Research On Diode Laser Absorption Spectroscopy For Non-steady Engine Combustion Diagnostics

The new generation of aero-engines?scramjet,aero-turbine engine,pulse detonation engine,etc.?are developing towards wide operation conditions and combustion diversity.The traditional contact measurement method can no longer meet the requirements of on-line monitoring and active combustion control in engine ground and flight tests.TDLAS?Tunable Diode Laser Absorption Spectroscopy?Wavelength Modulation Spectroscopy?WMS-2f?is well-known for its ability to significantly improve sensitivity and noise-rejection capabilities,and can be developed into online monitoring tool for the research of combustion organization and control in the development of high-performance engines,which is currently a hot topic in international research.TDLAS technique is based on the principle of resonance absorption of molecules on the laser transmission path to retrieve gas flow parameters,which can be used to measure the temperature,pressure,velocity,H₂O,O₂,CO₂and concentration of major pollutants in the inlet,interior and outlet combustion flow of the engine combustor.At present,TDLAS technique in atmospheric environment is mature and has developed TDLAT?Tunable Diode Laser Absorption Tomography?technique with two-dimensional spatial resolution measurement capability.However,WMS-2f technique still faces two major challenges in measuring unsteady combustion flow of the engine under wide operation conditions.First,the wide pressure range makes difficult to build an accurate physical model for the combustion parameters inversion.Second,the harsh combustion environment interferes with WMS-2f signal seriously.Therefore,WMS-2f technique needs further improvement and optimization in the aspects of absorption line selection,inversion physical model,key experimental techniques and data processing methods.In order to meet the requirements of on-line monitoring and active combustion control for the new generation engines,this dissertation has carried out WMS-2f technique research in a wide pressure range,established an accurate physical model for combustion parameter inversion and a real-time measurement device.The research results have been successfully applied to on-line measurement of pressure,temperature and water concentration in scramjet and aero-engine combustors.The main contents and conclusions of this dissertation are as follows:1.TDLAS theoretical research work?1?The absorption line selection criteria were designed for the wide operation conditions of engine combustion?pressure from 0.5 atm to30 atm,temperature from 500 K to 3000 K?,and two absorption lines of water at 6873.67 cm^-1and 7450.93 cm^-1were selected as detection targets by using the HITRAN 2012 and HITEMP 2010 spectral databases.?2?The influence of pressure,temperature,gas compositions on WMS-2f signal is analyzed by combining Voigt lineshape function with the principle of modulated absorption spectroscopy.The results show that:first,the width of WMS-2f signal can be monotonically changed with pressure by selecting the appropriate wavelength modulation depth,and it has a weak correlation with temperature and gas compositions.Second,at the same temperature,the intensity of WMS-2f signal decreases sharply with the increase of pressure,and different absorption lines decrease at different rates,which makes difficulty for accurate inversion of gas parameters by WMS-2f signal.?3?The pressure correction model of WMS-2f technique is established,and the inversion method of pressure,temperature and component concentration with explicit analytical calculation formula is given.Compared with the existing model,the pressure correction measurement model has the advantage of fast data processing,and is suitable for developing real-time TDLAS sensors to meet the application requirements of on-line monitoring and active combustion control of engine operation.According to the error transfer analysis,the standard uncertainty of temperature measurement can be controlled within 2.1%.2.TDLAS key experimental techniques research?1?Aiming at the difficulty of accurate measurement of dynamic wavelength of diode laser at high modulation frequency,a wavelength measuring device with high precision and high response rate is developed,the measuring precision can be better than 0.001 cm^-1and the response rate can be better than 25 MHz.A method for accurately measuring the free spectral range?FSR?of etalon by using characteristic absorption lines is proposed,and a wavelength inversion method is established by means of interference peak identification and sine function fitting,thus realizing high-precision measurement of dynamic wavelength and phase at wavelength modulation frequency from 1 k Hz to 500 k Hz.?2?The laser control waveform of time-division multiplexed WMS-2f technique is designed,and the measurement system of time-division multiplexed WMS-2f technique is established.An optical calibration device with high temperature,high pressure and high uniformity has been developed.The highest temperature is 1400 K,the highest pressure is 35 atm,and the temperature non-uniformity in the furnace is less than±3 K.?3?The calibration method of the pressure correction model is established,and the verification experiment and uncertainty evaluation of WMS-2f measurement system are carried out.The maximum temperature relative measurement deviation of the WMS-2f device is 2.5%under atmospheric pressure,and the maximum temperature relative measurement deviation is 3.1%in the pressure range of 1 atm to 12 atm.3.Research on engineering technique for engine applications?1?A small integrated WMS-2f device is developed,which mainly consists of an analog signal generation module,a laser current control module,a laser temperature control module,a laser transmitting and receiving module,a phase-locked detection module,etc.?2?A WMS-2f standard spectrum database under different pressures is established,and a data processing method using standard spectrum to fit the WMS-2f spectrum measured in the engine is proposed,which improves the on-line data processing speed and reduces the noise interference in harsh combustion environment.?3?In order to provide pressure information for CARS temperature measurement technique and facilitate the comparison of temperature measurement results between the two techniques.A collinear measuring device between TDLAS and CARS techniques has been developed to solve the problem that the detection areas of the two techniques are inconsistent.?4?A method of synchronously monitoring the background noise of WMS-2f signal is proposed to evaluate the uncertainties in the application of engine combustion measurements.The measurement precision of the developed device in the scramjet combustor is superior to 4 %.