Kurt Iskra

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.

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.

All-optical tunability of holographically multiplexed organic distributed feedback lasers.

We report the fabrication and characterization of optically pumped multiple grating distributed feedback lasers in dye doped organic thin films. Each multiplexed laser structure is inscribed at a different angle in the sample plane and possesses a unique emission wavelength. The polarization sensitivity of these structures with respect to the pumping light is exploited to enable simple and high-speed switching of the device emission wavelength.

Reductive Photopatterning of Phenylene-Vinylene-Based Polymers

Photochemical methods were developed to obtain a variation of the refractive index in aromatic polymer surfaces and a change in the photoluminescence characteristics of phenylenevinylene-based polymers. Films of aromatic polymers, among them polystyrene (PS), poly(2-vinylnaphthalene) (PVN) and derivatives of poly( p -phenylene-vinylene) (PPV) were UV irradiated in the presence of gaseous hydrazine (N 2 H 4 ). The photoreaction led to a strong reduction of the refractive index of the polymers due to a hydrogenation of the aromatic units. In the case of PPV, we observed reductive photobleaching. This new technique was employed to produce photogenerated patterns in PPV. The results are compared to oxidative bleaching.

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.

Development of a High Peak Power Solid-State Laser for Engine Ignition

A compact monolithic Nd:YAG-Cr4+:YAG high peak power, passively Q-switched, longitudinally diode-pumped laser was constructed for laser ignition. The system yielded pulses with energies of 8 mJ and durations of 1ns at 225W pump power.

Optical Diagnostics of Laser Ignition for Future Advanced Engines

A laser-based system should be advantageous to a spark-plug based ignition system. Free choice of the ignition spot and precise timing constitute two major advantages. Multi point laser ignition could lead to higher efficiencies, and laser ignition as such is capable of igniting leaner mixtures than a spark plug, thereby decreasing thermal NOx and soot emissions. This paper is devoted to advances in optical diagnostics of laser ignition for future internal combustion engines. The focus of this paper is on diagnostics at high pressures, that is engine-like conditions. Laser ignition tests were performed with the fuels methane, hydrogen and biogas in static combustion cells with dimensions comparable to stationary engines. A Nd:YAG laser (5 ns pulse duration, wavelength 1064 nm, 1–20 mJ pulse energy) was used to ignite gaseous fuel/air mixtures at initial pressures of 1–3 MPa. Schlieren photography and laser-induced fluorescence (LIF) were used for optical diagnostics (flame kernel development, shock wave propagation). The lean burn characteristics were investigated. Schlieren photography was used to determine the velocity of the shock wave and to study the influence of the shock wave on temperature rise and energy loss. Using planar laser-induced fluorescence (PLIF), the spatial distribution of the combustion intermediates OH and formaldehyde were recorded. The temporally resolved imaging shows that the initial stages of the flame front evolution closely follows the turbulence and density fluctuations caused by the shock and pressure wave induced by the laser spark. In this paper, results from LIF spectroscopy and Schlieren photography are compared. Depending on the laser pulse energy and focus size, at later stages after the ignition the flame front propagation approaches the laminar burning regime and flame front speed decrease. Flame front break up at lean conditions indicates the limit of the ignitable mixture fraction when the speed due to spark-induced convection exceeds the flame propagation rate.© 2004 ASME

Fabrication of refractive index and relief gratings in polymer films for DFB lasers

A styrene copolymer of 4-vinylbenzyl thiocyanate (PST- co -VBT) was employed as recording material for optical interference patterns with periods ∧ -2 , λ = 266 nm), refractive index gratings were produced in PST- co -VBT by an UV induced photoisomerization SCN - NCS. Subsequent modification of the patterns with gaseous amines yielded surface relief gratings via the formation of derivatives of thiourea. Laser irradiation with higher pulse energies (7 mJ cm -2 , λ = 266 nm) directly produced surface relief gratings (modulation depth 30 nm). These gratings were also reactive towards amine reagents and allowed a selective functionalization of the grooves of the relief (“reactive gratings”). Optically inscribed gratings in PST- co -VBT were employed as optical resonators for distributed feedback (DFB) lasing. With a laser dye (DCM) dissolved in PST- co -VBT, optically pumped DFB laser action was observed after inscribing index and relief gratings. The pumping threshold for lasing I th was 250 nJ cm -2 at λ = 532 nm.