Laser Spark Ignition Research Articles

Laser spark ignition: experimental determination of laser-induced breakdown thresholds of combustion gases

This short paper reports preliminary results on experimental measurements of breakdown threshold laser intensities of air, O 2, N 2, H 2, and CH 4 using a Q-switched Nd–Yag laser operating at 532 nm and 1064 nm and 5.5 ns pulse. The breakdown threshold intensities were measured for the range of pressure from 150 to about 3040 Torr. The results showed that the pressure dependence of breakdown threshold is I thr∝ p − n which is in agreement with the inverse bremsstrahlung absorption process creating breakdown. The degree by which I thr depends on pressure was found to be stronger at 532 nm than at 1064 nm indicating the important effect of diffusion loss.

98/02283 Laser spark ignition and combustion characteristics of methane-air mixtures

98/02283 Laser spark ignition and combustion characteristics of methane-air mixtures

Laser spark ignition and combustion characteristics of methane-air mixtures

Ignition breakdown kernels of methane-air mixtures initiated by laser-induced sparks and by conventional electric sparks arc compared during initial stages. Experiments were conducted using a four-stroke (Otto-cycle) single-cylinder typical high-pressure combustion chamber. The piston is cycled in the cylinder by using an electric motor driven hydraulic ram. An cxcimer laser beam, either produced from krypton fluoride gas (λ = 248 nm) or argon fluoride gas (λ = 193 nm), or a Nd:YAG laser beam (λ = 1064 nm) is focused into a combustion chamber to initiate ignition. Conventional electric spark ignition is used as a basis for comparison between the two different ignition methods and the resultant early breakdown kernel characteristics. A streak camera is used to investigate and record the initial stages of kernel formation. Both a breakdown and a radial expansion wave of the ignition plasma are observed for certain laser ignition conditions of methane-air mixtures under typical internal combustion (IC) engine conditions. Results indicate that only certain wavelengths used for producing laser ignition produce a radial expansion wave. Laser ignition kernel size is calculated and laser-supported breakdown velocity is calculated by using Raizer's theory and is compared with measured results. Laser ignition results in a 4–6 ms decrease in the time for combustion to reach peak pressure than is obtained when using electric spark ignition in the same combustion chamber and under the same ignition conditions.

95/06376 Time-resolved imaging of flame kernels: Laser spark ignition of H 2/O 2/Ar mixtures

95/06376 Time-resolved imaging of flame kernels: Laser spark ignition of H 2/O 2/Ar mixtures

Time-resolved imaging of flame kernels: Laser spark ignition of H 2/O 2/Ar mixtures

The shape and structure of developing flame kernels in laser-induced spark ignited hydrogen/air mixtures is investigated as a function of gas composition and time. Using planar laser-induced fluorescence (PLIF) to measure the spatial distribution of OH radicals produced inside the reaction zone, we have recorded the evolution of the nascent flame kernel in a series of images following the laser-induced spark. This series provides the rate of flame growth, the evolution of the flame shape, and the intensity of the PLIF signal as a function of time for both igniting flames and nonignition events. The reaction zones grow quickly at early times, but slowly decrease in propagation rate as the energy density within the flame kernel decreases. A distinct anisotropy is observed in the expanding spark and flame kernel. At short times ( t < 100 μs), a toroidal shape is observed similar to that seen previously for electrode-spark ignitions and for laser ignitions in methane/air. There is also a tendency for the flame to grow back toward the ignition laser. Successful ignitions appear virtually identical to failed ignitions during the first 100 μs. Significant differences, notably in intensity, appear between 100 and 500 μs following the spark. These observations imply that early flame kernel growth is dominated by gas motion induced by the short-duration spark. The ultimate fate of an ignition lies with the chemistry of the reactions which determines whether the gas undergoes a transition from hot plasma to propagating flame.

Laser spark ignition of chemically reactive gases.

Exploratory experiments on the ignition by laser-induced sparks of chemically reactive gaseous mixtures, both detollable and nondetonable, were performed. It was found that the minimum energy required to induce breakdown was of sufficient magnitude to generate a strong spherical blast wave. The ignition mechanisms are strongly coupled to the dynamics of the decaying blast. For nondetonable gases, it was observed that initially the blast wave is of sufficient strength to result in auto-ignition of the reactive media with the reaction front trailing immediately behind the spherical shock front. As the blast expands, the reaction front decouples rapidly from the shock and propagates as a deflagration wave dependent on the transport properties of the medium. For detonable mixtures there exists a critical energy above which direct initiation of a spherical detonation wave occurs. This critical ignition energy varies inversely with the initial pressure of the mixture showing the same qualitative behavior as the variation of the induction time with initial pressure. For ignition energy well below this critical value, similar behavior of rapid decoupling to a spherical deflagration wave as in the case of nondetonable gases was observed. For ignition energy close to the critical, decoupling followed by re-establishment to a spherical detonation wave occurs. I. Introduction W ITH the recent development of high-powered solid-state lasers, breakdown in gases can readily be achieved at the focal point when the laser beam is collimated by a simple convex lens. The basic mechanisms of electrical breakdown at optical frequencies and the subsequent phenomena associated with laser sparks are discussed in detail in a recent review article by Meyerand. 1 In the present paper some exploratory experiments on the use of laser-induced sparks for the ignition of chemically reactive gases are described. There are a number of important advantages associated with the use of laser-induced sparks for ignition studies. For example, the laser spark is almost a perfect point energy source in which the energy and the rate of its deposition can be controlled and measured accurately. Also, pure chemical ignition, entirely free of contamination by any electrode material, can be achieved. The complete absence of material surfaces in the vicinity of the ignition area eliminates the complex effects of heat transfer on the initial growth of the flame kernel. The previously mentioned properties of the laserinduced spark make it an ideal energy source for the study of the basic mechanisms of ignition and the subsequent combustion phenomena in a chemically reactive gas. It is particularly suitable for ignition experiments in supersonic streams where electrode interference effects would be significant. The present experiments are of a qualitative nature with the objective being to determine the ignition mechanisms and subsequent development of the combustion phenomena from a laser-induced spark in a combustible gaseous mixture. The particular emphasis is centered on detonable gases where it is of interest in the present program to determine the minimum energy level of a free spark that will result in the