Laser-induced Spark Research Articles

Effect of laser intensity on temporal and spectral features of laser generated acoustic shock waves: ns versus ps laser pulses.

Evolution of acoustic shock wave (ASW) properties generated during nanosecond (ns) and picosecond (ps) laser-induced breakdown (LIB) of atmospheric air at different input intensities is presented. The intensity is varied by changing the focal geometry of ns and ps pulses. The ASW pressures are observed to follow the dynamic interplay between the plasma density and recombination of plasma species. The conversion of laser energy to acoustic energy has increased from loose to tight focusing conditions. The central frequencies have moved toward the lower side with increasing laser intensities for both ns-LIB (76-48kHz) and ps-LIB (111.2-92.1kHz). The angular distribution of acoustic emissions was observed to follow the laser-induced plasma spark in both ns- and ps-LIB.

Wave-Front Measurements of a Supersonic Boundary Layer Using Laser Induced Breakdown

The aero-optical effect of a flat-plate adiabatic boundary layer has been measured using the light generated by a laser-induced breakdown spark. The measurements were performed in a blowdown wind tunnel at freestream Mach numbers of 3 and 4.38. The tests showed that the aero-optical effect of boundary layers with rms optical path difference as low as could be accurately measured using the laser-induced breakdown spark, including their deflection-angle spatial spectra. The results demonstrate that, using the laser-induced breakdown spark as a source of illumination, it is possible to make accurate measurements of low-amplitude aero-optical effects in a manner that is self-contained, nonintrusive, and suitable for a flight-test environment.

An experimental study on the ignition ability of a laser-induced gaseous breakdown

For examining the ability of laser ignition, a quiescent lean-fuel propane–air mixture was ignited in a constant-volume chamber by a spark induced by a pair of electrodes and an automotive spark driver, a spark induced by focused high-power laser, or a spark induced by focused high-power laser between a pair of dummy electrodes, where the deposited energy into the gas was the same in all cases. In the experiments, the mole fraction of the fuel in the mixture was varied, and the pressure history on the inner wall of the chamber, probability of successful ignition, and the high-speed multi-frame schlieren images of an ignition process were obtained. The results showed that the ignition ability of a laser-induced spark was superior to that of a conventional electrical spark in lean-fuel conditions near the ignitable limit, and that the higher ignition ability of a laser-induced spark was due not only to the lack of heat loss to electrodes but also to a large initial flame kernel whose effective energy was augmented by rapid heat release from the combustible mixture sucked into the kernel by a non-spherical inward flow created by a laser-induced spark. In addition, reinjection of the transmitted laser light into plasma strengthened the ignition ability of the laser. This was predominantly due to the increase of the total absorbed energy.

Comparative experimental study of ethanol-air premixed laminar combustion characteristics by laser induced spark and electric spark ignition

An experimental study of laminar combustion characteristics of ethanol-air premixed mixtures was conducted with different ignition methods, including laser induced spark ignition (LISI) and electric spark ignition (SI) at an initial condition of 358 K temperature and 0.1MPa pressure. Flame propagation with the two different ignition methods was analyzed and discussed. The laminar flame speed was extrapolated with a nonlinear extrapolation method. Results indicate that the laminar speed of ethanol-air mixtures with LISI is faster than that with SI at lean mixtures, but slower at stoichiometric and rich mixtures. The peak values of the laminar burning velocity for SI and LISI with nonlinear extrapolation are 50.1 cm/s and 47.6 cm/s at the equivalence ratio of 1.1, respectively. Laser-induced spark ignition is able to ignite leaner ethanol-air mixtures.

Dual-pulse laser-induced spark ignition and flame propagation of a methane diffusion jet flame

An experimental investigation was performed to study the ignition and flame propagation behaviors of a methane diffusion jet flame (Re= 5500) when dual pulse laser-induced spark discharges were introduced in a mixing layer. Time intervals (dt) of 50ns, 100µs, and 600µs between two laser pulses were tested, and the results were compared to a single pulse discharge case with the same total laser energy (60mJ). The laser-induced breakdown was introduced to the mixing layer at 9.5 jet diameters downstream from the jet exit. The chemical delay time scale under the flow condition was approximately 350µs (tcd). The disturbances in density fields generated by laser-induced breakdowns and chemical reactions were visualized using a high-speed schlieren imaging technique. The visualization at stationary air showed that interactions between two laser-induced breakdowns increased the surface area of hot plumes, but the effects of the interactions diminished when the breakdowns were introduced in a non-reacting air jet. The effects of the flow on the hot plumes prevailed the interaction effects between two breakdowns in non‐reacting air jet condition. However, when the dual pulse laser-induced spark discharges (dt= 600µs, dt/tcd= 1.71) were generated in a mixing layer of a methane jet, a rapid propagation of the flame was observed since the second breakdown enlarged the ignition kernel surfaces generated by the first breakdown. The results showed that using the dual pulse with intervals shorter than the electron lifetime scale (under 200ns) or longer than the chemical delay time scale could be beneficial in enhancing ignition and flame propagation processes due to the increased energy deposition or hot surface area, respectively.

