Measurements Of Soot Volume Fraction,Temperature And Gas Concentration Using Laser Absorption Spectroscopy In Flames

Combustion is a very complex process which consists of multi-dimensional interaction between fluid mechanics and chemical kinetics.Measurements of the key parameters such as temperature,gas concentration,and soot volume fraction with high accuracy and precision during combustion process is of great importance for improving combustion efficiency and reducing the production of pollutants,as well as futher understanding of combustion mechanisms.The purpose of this paper is to develop a method for simultaneous measurements of these multiple key parameters based on absorption spectroscopy,realizing the simulataneous reconstruction of the distribution of these parameters in combustion environment,and applying this new method for the study of chemical mechanisms of soot formation.Firstly,the fundamental of absorption spectroscopy was introduced in details.The line-shape functions,as the key spectral parameters,were well classified as Gaussian line-shape function,Lorentzian line-shape function and Voigt line-shape function.The characteristics of each line-shape function was discussed in details.Furthermore,three different techniques based on absorption spectroscopy,i.e.fixed-wavelength direct absorption spectroscopy,scanned-wavelength direct absorption spectroscopy,and modulation spectroscopy,were also well introduced.Meanwhile,the Mie scattering theory tegother with Rayleigh assumption were also introduced in details which were widely applied in particle diagnostics using optical techniques.Simultaneously sensing of CO₂ concentration and temperature in high temperature environment was realized by using one tunable diode laser working around 2 ?m.Absorption lines aroud 4996 cm-1 were studied and selected as the target detecting lines.The CO₂ concentration and temperature of CO₂/N2 mixed gas in gas cell at different temperature with different CO₂ content were measured.The measured values were very close to the setted values.The average deviation and peak deviation of the temperature values were 2.07%and 3.9%.The average deviation and peak deviation of the concentration values were 2.25%and 4.75%.Measurements were also taken in ethylene/air premixed flames,the average deviation and peak deviation of temperature between measured values and thermocouple values were 0.6%and 3.9%.And the measured values of CO₂ concentration in flames at different equivalence ratio were very close to the theoretical values.The selected absorption lines and measuring method were proved to be suitable for CO₂ measurement at high temperature environment with high accuracy.A noval method and application of simultaneously measuring soot volume fraction,H₂O concentration and temperature based on absorption spectroscopy by using one diode laser was proposed for the first time.The fundamental theory and the derivation process for obtaining and distinguishing the gas absorption coefficient and soot extinction coefficient were introduced in details.The measurements were taken in ethylene/air premixed flames at different conditions.The measured temperature values were very close to the thermocouple values with an average deviation less than 3%,and peak deviation less than 5%.The measured soot volume fractions and its deviation tendency at differenct positions in the flame were very similar to the literature values.The measured H₂O concentrations were close to the theoretical values with an average deviation less than 5%and peak deviation less than 10%.With the proposed method aforementioned,multiple paraemeters i.e.temperature,soot volume fraction,H₂O concentration,CO concentration and CO₂ concentration in ethylene/air diffusion flames produced on a modified Wolfhard-Parker slot burner were measured using three diode laser working at 1398.3 nm,2001.6 nn and 2302 nm.The H₂O and CO₂ were added into the air stream respectively in order to investigate the effections on soot formation process in flames and the chemical kinetics.There came a conclusion that both H₂O and CO₂ strongly suppressed the formation of soot by increasing the OH content which would decrease the PAHs concentration which were the precursors of soot during nucleation process.The method for multiple parameters measurement was combined with tomography technique to realize the simultaneous reconstruction of soot volume fraction,H₂O concentration and temperature distributions in flames.The basic theory tegother with the tomographic reconstruction model were introduced in details.Experimental reconstruction systems were well designed for in-situ measurement.The temperature and H₂O distribution in methane/air premixed flames were measured with high accuracy by using one diode laser.Measurements were also taken in axisymmetric ethylene/air diffusion flames.The reconstruction model as well as the least squares QR decomposition?LSQR?method were introduced.The distributions of soot volume fraction,H₂O concentration and temperature were simultaneously reconstructed.The reconstruction method shown great potential in combustion diagnostics.