance interferometry. We describe the physical principles, the optical arrangement, and the numerical apparatus, and we discuss the results and further possibilities.

Interferometric determination of the electron density in a high-pressure xenon lamp with a holographic optical element

A new setup for plasma diagnostics is presented that is based on real-time holographic interferometry. The hologram is used as a holographic optical element (HOE) that combines the properties of a hologram, of a lens, and of a grating simultaneously. The HOE is responsible for the formation of the interference pattern, and, in addition, acts as an imaging element and prevents most of the plasma radiation from reaching the interferogram detection system. The spectral and imaging properties of this HOE are calculated numerically, and this numeric procedure is tested experimentally. We applied the HOE-interferometry technique to the measurement of the electron density in a brightly radiating high-pressure xenon lamp. The principle of this experiment, two-wavelength interferometry, is described, and the results of the measurement are presented and discussed.

Combustion flame diagnostics using degenerate four-wave mixing : the dipole moment power law and rotational temperature for nitric oxide

We applied the recently developed method for determination of the power law in the degenerate four-wave mixing process in order to use a Boltzmann plot for determination of the rotational temperature of nitric oxide molecules generated in an oxyacetylene flame at atmospheric pressure. It is shown that only for a certain range of transition dipole moments can a simple power law be established and applied to temperature measurements. We checked for the influence of deviations from a simple power law due to different saturation intensities of the and lines.

Real-time measurements of electron density spatial distributions in a mini-arc plasma by dispersion interferometry

A new experimental technique suitable for real-time measurements has been developed which permits the determination of the spatial distribution of the electron density of a plasma with a time resolution commensurate with the 6 ns duration of a single laser pulse. The accuracy and sensitivity of the electron density measurements allow the application to low-temperature plasma diagnostics. The possibilities of the method were demonstrated by studying a steady state mini-arc argon plasma at atmospheric pressure. The measured electron densities with 2 cm plasma length spanned the range from to on the plasma axis.