Research On CO₂ Concentration Measurement Technology Of Inhaled Composite Engine Combustion Flow Field Based On TDLAS

The wide-speed-range air-breathing combined cycle engine is a significant strategic requirement for future aerospace integrated operations.During high and wide Mach number flight,the combined engine encounters various thermal-physical problems under extreme conditions such as high temperature,high pressure,high velocity,and high-intensity combustion.The control of these distributed parameters fundamentally relies on the control of combustion modes,as the safety and thrust performance of autonomous flight strongly depend on the control of combustion modes.Therefore,the precise identification and real-time closed-loop control of the combustion modes in air-breathing combined cycle engines have become urgent key bottlenecks to overcome.Combustion modes are essentially the intrinsic manifestations of engine operation,with the core being the coupling of temperature field,pressure field,material field,composition,and other characteristics.During combustion in the engine combustion chamber,a large amount of gases such as H₂O,CO,CO₂,and NO are generated.The variation in gas component concentrations resulting from combustion can serve as an important indicator for identifying engine combustion modes.Therefore,the perception of concentration changes in the engine combustion chamber holds significant military requirements and strategic significance for the precise identification of combustion modes.Currently,there are two methods for gas concentration measurement:contact gas sensors and non-contact optical measurement.When the air-breathing combined cycle engine is in combustion,conventional gas sensors are susceptible to interference from other gases,which affects the detection accuracy.Additionally,gas sensors intrude into the combustion flow field,leading to measurement errors.Moreover,sensor measurements are point measurements and cannot reflect the internal gas distribution in the combustion chamber.Most importantly,the extreme and complex environment inside the air-breathing combined cycle engine’s combustion chamber,characterized by high temperature,high pressure,diverse combustion products,and spatiotemporal transients,exceeds the working tolerance range of gas sensors,rendering them unable to function properly.Non-contact optical measurement methods include nondispersive infrared detection technology（NDIR）,differential optical absorption spectroscopy（DOAS）,Fourier transform infrared spectroscopy（FTIR）,differential absorption lidar technology（DIAL）,and tunable diode laser absorption spectroscopy（TDLAS）.Among these non-contact optical measurement methods,TDLAS offers advantages such as rapid response,high accuracy,and online monitoring capability.Based on this,the present study selects TDLAS technology for measuring CO₂ concentration inside the air-breathing combined cycle engine’s combustion chamber."In response to the difficulties in gas concentration detection in the high-temperature,high-pressure,and extremely complex environment of the air-breathing combined cycle engine’s combustion chamber,as well as the limited relevance of high-end diagnostic instruments and delayed response,this study conducts research on CO₂ concentration measurement techniques in the combustion flow field of the air-breathing combined cycle engine based on TDLAS.Firstly,the basic principles of TDLAS measurement technology are introduced,followed by a detailed discussion on the direct absorption method and wavelength modulation method in TDLAS measurement technology,with a thorough analysis and comparison of their advantages and disadvantages.Subsequently,the air-breathing combined cycle engine’s combustion chamber is simulated using Simulink simulation software.By utilizing the simulation model,the impact of changing parameters such as ambient temperature,ambient pressure,initial laser intensity,and optical path length on gas absorption curves and the resulting concentration measurement values are explored.Finally,an experimental test platform for tunable diode laser absorption spectroscopy is constructed,and concentration measurements of N₂ and CO₂ gas mixtures with different concentrations in the gas chamber are conducted using the direct absorption method and wavelength modulation method.Furthermore,by using a vibration motor,the vibration environment during the combustion of the air-breathing combined cycle engine is simulated,and the effects of changes in ambient pressure and vibration on CO₂ gas measurement are further investigated.The experimental results demonstrate an overall error control within 8%,confirming the feasibility of applying TDLAS technology for internal measurements in the combustion flow field of the air-breathing combined cycle engine and laying the foundation for subsequent research on the identification of combustion mode transitions in the air-breathing combined cycle engine.