Ernst Wintner

Laser ignition of methane–air mixtures at high pressures

Abstract Laser induced plasma ignition of methane–air mixtures at initial pressures up to 4 MPa was investigated experimentally using nanosecond Nd:YAG-laser pulses at 1064, 532 and 355 nm. Variations of the focusing system yielded, independent of the laser wavelength, an optimum focal spot diameter of about 20 μm under the experimental conditions, indicating that small impurity particles provide the seeds for laser plasma generation. The minimum laser pulse energy needed for ignition (ranging from 8 to 15 mJ) decreased with increasing initial pressure and with increasing equivalence ratio in the mixture regime of φ =0.91–0.56. Corresponding threshold intensities ranged from 10 10 to 10 11 W/cm 2 . Evidence for a non-resonant breakdown mechanism was established.

Laser breakdown with millijoule trains of picosecond pulses transmitted through a hollow-core photonic-crystal fibre

Sequences of picosecond pulses with a total energy in the pulse train of about 1 mJ are transmitted through a hollow-core photonic-crystal fibre with a core diameter of approximately 14 µm. The fluence of laser radiation coupled into the core of the fibre under these conditions exceeds the breakdown threshold of fused silica by nearly an order of magnitude. The laser beam coming out of the fibre is then focused to produce a breakdown on a solid surface. Parameters of laser radiation were chosen in such a way as to avoid effects related to the excitation of higher order waveguide modes and ionization of the gas filling the fibre in order to provide the possibility to focus the output beam into a spot with a minimum diameter, thus ensuring the maximum spatial resolution and the maximum power density in the focal spot.

Laser ablation of dental tissues with picosecond pulses of 1.06-μm radiation transmitted through a hollow-core photonic-crystal fiber

Sequences of picosecond pulses of 1.06-microm Nd:YAG laser radiation with a total energy of approximately 2 mJ are transmitted through a hollow-core photonic-crystal fiber with a core diameter of approximately 14 microm and are focused onto a tooths surface in vitro to ablate dental tissue. The hollow-core photonic-crystal fiber is shown to support the single-fundamental-mode regime for 1.06-microm laser radiation, serving as a spatial filter and allowing the laser beams quality to be substantially improved. The same fiber is used to transmit emission from plasmas produced by laser pulses onto the tooths surface in the backward direction for detection and optical diagnostics.

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.

Pico- and microsecond laser ablation of dental restorative materials

For biological hard tissues as well as for artificial restoration materials, ultra-short laser pulses (USLP) have proven their suitability for ablation with negligible collateral damage providing many advantages. For this paper, it was the aim to investigate the suitability of scanned USLP for the ablation of dental restorative materials compared to the microsecond pulses of conventional Erbium lasers. Scanning electron microscopy allowed analyzing the quality of the cavity surfaces with respect to structural properties as well as temperature exposition. Quantitative results about the ablation efficiency are given for different laser parameters on dental hard tissue and a broad variety of dental restoration materials, the latter being reported for the first time.

Low level light therapy by LED of different wavelength induces angiogenesis and improves ischemic wound healing

Low‐level light therapy (LLLT) has been revealed as a potential means to improve wound healing. So far, most studies are being performed with irradiation in the red to near‐infrared spectra. Recently, we showed that blue light (470 nm) can significantly influence biological systems such as nitric oxide (NO) metabolism and is able to release NO from nitrosyl‐hemoglobin or mitochondrial protein complexes. Therefore, the aim of this study was to evaluate and compare the therapeutic value of blue or red light emitting diodes (LEDs) on wound healing in an ischemia disturbed rodent flap model.

Laser ignition of engines - : a realistic option!

Due to the demands of the market to increase efficiencies and power densities of gas engines, existing ignition schemes are gradually reaching their limits. These limitations initially triggered the development of laser ignition as an effective alternative, first only for gas engines and now for a much wider range of internal combustion engines revealing a number of immediate advantages like no electrode erosion or flame kernel quenching. Furthermore and most noteworthy, already the very first engine tests about 5 years ago had resulted in a drastic reduction of NOx emissions. Within this broad range investigation, laser plasmas were generated by ns Nd-laser pulses and characterized by emission and Schlieren diagnostic methods. High-pressure chamber experiments with lean hydrogen-methane-air mixtures were successfully performed and allowed the determination of essential parameters like minimum pulse energies at different ignition pressures and temperatures as well as at variable fuel air compositions. Multipoint ignition was studied for different ignition point locations. In this way, relevant parameters were acquired allowing to estimate future laser ignition systems. Finally, a prototype diode-pumped passively Q-switched Nd:YAG laser was tested successfully at a gasoline engine allowing to monitor the essential operation characteristics. It is expected that laser ignition involving such novel solid-state lasers will allow much lower maintenance efforts.

Laser-Stimulated Ignition in a Homogeneous Charge Compression Ignition Engine

A laser-induced spark was generated inside the combustion chamber of a reciprocating engine running in Homogeneous Charge Compression Ignition (HCCI) mode to investigate the influence of the plasma ...

Hollow-core photonic-crystal fibres for laser dentistry

Hollow-core photonic-crystal fibres (PCFs) for the delivery of high-fluence laser radiation capable of ablating tooth enamel are developed. Sequences of picosecond pulses of 1.06 microm Nd:YAG-laser radiation with a total energy of about 2 mJ are transmitted through a hollow-core photonic-crystal fibre with a core diameter of approximately 14 microm and are focused on a tooth surface in vitro to ablate dental tissue. The hollow-core PCF is shown to support the single-fundamental-mode regime for 1.06 microm laser radiation, serving as a spatial filter and allowing the laser beam quality to be substantially improved. The same fibre is used to transmit emission from plasmas produced by laser pulses on the tooth surface in the backward direction for detection and optical diagnostics.