Investigations On The Laser Ignition Of Combustible Gas And Combustion Diagnosis Based On Femtosecond Pulses

With increasingly severe energy shortage and environmental pollution worldwide,it is urgent to develop efficient and clean combustion technologies to improve the combustion production capacity of fossil fuel and reduce the emission of combustion waste.On the one hand,the fuel ignition has a profound impact on the energy conversion efficiency and the subsequent combustion stabilization,hence a reliable ignition source contributes to an efficient and green combustion process.In recent years,considerable technical progresses have been made in laser ignition due to its advantages such as high ignition reliability and strong lean-fuel ignition capability.And the most widely used nanosecond（ns）laser-induced spark ignition has shown excellent performance in igniting multi-phase fuels like gas,solid and liquid,but it still meets limitations in terms of high energy demand,working distance,stability and so on.With the rapid progress in ultrafast laser technology,femtosecond laser pulse is gradually being used for assisted ignition but has enormous obstacles in triggering fuel ignition alone due to the low thermodynamic temperature and inferior energy deposition efficiency of femtosecond laser-induced plasma.In 2021,femtosecond laser filament ignition with energy threshold at the sub-millijoule level and robust 100%success rate in the lean-fuel combustible gas mixture was realized for the first time,in which a critical balance between the length and internal plasma density of the filament is considered to play a crucial role in achieving successful ignition.However,the mechanism of femtosecond laser filament ignition and subsequent flame kernel evolution are still unclear,as well as the application potential of femtosecond laser ignition requires to be further explored.On the other hand,monitoring and diagnosing the combustion process conduce to a deep understanding of the combustion mechanism and optimize the combustion technology,which are necessary for improving the combustion efficiency and controlling the emission of pollutants.Currently,the combustion diagnosis technologies based on nanosecond laser have been widely utilized in the measurement and characterization of various parameters of the combustion flow field.However,due to the long duration of nanosecond pulse,the ns laser-based technologies cannot characterize the ultrafast time-scale（picosecond or femtosecond order）evolution information of incomplete combustion products such as soot particles.Therefore,the femtosecond laser combustion diagnosis technologies are imperative to meet the measurement requirements of ultrafast-dynamic information of combustion products.The present thesis examines the mechanism,spatiotemporal evolution of flame kernel and remote ignition of femtosecond laser filament ignition,offering theoretical and data support for further understanding and application of femtosecond laser ignition technology.Meanwhile,the ultrafast dynamic behavior of multi-size soot particles in the combustion field under the excitation of the filament is studied by femtosecond laser sensing technology,which provides a feasible method for online and in-situ measurement of ultrafast time-scale evolution information of nanoparticles.The innovative achievements of this thesis are as follows:1.Research on the mechanism of femtosecond laser filament ignition.We performed the filament ignition of a premixed lean-fuel mixture using an 800 nm and400 nm dual-color femtosecond laser scheme,and it was found that the ignition performance is strongly dependent on the photon energy of the incident laser pulse.By analyzing the fluorescence intensities of N₂⁺and early-stage OH radicals as a function of the delay time between the 800 nm and 400 nm pulses,it showed that the ionization enhancement effect during the filament ignition in the dual-color scheme is able to produce more abundant molecules/ions such as O₂⁺and CH₄⁺,which promote the formation of OH and other active free radicals through dissociation and subsequent chain reactions,resulting in the enhancement of the oxidative exothermic reactions,and eventually the successful ignition.Therefore,the addition of the 400 nm pulse in the dual-color scheme for ignition can efficiently decrease the minimum ignition energy（MIE）as compared to that in the 800 nm single-color scheme.The research results clarified the essential contributions of the non-resonant photochemical reaction generated by the filament-induce“cold”plasma to the robust ignition ability and the ultralow energy threshold of femtosecond laser ignition,further revealing the mechanism of femtosecond laser filament ignition and providing a solution for igniting the lean-fuel combustible gas with the ultralow ignition energy threshold.2.Research on the development characteristics of flame kernel in femtosecond laser filament ignition.We employed a high-speed camera to image the chemiluminescence of CH*in the