Near-Real Time Measurement of Carbonaceous Aerosol Using Microplasma Spectroscopy: Application to Measurement of Carbon Nanomaterials.

A sensitive, field-portable microplasma spectroscopy method has been developed for real-time measurement of carbon nanomaterials. The method involves microconcentration of aerosol on a microelectrode tip for subsequent analysis for atomic carbon using laser-induced breakdown spectroscopy (LIBS) or spark emission spectroscopy (SES). The spark-induced microplasma was characterized by measuring the excitation temperature (15,000 - 35,000 K), electron density (1.0 × 1017 - 2.2 × 1017 cm-3), and spectral responses as functions of time and interelectrode distance. The system was calibrated and detection limits were determined for total atomic carbon (TAC) using a carbon emission line at 247.856 nm (C I) for various carbonaceous materials including sucrose, EDTA, caffeine, sodium carbonate, carbon black, and carbon nanotubes. The limit of detection for total atomic carbon was 1.61 ng, equivalent to 238 ng m-3 when sampling at 1.5 L min-1 for 5 min. To improve the selectivity for carbon nanomaterials, which consist of elemental carbon (EC), the cathode was heated to 300 °C to reduce the contribution of organic carbon to the total atomic carbon. Measurements of carbon nanotube aerosol at elevated electrode temperature showed improved selectivity to elemental carbon and compared well with the measurements from thermal optical method (NIOSH Method 5040). The study shows that the SES method to be an excellent candidate for development as a low-cost, hand-portable, real-time instrument for measurement of carbonaceous aerosols and nanomaterials.

Effect of water mist on minimum ignition energy of propane/air mixture

Water mist is anticipated to be an alternative of halogenated hydrocarbon fire suppressants because it has high thermal and chemical potential to prevent the ignition of a combustible mixture and it is environmentally acceptable. The minimum ignition energy (MIE) is an important property for designing safety standards and understanding the ignition process of combustible mixtures. In the present investigation, the effect of polydisperse water mist on MIE of propane/air mixture is investigated experimentally. Instead of the electric spark or laser-induced spark, the mixture is ignited by a fine Nichrome wire which is heated and melted electrically. Ignition energy of propane/air mixture is measured from the energy released by melting of a fine Nichrome wire. The discharge duration of wire melting is longer than that of electric spark or laser induced spark, resulting in the larger MIE. The ignition probability follows the cumulative probability distribution function of net discharged energy. The minimum energy density, i.e. energy per unit volume is found to depend on the energy discharge duration. Volumetric energy release rate (VERR), i.e. energy released per unit volume and unit time, is the dominant factor for the ignition probability. For lean propane/air mixtures, water mist increases minimum VERR by 50% when water mist mass fraction Y0 is 0.17 as compared to Y0=0. The ignition probability vanishes when Y0>0.2. On the other hand, for rich mixtures, the reduction of the minimum VERR at ignition is small but definite. The minimum VERR for wire melting is one or two orders of magnitude larger than that calculated for electric spark due to the longer ignition delay. Water mist is more effective in suppressing the explosion of combustible mixtures for lean mixtures than for rich mixtures.

