Research On Hybrid Fs/Ps-CARS Technique For Gas Thermometry Under Confined Space Condition

With the development of laser diagnostics techniques,they have been widely applied in the measuring of different key parameters of combustion.Among these techniques,the coherent Anti-Stokes Raman Scattering?CARS?is an important method for its high precision and high signal to noise ratio,which is widely applied in the measurement of the temperature,concentration,and pressure of the reacting flow and non-reacting flow.With the development of femtosecond laser technique,a lot of attention has been given to the femtosecond CARS for its high repetition and low the interference interference.According to the research background of aero-engine and gas turbine,the thermometry technique with high time and frequency-domain resolution and large dynamic range under confined space condition is highly needed.So the hybrid fs/ps-CARS under confined space condition was selected as the research topic of this dissertation.Because of the sensitive of the femtosecond CARS in time-domain,the time-domain nonlinear optical model of femtosecond CARS was studied.For the absence of the input pulse characteristics influenced by the confined space,the parameters such as pulse delay jitter and pulse chirp were introduced to modify the current model.The effects caused by the confined space window dispersion and turbulent pulse jitter can be fully described by the modified model.Also the Raman transition model of diatomic molecule gas had been studied which was the main measuring target of CARS combustion diagnostics.During the study,the key spectral parameters such as the frequency of ro-vibrational and pure-rotational Raman transition,the Raman cross section,and the Raman linewidth were calculated.Then using the modified model three variants of femtosecond CARS were simulated with the uniform model by only changing the input pulse characteristic parameters.And basing on the simulation code we developed a experimental data fitting program with user interface.Basing on the modified femtosecond model,we studied the influence caused by the confined space affected input pulse features such as the Stokes delay,the pump and Stokes pulse width and chirp,the probe delay,and the probe pulse chirp.The effects of these features on confined condition thermometry were analyzed by generating influenced simulated spectra and fitting with different fitting models.The fitting errors introduced by these features were studied with this method.A second harmonic bandwidth compressor were designed and built,and with this bandwidth compressor we built a pure-rotational hybrid fs/ps CARS experimental plat-form.The influence of confined space condition on thermometry were studied by using this CARS experimental platform.The pump and Stokes pulse chirp and delay jitter in-fluence on thermometry were studied.And also the fitting residuals introduced by probe pulse chirp were researched under different probe delay.These distortion of input pulses usually are caused by window material for the seal of confined space.In this paper,we have given out the directly explanation of the theoretical illustration of the fitting residuals periodical fluctuation with probe delay.And the theoretical illustration was verified by comparing the theoretical and experimental results.Basing on these research,the probe pulse chirp was set as a fitting parameter to solve the inherent fitting errors of N₂confined space CARS spectra in published results.After comprehensively considered the effect of pressure and input pulse chirp,the confined space thermometry was executed in a standard temperature control chamber by using pure-rotational hybrid fs/ps CARS.To verify the validity of the CARS thermometric system under larger temperature range,it was also applied to a McKenna burner and the results showed a good agreement with the TFP measuring results and the publications.