Studies On Laser Spectroscopy Technology And Its Application To Combustion Diagnosis In The Jet Engine

For the purpose of the temperature and species concentration measurement, Coherent Anti-Stokes Raman Spectroscopy (CARS), Laser-Induced Fluorescence Spectroscopy (LIFS) and Planar Laser-Induced Fluorescence (PLIF) were studied systemically. Emphases of these studies were put on the single pulse CARS measurement, the accuracy of CARS and the rotational temperature measurement using LIFS. On the basis of above studies, laser spectroscopy technologies were applied to supersonic combustion diagnosis and research on combustion in the rocket engine. And then, mechanisms of the formation of flame structure in supersonic combustion, influence of cavities and so on were discussed.After the theory of CARS and methods of theoretical simulation of CARS spectrum were summarized. The experimental measurements were studied and improved. After setups of some key parameters of CARS system have been optimized, the single pulse measurement of temperature of CARS was realized. Results of temperature measurement using single pulse and multi-pulse mode nitrogen Q-branch CARS were then studied in detail. The accuracy of measurements was discussed as an emphasis. Discrepancy between temperature measured respectively by thermocouple and CARS in a plane flame was discussed. It was found that accuracy of temperature measurements of CARS in flames is probably influenced by the state of thermal equilibrium in area where reaction is still going on. This is a new theory hypothesis of CARS measurements.It was studied that the method of temperature measurement using LIFS. It was proposed that a new method based on that double lines were excited by means of laser wavelength scanning and intensity of broad band fluorescence was expressed with spectral integral. The measurement principle of this method was deduced from basic theory of Laser-Induced Fluorescence (LIF). The rotational temperature of OH in the plane flame was measured by LIFS after line Q1(5) and Q1(8) of transition A2∑ ← X2 (?) (1, 0) were excited. The LIFS measurement supported the hypothesis about that the accuracy of CARS measurements could be influenced by the thermal non-equilibrium state of the flame.The PLIF technology to measure the distribution of OH by means of the single transition being excited was studied. After Q1(8) of the transition A2∑← X2 (?) (1 , 0) of OH had been excited, PLIF images of OH distribution in flames were got. While such PLIF images of OH in several kinds of typical flames were compared to the spontaneous emission images of radicals,flame structures of these typical flames were analyzed.The OH PLIF technology was then used to study the flame structure of H2/air supersonic combustion. Those important factors that maybe influence the combustion performance were analyzed. Especially, it was concluded that the turbulent diffusion is the most significant process of this supersonic combustion. While the flame structure was analyzed, comparison was done between the PLIF image and the CFD result. It is found that simulation using the 8 equations model of hydrogen and oxygen reaction and the standard double equations turbulent model can describe the main topologic structure of supersonic flame. However, it can not describe the detail and instantaneous structure of turbulent flame.The mechanism of the cavity to improve ignition performance and to hold the flame in the supersonic combustion flow was studied using OH PLIF technology. It is believed that the relation between the location of the cavity being installed and the location hydrogen being sprayed will probably influence the performance of cavity to ignite the flame. And also, it is believed that cavities hold the flame by means of "igniting" the main flow continuously. The influenc of the ratio of length to depth and obliquity of back edge of the cavity were studied. It is found that the flame structure is sensitive to the obliquity of back edge of the cavity. Influenc of the scheme of fuel injection and the disturbance on flow were analyzed with OH PLIF images. The results of analyses showed that disturbance on shear layer near cavity can effectively change mixing and diffusing process of the fuel, and then improve combustion. Research was conducted on alcohol/air supersonic combustion ignited by the hydrogen flame. Impacts upon ignition and forming of flame structures of alcohol injection, evaporation and so on were studied.Temporally-resolved temperature measurements were carried out in a H2/air supersonic combustion using the single pulse N2 Q-branch CARS. In comparison with the measurement results in the stable flame, temperature in the supersonic flame is obviously fluctuating. The measurements also manifested that temperature is not equivalent spatially.By means of the comparison being done among OH PLIF images, spontaneous emission images and CFD results, distribution of OH radicals and temperature of plume were studied. PLIF images showed that processes of the hydrogen/oxygen combustion and the hydrogen/-oxygen/alcohol combustion are under the different controlling mechanism respectively. The intensive spontaneous emission of flames in the rocket engine makes it possible to diagnose combustion using the spontaneous emission. At the mean time, flame structures of the combustion of the different combination of kerosene, alcohol, oxygen and hydrogen were studied using the ultraviolet spontaneous emission. The feasibility of CARS measurements was demonstrated in a model tri-propellant rocket engine. Measurement results of nitrogen Q-branch CARS showed that temperature and species concentration will be affected by the hydrogen massflowrate in the model tri-propellant rocket engine.The work of this paper can be divided into two parts: studies of laser spectroscopy technologies and studies of application of these technologies to combustion diagnosis in a jet engine. Both parts of work concluded some innovative results, especially in researches on optimization of CARS measurements, CARS measurement accuracy, temperature measurements using LIFS, mechanism of the supersonic combustion, and so on.