Evolution of flame-kernel in laser-induced spark ignited mixtures: A parametric study

The present work focuses on the early stages of flame-kernel development in laser-induced spark ignited mixtures issuing out of a Bunsen burner. The time-scale of 3 µs to 1 ms associated with the flame-kernel evolution stage of an ignition event is targeted in this work. A CH4/air mixture (equivalence ratio ϕ = 0.6) is studied as a base case, and compared with CH4/CO2/air (mole fractions = 0.059/0.029/0.912, respectively) and CH4/H2/air (mole fractions = 0.053/0.016/0.931, respectively) mixtures for nearly the same adiabatic flame temperature of 1649 K. The spatio-temporal flame-kernel evolution is imaged using planar laser induced fluorescence of the OH radical (OH-PLIF), simultaneously with H-alpha emission from the plasma. The H-alpha emission suggests that the plasma time-scale is well below 1 µs. The PLIF images indicate all the stages of kernel development from the elongated kernel to the toroidal formations and the subsequent appearance of a front-lobe. The different time-scales associated with these stages are identified from the rate of change of the kernel perimeter. The plasma is followed by a supersonic kernel-perimeter growth. Larger flame-kernel spread is found in the case of CH4/H2 mixtures. A distinct shift in the trends of evolution of LIF intensity and kernel perimeter is observed as the fuel concentration is varied near the lean flammability limit in CH4/air (ϕ = 0.35–0.65) and H2/air (ϕ = 0.05–0.31) mixtures. The flow velocity (Reynolds number, Re) effect in both laminar and turbulent flow regimes (Re = ∼600–6000) indicates that the shape of the flame-kernel changes at higher velocities, but the size of the kernel does not change significantly for a given time from the moment of ignition. This could be due to a balance between two competing effects, namely, increase in the strain rate that causes local extinction and thus decreases the flame-kernel growth, and increase in the turbulence levels that facilitates increased flame-kernel surface area through wrinkling, which in turn increases the flame-kernel growth.

Benefits and applications of laser-induced sparks in real scale model measurements.

The characteristics of using a laser-induced spark as a monopole source in scale model measurements were assessed by comparison with an electric spark and a miniature spherical loudspeaker. Room impulse responses of first order directivity sources were synthesized off-line using six spatially distributed sparks. The source steering direction was scanned across the horizontal and vertical plane to assess the origin of early reflections. The results confirm that the characteristics of the laser-induced spark outperform those of typical sources. Its monopole characteristics enable the authors to synthesize room responses of directional sources, e.g., to obtain directional information about reflections inside scale models.

Multi-point laser spark generation for internal combustion engines using a spatial light modulator

This paper reports on a technique demonstrating for the first time successful multi-point laser-induced spark generation, which is variable in three dimensions and derived from a single laser beam. Previous work on laser ignition of internal combustion engines found that simultaneously igniting in more than one location resulted in more stable and faster combustion – a key potential advantage over conventional spark ignition. However, previous approaches could only generate secondary foci at fixed locations. The work reported here is an experimental technique for multi-point laser ignition, in which several sparks with arbitrary spatial location in three dimensions are created by variable diffraction of a pulsed single laser beam source and transmission through an optical plug. The diffractive multi-beam arrays and patterns are generated using a spatial light modulator on which computer generated holograms are displayed. A gratings and lenses algorithm is used to accurately modulate the phase of the input laser beam and create multi-beam output. The underpinning theory, experimental arrangement and results obtained are presented and discussed.

Investigation on minimum ignition energy of mixtures of α-pinene–benzene/air

Minimum ignition energies (MIE) of α-pinene–benzene/air mixtures at a given temperature for different equivalence ratios and fuel proportions are experimented in this paper. We used a cylindrical chamber of combustion using a nanosecond pulse at 1064nm from a Q-switched Nd:YAG laser. Laser-induced spark ignitions were studied for two molar proportions of α-pinene/benzene mixtures, respectively 20–80% and 50–50%. The effect of the equivalence ratio (Φ) has been investigated for 0.7, 0.9, 1.1 and 1.5 and ignition of fuel/air mixtures has been experimented for two different incident laser energies: 25 and 33mJ. This study aims at observing the influence of different α-pinene/benzene proportions on the flammability of the mixture to have further knowledge of the potential of biogenic volatile organic compounds (BVOCs) and smoke mixtures to influence forest fires, especially in the case of the accelerating forest fire phenomenon (AFF). Results of ignition probability and energy absorption are based on 400 laser shots for each studied fuel proportions. MIE results as functions of equivalence ratio compared to data of pure α-pinene and pure benzene demonstrate that the presence of benzene in α-pinene–air mixture tends to increase ignition probability and reduce MIE without depending strongly on the α-pinene/benzene proportion.