flame kernel generated by femtosecond laser filament,and studied the spatiotemporal evolution characteristics of the flame kernel.It was found that the upper and lower fronts of the flame kernel in femtosecond laser filament ignition develop unidirectionally along the direction of gas flow throughout their entire life-cycles.With the increase of gas flow speed,the displacement speeds of the upper and lower fronts of the flame kernel also increase;the upper front is in uniform motion and the lower front is in variable motion.Moreover,we studied the effect of laser pulse repetition rate on the evolution of flame kernel in the filament ignition,and found that the displacement and development processes of the upper and lower fronts of flame kernel were basically the same under different pulse repetition rates,and as laser pulse repetition rate increased,the catch-up distance between the existing and newly-formed flame kernels gradually decreased,indicating that the ignition stability can get enhanced under high laser pulse repetition rate.Compared to the nanosecond laser-induced spark ignition with the bidirectional development of the early-stage flame kernel,the femtosecond laser filament ignition with the characteristic of unidirectional development of flame kernel throughout the entire life-cycle exhibits a better ignition enhancement performance with high pulse repetition rates,hence a more stable and effective ignition is able to be achieved by expanding the upper limit of the laser pulse repetition rate.With the progress of high-repetition-rate femtosecond fiber lasers,femtosecond laser ignition is expected to play an important role in practical engineering.3.Research on remote ignition application of energy deposition characteristics in a telescopic filament.We first presented a comparative study on the energy deposition characteristics of the filament under the focusing conditions of the telescope system and the single-lens system by measuring the length,N₂⁺fluorescence intensity distribution and energy deposition efficiency of the filament plasma.It was found that the energy deposition efficiency of the filament plasma can be effectively modulated by changing the distance between the two lenses in a telescope system,and as the effective focal length of the telescope system changes,there exists an optimal plateau region for the average lineic energy deposition in the filament,which differs remarkably from the monotonic change trend of energy deposition with the focal length in a single-lens focusing system.Furthermore,based on the energy deposition characteristics in the telescopic filament,high-intensity plasma filaments were remotely generated by adjusting the distance between the two lenses in a telescope system,and we extended the ignition operation distance of the filament in a premixed lean fuel/air mixture to twice that of a single-lens focusing system under the same incident laser energy condition and effectively decreased the minimum pulse energy for remote ignition.The research results shed new lights on the energy deposition characteristics of filamentation over a long distance under a telescope system,and provide a strategy for the realization of remote femtosecond laser filament ignition.4.Probing dynamics of ultrafast structural changes of multi-size soot particles based on femtosecond laser.We investigated the ultrafast dynamics of soot particles in laminar diffusion flames using a pump-probe approach,in which a strong near-infrared femtosecond laser at 800 nm was used as the pump to excite the soot particles,and multi-color femtosecond laser pulses at the wavelengths of 267 nm,400 nm,and600 nm were used as the Rayleigh scattering probes to detect the excited soot particles.Based on the selectivity of the probe light wavelength on the size of soot particle,the swelling at femtosecond time scale and subsequent shrinking at picosecond time scale of soot particles under the excitation of the pump filament were confirmed by analyzing the intensity variations of the scattered probe laser pulses as a function of the pump-probe delay time.The ultrafast dynamic process is strongly dependent on the initial size of the nanoparticle,that is,the swelling rise time and the shrinking decay time monotonically increase as the initial size of soot particle increases.In addition,by comparing the scattered probe signal intensities at different wavelengths along the flame axis with and without the pump laser excitation,it was found that the two-dimensional spectroscopy in the temporal and spectral domains can better reveal the distribution and dynamics of soot nanoparticles with different sizes in the flame.The research results offer the information on the structural dynamics of soot particle at the ultrafast time scale,providing a viable method for online and in-situ diagnostics of soot dynamics,and revealing that the pump-probe approach holds promising applications in characterizing the ultrafast time-scale evolution of various nanoparticles in the combustion field.