Numerical Simulation And Experimental Investigation Of The Mixing And Reacting Mechanisms Associated With The Transverse Injection In HYLTE Nozzle

Hypersonic low temperature (HYLTE) nozzle is an advanced nozzle for high power chemical laser, which has great influences on gas transporting, mixing and reacting. Many complicated processes are found to exist in this nozzle and contiguous cavity, including gas dynamics, nonequilibrium chemical kinetics, and laser physics of gain mediums. It is important to investigate into these key processes systematically. Using a combination of theoretical analysis, numerical simulation and experimental observation, the mechanism of mixing, reacting and radiating processes in the coupled section of HYLTE nozzle and cavity were studied.The numerical simulation program suitable for the nozzle and optical cavity was developed, mainly based on the method of hybrid RANS/LES. Utilizing experimental techniques and numerical methods, the flow patterns and mixing characteristics of supersonic angled jets into a supersonic crossflow were investigated and analyzed comprehensively. The changing tendencies of the mixing characteristics with the altering of secondary injection angle, secondary orifice separation and injection stagnation pressure were examined. In order to enhance the mixing effect of multiple oblique injections in HYLTE nozzle, the penetration depth, the contacting area of jets and crossflow, the total pressure loss, the mixing length and the local pressure matching should be considered simultaneously.The forepart of HYLTE oxidant nozzle was simulated numerically with the purpose of obtaining the inlet flow parameters of the coupled section. The influences of the factors including the inlet stagnation pressure, the throat width, the expanding angle of supersonic section and the recombination effect of F atom on the thickness of the boundary layer and the homogeneity of the gas flow were analyzed.The three-dimensional unsteady mixing flowfields generated by supersonic transverse injections in HYLTE nozzle with several typical configurations were studied. Results showed that streamwise vortices, contacting surface distorting and stretching dominated and accelerated mixing. Geometric parameters, such as secondary injection angle, tandem distance and parallel distance affecting the penetration depth, the mixing efficiency and the total pressure loss were investigated. The strength of the counter rotating pairs of vortices, and as a result, the reactant surface entrainment increased with increasing injection angle. At the same time, the injectant penetration and mixing rate increased, which was consistent with the increase in total pressure loss. Neither the narrow nor the broad tandem/parallel distance would improve the mixing efficiency and pressure recovery coefficient. The parameter which made an obvious impact on HYLTE nozzle mixing performance was the injection angle of secondary nozzles.Based on the detailed DF chemical kinetic models of two different reactant systems, the relevant simplified chemical kinetic mechanisms were presented. The simplification affecting the calculation of the combustion temperature, the thermodynamic parameters, the DF mole fractions, the small signal gain and the refractive index were presented. Compared with the detailed mechanism, the simplified mechanism described the general DF combustion process effectively, and at the same time elevated computational efficiency to a great extent, therefore it was more suitable to couple with three-dimensional flowfield numerical simulation code or software.The reacting flowfields under three different injection angles were simulated with the simplified chemical kinetic model and hybrid RANS/LES method. Results showed that the strength of the streamwise vorticities was reduced as a result of heat release, but the existing of counter rotating pairs of vortices would distort the contacting surface and enhance the mixing of fuel and oxidant, and therefore accelerate the reacting process and the production of gain medium. Combusting flowfields could induce low-frequency pressure instability and higher total pressure losses in the cavity. Moreover, the small signal gain coefficient distributed in a narrow center region in the cavity. As the injection angle increased, the high gain zone appeared to be shorter and closer to the nozzle exit plane. Generally, larger injected angle was associated with greater total pressure loss and nonuniformity of the gas flow. The high gain zone distribution was appropriate and its length reached a maximum with the injection angle of 40 degree, which could reduce the lasing load of cavity mirrors efficiently.Geometric optical model was coupled to the reacting flowfield computation, the variation of flow and optical fields from the establishment to the stabilization status in supersonic DF chemical laser were simulated. It was observed that laser radiation generally had dominant effects only on the concentrations of the lasing species, and it had relatively minor effects on the basic fluid dynamic parameters. For the case without lasing, the complete population inversion phenomena could be found in wider range, which did not occur for lasing. The lasing output was based on the partial population inversion of the vibration- rotation transition in DF molecules.A kind of supersonic DF-CO₂ transfer chemical laser based on HYLTE nozzle was presented. The variations in the reacting flowfield and gain performance under different chemical kinetic models, secondary species ratios and operation parameters were obtained. Results showed that the simplification of kinetic model lent itself to easy physical interpretation and permitted efficient and accurate prediction of performance characteristics. Reasonable choices of secondary species ratios and operation parameters could acquire ideal small signal gain uniformly distributed.