Study On Concentration Distribution Measurement Based On Ambient Gas Traced Laser Induced Fluorescence

In-cylinder direct injection technology can improve engine thermal efficiency by increasing the compression ratio and reducing the pumping loss,and has great advantages in reducing the emission of pollutants such as nitrogen oxides.Concentration distribution information of stratified mixture is very helpful to research the DI engine.Laser-induced fluorescence(LIF)is used to mesure the concentration distribution of the fuel.This thesis focuses on the research work of ambient gas traced laser-induced fluorescence(LIFA).In conventional LIF measurement with tracer added into fuels,selection of tracer is limited by its traceability including similar boiling point and diffusion coefficient etc.compared with targeted fuels,and pre-calibration between the LIF signals and the concentration must be done to obtain a quasi-quantitative information,limiting its application from wider fuel range.In this study,acetone is used as the tracer and it is introduced into the ambient gas.With accurate control of the mass fraction of acetone in the ambient gas,the on-site calibration between the LIF signals and acetone concentration can be implemented.Subsequently,when fuel is issued into the ambient,the quantitative fuel concentration can be obtained from the LIF signals of acetone,regardless of fuel properties.Besides,based on the LIFA method,the concentration mesurement system is designed to meet various conditions,including the injection system,the optical system and the synchronization system.With the help of this system,the 2-D image can be aqquired in different time after start of injection to describe the structure of instantaneous flow at different conditions.Finally,as a preliminary study,nitrogen as a representative of gas fuels is injected into the ambient of acetone and nitrogen by a high-pressure injector with a hole diameter of 0.2 mm.The fluorescence of the tracer is excited by the 266 nm light from the 4th harmonic of an Nd: YAG laser and recorded by an ICCD camera.The deduction of the fuel concentration from the tracer signals is described in detail,the on-site calibration processes and uncertainties are also discussed.Finally,the mixture formation processes of the nitrogen gas jets with different injection pressures are visualized and quantified,which are compared to argon jets to show the LIFA abilities.