Laser-Induced Spark Ignition of Gaseous and Quiescent N-Decane–Air Mixtures

Non-resonant breakdown ignition of quiescent mixtures of n-decane vapors and air is characterized by focusing the beam of a Nd:YAG laser operating at 1064 nm within a closed vessel. Breakdown and ignition are considered following a statistical approach: percentages of ignition are determined for different fuel equivalence ratios and pressures (Φ = 0.65–2; P0 = 0.5–4 bar) at constant initial temperature T0 = 347 K. Significant influence of these parameters is reported. The domain of systematic ignition is characterized in terms of equivalence ratio and energy delivered by the laser, i.e., parameters relevant to propulsion applications. From a more fundamental point of view, absorbed energy is considered and the minimum ignition energy value equals 0.95 mJ. Effects of air chemical composition on ignition probabilities are evidenced by employing ambient or synthetic air, with argon or water vapor. This work brings useful complements to existing studies mainly focused on decane spray ignition.

Minimum Ignition Energy Measurements for α-Pinene/Air Mixtures

The aim of this work is to determine the minimum ignition energy (MIE) of the α-pinene/air mixture using a laser-induced spark ignition. The α-pinene is a volatile organic compound (VOC) widely found in air freshener, home fragrance, and household products and also emitted by several vegetal species. However few studies deal with this material despite its flammability limits lower than 1% at various temperatures and its high laminar burning speeds. Moreover, some studies assume that the α-pinene/air mixture can play a role when an accelerating forest fire (AFF) occurs. This experimental work highlights the low values of MIE for seven equivalent ratios and also pinpoints the quick rise of ignition probability on a short range of energy absorbed. This study shows the potential threat of the α-pinene/air mixture.

Effect of flow velocity and temperature on ignition characteristics in laser ignition of natural gas and air mixtures

Laser induced spark ignition offers the potential for greater reliability and consistency in ignition of lean air/fuel mixtures. This increased reliability is essential for the application of gas turbines as primary or secondary reserve energy sources in smart grid systems, enabling the integration of renewable energy sources whose output is prone to fluctuation over time. This work details a study into the effect of flow velocity and temperature on minimum ignition energies in laser-induced spark ignition in an atmospheric combustion test rig, representative of a sub 15MW industrial gas turbine (Siemens Industrial Turbomachinery Ltd., Lincoln, UK). Determination of minimum ignition energies required for a range of temperatures and flow velocities is essential for establishing an operating window in which laser-induced spark ignition can operate under realistic, engine-like start conditions. Ignition of a natural gas and air mixture at atmospheric pressure was conducted using a laser ignition system utilizing a Q-switched Nd:YAG laser source operating at 532nm wavelength and 4ns pulse length. Analysis of the influence of flow velocity and temperature on ignition characteristics is presented in terms of required photon flux density, a useful parameter to consider during the development laser ignition systems.

Absorption of laser-radiation energy by femtosecond laser spark in air

The fraction of focused femtosecond laser radiation energy absorbed by a laser-induced spark in air has been studied. It is established that the absorbed energy exhibits nonlinear growth with increasing energy of the laser pulse. The threshold power for laser-induced spark formation in air is 5.2 GW.

Ignition by Electric Spark and by Laser-Induced Spark of Ultra-Lean CH4/Air and CH4/CO2/Air Mixtures at High Pressure

This study investigates the lean flammable limit (LFL) of CH4/air and CH4/CO2/air mixtures, using two different ignition devices: electrical discharge and laser-induced spark (LIS). The experiments were performed for CH4/air and (0.6 CH4 + 0.4 CO2)/air mixtures in ultra-lean conditions (0.48 ≤ φ ≤ 0.67) at different initial pressures (P = 100–700 kPa) and at an initial temperature of 298 K. In these conditions, all observed flames were ascending. The minimum laser pulse energy for ignition (MPE) and the maximum overpressure (MO) were determined. The LFL increases with the initial pressure and with the addition of CO2.

Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames: New insight into ignition transition

Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion for a wide range of equivalence ratios. Our study used a similar approach to present an additional and complete MIE data set for laser-induced spark ignitions in a flow configuration and under turbulent conditions (i.e., for different ranges of length and time scales) that differed from those chosen by Shy and his colleagues. To extend the previous analyses to very lean premixed methane/air flames, the study examined a characteristic chemical time (τCB), which was defined as the start of the chain-branching reactions. Physically, this chemical time corresponded to the time when the initial chemical reactions of the ignition process released enough heat to compensate for heat losses and to enable a self-sustained reaction in successful ignition. Values for τCB, which were experimentally obtained from the temporal evolution of the mean hot kernel emissions (in laminar flow), strongly decreased with the equivalence ratio Φ for very lean flames and remained nearly constant and equal to 150μs for Φ>0.7. We obtained a clear turbulent ignition transition on the MIE as a function of the rms velocity (u′) and reconfirmed the two modes of ignition. Temporal kernel emission recordings served to describe the transition through the study of the interaction between the turbulence and the hot kernel at the time t=τCB. For low turbulence, when the smallest time scales of the turbulence τK were larger than the τCB values, the hot kernel/turbulence interaction occurred after the initiation of the chemical reactions. The amount of deposited energy required to initiate the reactions and to attain a self-sustained flame kernel was consequently similar to that found under laminar conditions. After the transition, the smallest time scales of the turbulence were smaller than the τCB values. Turbulence may have affected and interacted with the hot kernel before the initiation of chain-branching reactions occurred. Larger amounts of deposited energy were therefore required to compensate for this turbulent dissipation and to attain a self-sustained flame. A Peclet number (PeCB) was introduced, which is equal to the ratio of the turbulent diffusivity at the time of the initiation of the chemical reactions and the thermal diffusivity. These very scattering MIE data (dependent on Φ and u′) are plotted as a function of PeCB and collapsed into a single curve with two drastically different increasing slopes showing the ignition transition.

Influence of laser pulse parameters on characteristics of a source of multicharged metal ions based on laser-induced medium-power spark discharge

The dynamics of laser-induced vacuum spark discharge with storage energy not exceeding 25 J has been studied in a broad range of laser pulse energies and power densities. It is shown that, using discharge-initiating laser pulse of nanosecond duration, it is possible to obtain stable single pinching of cathode jet plasma at a storage voltage above 10 kV. Plasma pinching is accompanied by the generation of a beam of multicharged ions of the cathode material (aluminum) up to Al8+. The maximum energies of ions obey the scaling relation Emax = 5ZeU0 that has been obtained previously for a low-voltage discharge. An increase in the laser pulse energy leads to growth of the average beam charge and a sharp decrease in the ion energy.

Laser-Induced Spark Ignition of Premixed Confined Swirled Flames

Optimization of the ignition location is of crucial importance for many combustion systems and requires advanced knowledge of the ignition process for both fundamental and applied configurations. In this article, a premixed swirl burner was designed to experimentally study the impact of the spark location on successful ignition and to detail the scenario from ignition to flame stabilization. Two swirl numbers were investigated to evaluate their impact on the ignition process. Particular attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic particle image velocimetry) as well as on ignition conditions. Ignition probability maps were obtained for a constant level of deposited energy. Contrary to previous studies, no correlation between local turbulent kinetic energy and ignition probability was observed, and a deeper analysis of the temporal evolution of the flame kernel within the combustion chamber is required. Coupling fast flame visualization with the corresponding pressure signal demonstrated that the efficiency of the ignition location was not only controlled by the local flow properties, but also by the early flame kernel development, linked by its typical trajectories within the combustion chamber.

Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames

Spark ignition in lean and highly turbulent premixed flows is experimentally investigated by measuring the Minimum Ignition Energy (MIE), for laser-induced spark ignition in methane/air mixtures, as a function of the velocity fluctuation u′, the equivalence ratio and the focal length of the focusing lens. The burner provides a 2D stationary flow with a homogeneous and isotropic turbulence.A clear transition on the MIE values is observed, when u′ increases. Before the transition, the MIE increases gradually with u′, whereas across the transition (for u′∼1 ms−1), the MIE increases strongly. As the MIE is proportional to the flame thickness to the power of three, this slope breakdown could indicate the existence of two distinct modes of flame structure. From a certain threshold of turbulence intensity, the flame kernel no longer develops in a flamelet regime, rather in a regime where its structure is strongly modified by the turbulence: the flame front would undergo an intense mixing by all the eddies of the flow (Da<1) and a thickening by the small eddies (Ka∼10), leading to an intense stretch and possibly local extinctions. This ignition transition phenomenon in turbulent mixtures has already been reported by Shy et al. (2010) [1] and Huang et al. (2007) [2], but for different experimental conditions.The present study also reveals that there is a common criterion for the ignition transition phenomenon: whatever the focal length of the focusing lens and the equivalence ratio of the mixture, an ignition transition occurs when the turbulent flow reaches a Karlovitz number of the order of 10.