Investigation On Flowfield Of Combustion Driven CW DF/HF Chemical Laser Employing HYLTE Nozzle

The DF/HF chemical laser is attractive because of its high output power, good wavelength, excellent performance in magnification, etc, the parameters of the flowfield in the cavity are very important to the laser's capability. In this dissertation, the experimental, simulative and theoretical investigations are carried out to study the flowfield of HYLTE nozzle which is widely used in this type of laser.The reactant mixing rate has a strong influence upon the chemical efficiency, and laser induced iodine fluorescence (LIIF) is a very good method to study it. In order to optimize LIIF, the relation between the fluorescence intensity and the wavelength of the exciting laser is theoretically and experimentally investigated. It is concluded that the 532nm solid laser is a good exciting laser for LIIF in some conditions. The fluorescence intensity excited by 514.5nm laser with different power is measured, and the dependence of fluorescence on the power of exciting laser is obtained.Although the LIIF has been used to visualize the flowfield in DF/HF chemical laser for a long time, but until now, the precision of this method hasn't been reported. The flowfield in DF chemical laser and that in the LIIF experiment are simulated through the CFD software FLUENT. The two flowfields are analyzed, and the conclusion is deduced that when the massfraction of iodine is not too high, it is nice to use LIIF in studying the mixing performance of the HYLTE nozzle employed by DF/HF chemical laser.The pressure at the nozzle's inlet affects the mixing condition of the flowfield, but in one HYLTE nozzle blade, the pressure is nonuniform at the secondary nozzles' inlets. The static and total pressure of the flowfield in a pipeline which has ten branches are measured. The result is that along the direction of the flow, the static pressure increases while the total pressure decreases. The mass flowrate coefficient of different gases (D2, H2, He) in different Laval nozzles is computed, the result shows it is mostly affected by the boundary layer. The coefficient increases when the pressure at the inlet augmenting, it increases first and then decreases while the nozzle's throat height increasing. A formula is given to indicate the relation among the mass flowrate coefficient, the pressure at the nozzle's inlet and the throat's height.The whole system that used LIIF to measure the mixing performance of HYLTE nozzle is designed. From the experiment the following results can be concluded: the fuel streams can penetrate through the oxidant flow in the symmetry plane of the secondary nozzles, its diffusion is not obvious, the shapes of fuel nozzles have been changed because the laser has been used for a long time; several fuel nozzles are filled, the point at which the fuel and oxidant mix completely moves downstream when the pressure at the fuel nozzles' inlets decreases. Based on this experiment, the application of LIIF in nozzle's development can be explored.A device is designed to investigate the condition of nozzles. It can be used to obtain the configuration of the nozzle and the distribution of the pressure at the nozzle's inlet. The flowfield in our experiment is simulated by CFD software FLUENT, and the simulation result is very close to the experiment result. The fluorescence is analyzed in detail, and it is concluded that the distortion of the interface between the reactants and the crossflow vortex are the main means to accelerate the mixing rate in HYLTE nozzle.The heat released from the reaction of the oxidant and the fuel makes the flow expand transversely, thus the mixing performance in cold flowfield is different from that in reactive flowfield. Chemiluminescence is a good method to study the reactive flowfield, and it is used to investigate the mixing performance of oxidant and fuel when the reaction is going on in the CW HF chemical laser. The result shows that the two streams begin to mix at the nozzle exit plane, at the place where the lasing axis locates, the two fuel streams which border upon each other almost meet together. The flowfield in the experiment is simulated by the CFD software FLUENT, its results are qualitatively consistent with those of measured.The theory that using LIIF to measure the velocity, temperature and pressure of the flowfield is studied systematically. The values and the distributions of these three parameters of the flowfields in the DF chemical laser and in the experiment are compared, The result shows that the LIIF can be used to measure the velocity and pressure, but can't be used to measure the temperature in the DF chemical laser.