Investigations Of Femtosecond-Laser-Based Diagnostic Techniques In Combustion Science

Laser-based techniques are critical diagnostic tools in combustion science.Nanosecond（ns）laser-based techniques appeared in the 1980s and have been applied to measure some key parameters in combustion flow fields.However,with the increasing requirements for diagnostic efficiency,spatial resolution,measurement dimensionality,and simplicity of the optical system,ns-laser-based techniques suffer from some limits.Currently,ns-laser-based techniques are no longer sufficient to support combustion research adequately.The advent of femtosecond（fs）lasers has provided a new idea for the developments of laser-based combustion diagnostics.Femtosecond laser is the pulsed laser with its duration on the scale of femtoseconds.Due to its short pulse duration,high peak power,low pulse energy,and broad bandwidth,fs laser shows unique advantages in combustion diagnostics.In this dissertation,based on the advantages of fs lasers and a fundamental physical issue that the interaction between fs-laser photons and electrons,atoms,molecules,radicals,and other components in gaseous flow fields,we developed a series of novel fs-laser-based techniques for measuring key parameters,such as species,mixture fraction,velocity,and temperature.The main contents are as follows.For species measurements,to break the bottlenecks of the existing ns-laser-based techniques and the research gap,the present work takes NH₃ measurements as an example to investigate the fs-laser-based techniques.The work can be divided into two parts:a resonant excitation method and a non-resonant excitation method.For the resonant excitation method,ns two-photon laser-induced fluorescence（TPLIF）has low excitation efficiency.In this part,the fs-TPLIF technique was expanded to NH₃measurements in combustion flow fields.Compared with ns-TPLIF,the excitation efficiency of fs-TPLIF can be increased by two orders of magnitude.For the non-resonant excitation method,there are few studies about non-resonant fs-laser-based techniques for polyatomic gaseous molecules measurements.In this part,a technique named femtosecond laser-induced plasma spectroscopy（FLIPS）was developed,which fills this research gap.In this part of the work,we utilized the characteristics of the high peak power and the low pulse energy of the fs laser to extend the application scope of the resonant fs-TPLIF technique,and after that,we proposed a novel non-resonant fs-laser-based technique for species measurements.For mixture fraction measurements,the most commonly used ns-laser-based technique is laser-induced breakdown spectroscopy（ns-LIBS）.However,in general,ns-LIBS can only achieve one-point measurements with a spatial resolution on the scale of millimeters.In this part,femtosecond laser-induced plasma spectroscopy（FLIPS）was developed.An fs laser was used to generate a one-dimensional plasma channel with a uniform intensity distribution in the flow field.By measuring spatially resolved spectra and quantitative analysis,the mixture fraction was calibrated with the spectral intensity ratio of the plasma emissions.Compared with ns-LIBS,FLIPS features the ability of one-dimensional measurements and can improve the spatial resolution by one order of magnitude.In this part of the work,we utilized the characteristics of the short pulse duration and the high peak power of the fs laser to propose a promising fs-laser-based technique for mixture fraction measurements.For velocity measurements,particle imaging velocimetry（PIV）and ns-laser molecular tagging velocimetry（ns-MTV）interfere with strong laser stray light in near-wall flow fields.Femtosecond laser electronic excitation tagging（FLEET）is one of the most advanced molecular tagging velocimetric techniques in gaseous flow fields.In this part,based on systematic investigations of FLEET,a novel velocimetric technique with cyano（CN）as tracer molecules was proposed,which is named femtosecond laser-induced CN chemiluminescence（FLICC）.Through experimental validation,FLICC was found to be an ideal technique for near-wall velocity measurements.In this part of the work,we utilized the characteristic of the high peak power of the fs laser to systematically investigate an existing velocimetric technique,and we proposed a novel velocimetric technique.For temperature measurements,coherent anti-Stokes Raman scattering（CARS）is a golden-standard thermometric technique.However,CARS has a relatively complex optical system and generally can only achieve one-point measurements.In this part,femtosecond two-photon laser-induced fluorescence（fs-TPLIF）with CO as indicator molecules was demonstrated to achieve temperature measurements in combustion flow fields.The conventional bands and the hot vibrational bands of CO molecules can be simultaneously excited by one fs-laser beam.As a result,the temperature-dependent Boltzmann distribution can be obtained from the relative fluorescence intensity related to different ro-vibrational states,and the temperature can be extracted from fluorescence spectral analysis.CO-fs-TPLIF thermometric technique can achieve one-dimensional temperature measurements with only one fs-laser beam.In this part of the work,we utilized the characteristic of the broad bandwidth of the fs laser to propose an fs-laser-based thermometric technique with a simple optical system.In summary,this dissertation focuses on the measurements of some key parameters in combustion flow fields.Taking advantage of the characteristics of the fs laser,a series of fs-laser-based techniques were investigated and developed to break the technical limits of the current ns-laser-based techniques.The fs-laser-based techniques can provide more efficient and reliable technical support for the research in combustion science.