| Laser induced plasma ignition (LIPI) is a new ignition technique which is able to realize stable and reliable ignition of the lean-burn combustion and rocket engine systems. It has many advantages over traditional ignition systems, including easier control on the ignition position, better adaptability to working conditions, feasibility of multipoint ignitions, etc. Aimed to solve the problems involved with the parametric and kinetic investigations of the LIPI, such as the imperfect ignition model which requires improvement, the lack of parametric data for the LIPI of diffusion mixtures, and the too high minimum laser pulse energy (MPE) of the direct gas breakdown ignitions which limits the practical applications of the LIPI, this dissertation will concentrate on the parametric and kinetic investigations of the LIPI of methane/air mixtures.The dissertation firstly summarizes the developments of the LIPI, proposes the problems related to the parametric and kinetic investigations of the LIPI, and then presents the main research contents of the work. Concerned with the limitation of the existing indirect ignition model in which the chemical reaction is not considered and the unknown key factors affecting the ultimate fate of the LIPI, by taking into account both the thermal effect and combustion chemical reaction caused by the laser induced plasmas (LIPs), the dissertation then performs combustion chemical kinetic simulation on the LIPI of premixed methane/air mixtures using Taylor’s blast wave theory and CHEMKIN software. The effects of laser spark energy and equivalence ratio on the OH radical concentration in the initial flame kernels are analyzed and the effects of OH radical on the LIPI processes are investigated. The simulation results show that the LIPI is a kind of forced ignition with the combined action of the thermal effect and combustion chemical reaction induced by the LIPs. The role played by the chemical reactions especially the OH radical in the LIPI processes should be taken into account when analyzing the LIPI mechanism.Secondly, the properties of the LIPs in nitrogen, oxygen, air and methane, including the breakdown threshold, electron temperature, electron density and rotational temperature, and the effects of laser pulse energy and gas pressure on these properties, have been investigated systematically, providing important reference data for the hydromechanical simulation of the LIPI processes. A new relative spectral response calibration method based on the argon branching ratios is proposed for the laser induced breakdown spectroscopy (LIBS) system, and the relative spectral response in the wavelength range of221-617nm is calibrated with an uncertainty of~11%. The electron temperature and electron density of the LIPs in nitrogen, oxygen, and air are obtained using the Boltzmann plot method and the Stark broadening of the ionic lines, respectively. The self-absorption to the Hα line in the methane LIPs is evaluated and corrected using the duplication mirror method, and the electron density of the methane LIPs is determined using the Stark broadening of the Hα line. By comparing the measured and simulated C2(0,0) band of the Swan system with the rotational temperature as the optimization parameter, the rotational temperature of the methane LIPs is determined.Thirdly, the method to measure the equivalence ratios of the combustible mixtures is investigated. The calibration relationships between the intensity ratio of Hα line to the atomic oxygen triplets around777nm and the equivalence ratio are obtained with a premixed laminar methane/air flame as the standard, and the effects of laser pulse energy, detection position and flow rate of the gas mixtures on the calibration relationships are determined. The local equivalence ratios of a premixed methane/air flame and a methane diffusion flame are determined using the calibrated relationships, thus providing an effective method to measure the local equivalence ratios for the kinetic investigations of the LIPI.Fourthly, the parametric and kinetic investigations of the LIPI of premixed methane/air mixtures are performed. The minimum ignition energy (MIE), ignition time and blow out time of the LIPI and their dependence on the experimental conditions, including the equivalence ratio, flow rate, ignition position, and laser spark energy have been obtained. The MIE obtained agrees well with the existing results and is one order of magnitude higher than that of the electric spark plug ignition. With increased laser spark energy, the ignition time decreases gradually and then approaches a constant value, while the blow out time increases gradually before it approaches a constant value. The key factors affecting the ultimate results of the LIPI of premixed methane/air mixtures are determined by simultaneously measuring the three relevant parameters of the LIPI, namely, the spark energy, the local equivalence ratio, and the OH radical emission signal profile during the early times (μs scale) after the onset of laser spark, using a combination of the LIBS, emission spectroscopy and single-photon counting techniques, and performing a two-dimensional correlation analysis of these parameters with the ultimate ignition results. The OH radical concentration level in the initial flame kernel at early times (μs scale) following the generation of the LIPs is found to be the key factor determining the ultimate fate of the ignition. The chemical reactions in the initial flame kernels during the early time following the laser sparks determine whether the initial flame kernel can develop into a sustainable flame. Combining the combustion chemical kinetic simulation results, the experimental results and existing results, the indirect ignition model by the residual hot-gas after the shock wave is improved and a new joint thermal/chemical-reactional ignition mechanism for LIPI is proposed. With the joint function of the thermal effect and combustion chemical reaction induced by the LIPs, a higher laser spark energy (corresponding to a higher hot gas temperature) and a local equivalence ratio near stoichiometric condition will generate a higher OH radical concentration in the initial flame kernel at early times following the onset of the LIPs. The higher OH radical concentration can help to maintain and improve the concentrations of H, O and OH radicals in the flame kernel, and improve the flame propagation speed, thus promoting the chain reactions in the initial flame kernel, favoring the development and propagation of the initial flame kernel, and finally leading to successful ignitions.Finally, the parametric investigations of the LIPI of more practical coaxial methane/oxygen-enriched-air diffusion mixtures are performed for the first time. Both the MIE and ignition time show notable spatial distributions in the diffusion area, and values of~8-18mJ and~140-2800μs are obtained respectively. The distribution of local equivalence ratios in the diffusion area is simulated using the computational fluid dynamics software FLUENT, which indicates that the spatial distribution of the LIPI parameters is closely related to the local equivalence ratios at the ignition spots. Meanwhile, to reduce the MPE, the laser ablation plasma ignition (LAPI) of premixed methane/oxygen-enriched-air mixtures is proposed for the first time. For gas mixture with an oxygen concentration of30%, a flow speed of282cm/s, and an equivalence ratio of0.8-1.6, the MPE of the LAPI is obtained as2-5mJ, which is reduced by more than74%from that of the direct gas breakdown ignition. The reduction in MPE is of great importance for the practical applications of the LIPI. The ignition time of the LAPI decreases gradually with the increasing equivalence ratio and approaches a constant of~50μs when the equivalence ratio is larger1.3.The results presented in the dissertation will be of importance, both scientifically and practically, to improve the understanding on the LIPI mechanism, to guide the design and optimization of the LIPI system, and to promote the practical applications of the LIPI. |
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