Theoretical Study The Effect Of Temperature On Fine Electronic Spectra Of Combustion Intermediates

The complexity of fossil fuel combustion makes it difficult to build a detailed combustion mechanism.Hence,to identify intermediates produced in combustion will help to build a correct combustion mechanism and understand the combustion characteristics of hydrocarbon fuel.However,online detection of combustion intermediates is limited,for which experimental instruments are not amenable to operating at high temperature（e.g.～2000 K）.Because molecular spectrum is the messenger of molecular structure,its characteristic spectrum reflects the specific structure of molecule,many theoretical methods have developed to simulate electronic spectrum of molecule.Unfortunatly,the above methods without considering influence of temperature that can not accurately reflect the spectral characteristics of intermediates.Based on the Franck-Condon principle,a spectral method including temperature effect was developed and programmed to calculate fine electronic spectra of combustion intermediates,and used to simulate spectra of a series of important combustion intermediates.First,the electronic structure of typical intermediates by hydrocarbon fuel combustion,including free radicals,positive ions and neutral molecules（which have experimental spectra for comparison）,were calculated by Complete active space field（CASSCF）method,and the results were agreed well with the available experimental values.Then,we calculated fine electronic spectra of combustion intermediates at different temperature（such as 0 K,300 K,800 K,1500 K and 2000 K）,analyzed the results in detail,and studied the influence of temperature on spectra.The above results demonstrate that the simulated spectra are in good agreement with the experimental ones,suggesting that the spectral method we developed is reliable.Using the method,the spectra of some combustion intermediates which lack of experimental reports were given.The present work provides theoretical guidance for experimental detection of combustion intermediates and establishment the correct combustion mechanism.The main contents of this paper are as follows:1.As one of the important combustion intermediates,free radicals are the basis of construction combustion mechanism.Simulating fine electronic spectrum of radical is of great significance for combustion diagnosis.The spectra of CN,C₂,OH and CH radicals were simulated by the spectral method incorporating influence of temperature.At low temperature,the simulated spectrum of CN radical was inconsistent with the experimental one,but the simulation reproduced experimental spectrum（which was performed at 1700 K）very well when temperature was 1500 K,which indicates the spectral method is reliable.More importantly,the results showed that the spectral transition peaks increased with the increase of temperature,which reflects the influence of temperature on the spectra of radicals.Furthermore,the combustion temperature can be roughly predicted（the temperature difference between prediction and the experiment is 200 K）.Similar conclusions were obtained in the simulation of C₂,OH and CH radicals.This work provides a method for detecting combustion intermediates across a wide temperature range that can be extended to other intermediates such as macromolecular free radicals,neutral molecules and positive ions.2.Other combustion intermediates such as positive ions and neutral molecules are important to construct combustion mechanism and understand the essence of combustion.Because of the differences in molecular structure and charge between positive ions and neutral molecules and free radicals,the fine electronic spectra of CO⁺,N₂⁺,C₂N,NO₂,OOH and HCO from hydrocarbon fuel were simulated,respectively.It is used to prove the applicability of the method.The simulation showed that the spectral transition peaks of different molecules increased with the increase of temperature,which reflects the effects of temperature on spectra.In the simulation of positive ions（such as CO⁺）,the spectra at low temperature could not reflect the experimental spectrum.When at a certain temperature（like 2000 K）,the spectrum was basically consistent with the experimental spectrum,which indicates the method is appropriate to simulate spectra of positive ions,as well as neutral molecules like HCO.3.In order to expand the fine electronic spectra data of combustion intermediates,the spectra of NH,NO⁺,CH₂,CH₃,C₂H and C₂H₃ were simulated.Among these simulation,the vibrationally resolved electronic spectrum of NO⁺ions reproduced satisfactorily the experimental spectrum with nearly the same peaks at（1-0）215 nm（0-0）226 nm,（0-1）238 nm,（0-2）251 nm and（0-3）268 nm at temperature of 2000 K.And the simulation of NH radical was in good agreement with the experimental one.Meanwhile,the emission spectra of other combustion intermediates,CH₂,CH₃,C₂H and C₂H₃ were also presented and their main transition bands were assigned.