Research On Propulsion Mechanism And Performance Optimization Of Polymer Ablation By Pulsed Laser Irradiation

This dissertation is laid in the background of nano and pico satellite launchings with laser propulsion; and concentrates on the pivotal phenomena and problems in the process of laser propulsion, such as polymer pyrolysis, recoil effect of the ablation products, generation and propagation of the laser supported combustion and detonation waves, denudation of the metal doped polymer and development of the plume adjusted by metal grid. Using numerical simulation and theoretical analysis as the mainly research techniques, assisting with experimental verification, the propulsion mechanism and performance optimization are investigated in this dissertation.A slicing response mass ablation model is carried out to describe the mass loss process of the working substance irradiated by pulsed laser, and an analytic method is set up to calculate the propulsion performance of laser ablation propulsion controlled by the process of ablation. By comparing the predictions with the experimental data, the accuracy and applicability of the analytical model are checked. The dependences of the LAP performance on laser intensity, laser pulse width, absorption coefficient, thermal conductivity and specific ablation energy are also discussed are obtained.The impulse generation model of polymer ablation by laser irradiation is established, which can describe the ablation mechanism change induced by the variation of pyrolysis rate. The equilibrium composition of the plume with 14 components, ionization of the ablation products, plasma evolution and its dynamic shielding effect are considered in the model. By the cooperation of self-programming software and business software, a multi-disciplinary couping numerical simulation system is set up. Typical validation examples and experiments of high-speed photography and impulse measurement are designed to verify the functions and accuracies of the system. The results show that, the system runs steadily and the calculation results are appropriate.The propulsion mechanism and performance of polymer ablation induced by single pulsed laser are analysed numerically. The effect of the laser fluence and laser energy temporal distribution on the impulse generation process and the propulsion performance are discussed, and the quadratic regression model of LAP performance on laser intensity and pulse width are obtain as well as the optimal parameters. The ablation process of polymer irradiated by double pulsed lasers are analysed from laser transmission efficiency and mass ablation density; and the development of the plume and the impulse generation mechanism of the double pulsed laser working process are investigated comprehensively. Consequently, the contributions of both pulses to the improvement of the propulsion performance are clear. The first pulse provides sufficient ablation products; the second pulse ionizes the ablation product plume produced by the first pulse rapidly and maintains a Laser-Supported Detonation wave(LSD) moving towards the incidence laser. Therefore, the emission speed of the product ablated by the first pulse laser increases and the mass ablated by the second pulse decreases evidently, which leads to a higher specific impulse. Using the orthogonal experimental design method, the laser parameters of both pulses are evaluated to optimize the propulsion performance.An ablation and impulse generation numerical model for the metal particle doped polymer irradiated by pulsed laser is set up, which considers the mecro distribution of the metal particle in the polymer base. The effect of metal particle to the ablation process, plume development and shielding effect are discussed. Because of the distinct diversities of the material parameters, the metal particles are mainly ablated in the form of denudation; the utilization rate of the metal particle is so inefficient that the effect of metal doping to the propulsion performance is negative.An impulse generation numerical model for the metal grid assisted polymer irradiated by pulsed laser is established, which can take account of the characteristics of the ablation process and plume induced by the metal grid. The results of the metal grid assisted polymer show that metal grid prompts a high pressure region beside the metal grid and a low speed region between the metal grid and the ablation surface. When the laser is turned off, the two regions slow up the pressure attenuation noticeably, this involves the improvement of the propulsion performance comprehensively. The polymer ablation velocity is the key acting factor, and the metal grid adjusts the impulse characteristic on the basis of the polymer properties. Finaly, using the orthogonal experimental design method, the laser parameters and the metal grid parameters are evaluated to optimize the propulsion performance and the optimal parameters are obtained.As a summary, this dissertation has studied the propulsion mechanism and the parameter dependence of LAP performance systematically. The conclusions can be applied to direct the design and parameter selection of LAP thruster, and can support the development of micro satellite launching with laser propulsion from the aspect of theory.