Research On Gas Temperature Measurement Based On Laser Induced Thermal Grating Spectroscopy

As one of the most important parameters in various thermodynamic processes,temperature is the most intuitive description of the combustion process.Accurate gas temperature measurement is necessary for the optimal design and model validation of low emission internal combustion engines,gas turbines and aero-engines.However,high temperature,high pressure and complex flow field condition make the measurement of gas temperature in these occasions particularly difficult.Laser Induced Thermal Grating Spectroscopy,as a non-contact optical measurement technology,can be used to solve the problem with high precision,high pressure resistance and non-contact.Therefore,in this thesis,the laser induced thermal grating spectroscopy is researched as followed.Based on the four-wave mixing of laser and gaseous hydromechanics theory,the basic theory of laser induced thermal grating spectroscopy is analyzed.NO₂ is selected as the excitation gas.the Nd:YAG laser at 532nm is used as the pumping laser and the 671nm continuous laser is used as the detection laser.A laser induced thermal grating spectroscopy system is established for temperature measurement.The phase matching issue and beam focusing adjustment are well addressed to ensure generation of stable and high signal-to-noise-ratio（SNR）signals.Various calibration methods of grating spacing are proposed.Based on the theoretical model,the LITGS signals under different working conditions are simulated.Various data processing algorithms such as Fourier transform method are used to process the analog signals and compare the advantages,disadvantages and applicable scenarios of different algorithms.Finally,a software program matching the experimental system is developed.The speed of sound of mixed gases is measured by LITGS system.It is found that the measurement results agree well with the theoretical calculation under the same condition,with deviation values less than 2%.Meanwhile,the temperatures of NO₂/air mixture gas in pressure vessels are investigated.The results show high consistency with the one conducted by thermocouple measurement,with deviation values less than 3%and standard deviation of 3K,when gaseous temperature vary from 290K to 450K under standard atmosphere pressure.Furthermore,gaseous pressure is found to have a large degree of effect on raw signal features,measurement accuracy and measurement uncertainty.When the gaseous pressure increase in the range 0.4 bar to 3bar,the signal duration increase significantly.Moreover,the measurement accuracy and standard deviation are improved,while uncertainty is greatly suppressed.It is preliminarily proved that LITGS technology has better performance in high-pressure conditions.In this thesis,the laser induced thermal grating technology,a photoacoustic method for measuring gas temperature and sound velocity,is investigated.Through the research to this technology,it is preliminarily proved that this technology has the potential of gas temperature measurement under high temperature and high pressure conditions,and is of great significance to the high-precision measurement of gas temperature in internal flow field of internal combustion engine,gas turbine and aero-engine.