Research On The Diagnosis Of Complex Flow Field Using Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy(TDLAS)technique can be utilized to measure multiple flow parameters including gas temperature,species'concentration and flow velocity simultaneously.It has been extensively used in flow diagnosis due to its merits of non-invasion,fast response,high sensitivity and good stability.The combination of TDLAS and computer tomography can overcome the disadvantage of TDLAS measurement of being light of sight measurement,and can be used to measure the two dimensional distribution of flow parameters.This dissertation focuses on the diagnosis of complex flow field such as high speed flows and complex combustion flows using tunable diode laser absorption spectroscopy.Unsteady water vapor condensation phenomena in high speed expansion flow are investigated using laser absorption spectroscopy technique.In specific,unsteady condensation process under different expansion time scales and initial content of water vapor in carrier gas are analyzed.Numerical simulations are carried out to study the tomographic algorithms used in tunable diode laser absorption tomography(TDLAT).In the meantime,measurement of two dimensional distribution of gas temperature and water vapor concentration are conducted within the combustion flow filed generated by a flat flame burner to verify the effectiveness of the tomographic algorithms established before.The main works of this dissertation are as follows:(1)Unsteady water vapor condensation in high speed expansion flow is investigated using TDLAS technique.We proposed a novel and effective experimental method and designed a convenient expansion system,in which TDLAS measurement setup and light setup are integrated.Two near infrared water vapor absorption transitions and a near infrared methane absorption transition are used to monitor the condensation phenomenon under different expansion time scales and different initial content of water vapor in the carrier gas.The light sheet results visually reveal the condensation phenomenon while TDLAS measurements quantitatively follow the transient gas temperature and water vapor concentration during the expansion.Computational Fluid Dynamics(CFD)simulations are carried out to verify the results of TDLAS measurement and to analyze the influence of temperature gradient in the optical path.(2)Two different kinds of tomographic algorithms are established with the help of Matlab.Numerical simulations are carried out to study the influence of beam arrangement and number of transitions on the reconstruction quality of these two tomographic algorithms.Noise tolerance of these two algorithms is also investigated in noise perturbed environment.Further,the measurement noise in TDLAS measurement is analyzed by numerical works and the noise restraining mechanisms of direct absorption spectroscopy and wavelength modulation spectroscopy are investigated.Further,tomographic method based on wavelength modulation spectroscopy is established with the help of Matlab.Numerical simulations are carried out under different level of measurement noise to evaluate the influence of measurement noise on the reconstruction quality of both direct absorption spectroscopy based and wavelength modulation spectroscopy based tomographic methods.Greater noise immunity is found in wavelength modulation spectroscopy based tomographic method.Further,the reconstruction quality of linear tomographic method and nonlinear tomographic method are investigated in complex flow field with limited optical beams.It turned out that the nonlinear tomographic method outperformed the linear tomographic method with limited optical beams.(3)An experimental system for measurements of two dimensional temperature and water vapor concentration distributions are designed with a Mckenna flat flame burner.Two near infrared water vapor absorption transitions near 1392nm and 1395nm along with linear tomographic method are used in the experiments for the measurements of the 2D temperature and water vapor distributions under different combustion conditions.The temperature distributions derived by spectroscopic method are compared with those measured by thermocouple to evaluate the measurement accuracy of laser absorption tomographic method.In both combustion conditions,the maximum deviation between results measured by laser absorption tomographic method and results by thermocouple are less than 10%.In the meantime,the influence of beam arrangements are investigated with different beam arrangement ways,it turned out that the more the number of beam the better the reconstruction quality,which is the same with the simulation findings.A novel method for flow parameter solving in wavelength modulation spectroscopy is proposed.The verification experiment under atmospheric environment is carried out to verify the validity of the proposed method.Two dimensional distribution of gas temperature is measured with wavelength modulation spectroscopy based tomographic method in the flat flame experimental system.The maximum deviation between thermocouple measured result and result by wavelength modulation spectroscopy based tomographic method is 5.9%,which is much less than those with direct absorption spectroscopy tomographic method under the same combustion condition.