Theory And Application Research Of Gas Parameter Diagnostic Based On Self-absorbed Emission Spectroscopy

Recently,accurate measurement of gas temperature and species concentration is one of the key issues in the field of combustions and plasmas.Although the molecular optical emission spectrum（OES）is regarded as the most direct and effective technique due to its simple operation and high tolerance on environment,it still faces some problems in practical applications.To solve these problems,a self-absorbed OES model,which contains the information of the excited states,ground state and the concentration of molecules,was proposed in this thesis.Further,inspired by the proposed model,a novel method was established to simultaneously characterize the gas temperature and concentration.This method has the advantages of wide application scope and easy operation.The main research contents are summarized as follows:Theoretical research:The emission light intensity and absorption coefficient were first calculated based on the Einstein radiation theory.In combination with the Beer-Lambert law,a model to calculate the absorption spectrum with a structured light source was then proposed to quantify the spectral coupling between emission and absorption processes.Meanwhile,the temporal and spatial coupling between emission and absorption was analyzed using the theory of radiative transfer.Finally,a molecular self-absorbed OES model,which is suitable for practical applications,was established.In addition,based on this model,two methods were proposed to quantify the gas temperature and concentration according the specific spectral structures.To further address the non-equilibrium phenomena in the excited state,a superposition of two Boltzmann distributions was applied to describe the population in the excited state.Wiht this imporved self-absorbed OES model,information from both exicited and ground states can be inferred simultaneously.Experimental research:（1）The optical emission spectrum of CO?ngstr?m（B¹Σ⁺→A¹Π）system（1,0）transition was calculated and was used to determine the gas temperature in the plasma assisted CO₂ reforming experiment.The measured temperatures were in accordance with the results from thermography,which validated the emission light intensity model and laid the foundation for the molecular self-absorption OES model.（2）The NO self-absorbed OES in the plasma assisted denitrification experiment were analyzed based on the structured-light-source absorption spectrum,which enables a calibration-free measurement of NO concentration using the characteristic peaks.The effect of collision coefficient and rotational temperature on the accuracy of NO concentration was further discussed as well.By comparing with the given NO concentrations,the correctness of structured-light-source absorption spectrum was confirmed.In addition,real-time monitoring of NO concentration in the plasma assisted denitrification was conducted through the proposed method.（3）The self-absorbed OES of OH radical in an atmospheric pressure He-H₂O RF discharge was calculated with the proposed model.By collecting ultra-high resolution spectra and eliminating the non-equilibrium spectral lines,the experimental spectra with low-rotational（J’≤6.5）equilibrium lines were fitted.The OH radical concentration and gas temperature were simultaneously inferred.Finally,with the improved self-absorbed OES model,the formation mechanism of the OH（A）excited state in H₂O-containing plasma was studied with the proposed method,which could provide experimental verification for the theoretical reaction paths.